Hello!
This series provides LKMM updates:
1. fix references for DMA*.txt files.
2. Replace HTTP links with HTTPS ones: LKMM.
3. tools/memory-model: Update recipes.txt prime_numbers.c path.
4. tools/memory-model: Improve litmus-test documentation.
5. tools/memory-model: Add a simple entry point document.
6. tools/memory-model: Expand the cheatsheet.txt notion of relaxed.
7. tools/memory-model: Move Documentation description to
Documentation/README.
8. tools/memory-model: Document categories of ordering primitives.
9. tools/memory-model: Document locking corner cases.
Thanx, Paul
------------------------------------------------------------------------
Documentation/litmus-tests/locking/DCL-broken.litmus | 55
Documentation/litmus-tests/locking/DCL-fixed.litmus | 56
Documentation/litmus-tests/locking/RM-broken.litmus | 42
Documentation/litmus-tests/locking/RM-fixed.litmus | 42
Documentation/memory-barriers.txt | 6
tools/memory-model/Documentation/README | 86 +
tools/memory-model/Documentation/cheatsheet.txt | 27
tools/memory-model/Documentation/litmus-tests.txt | 1078 ++++++++++++++++++-
tools/memory-model/Documentation/locking.txt | 320 +++++
tools/memory-model/Documentation/ordering.txt | 462 ++++++++
tools/memory-model/Documentation/recipes.txt | 4
tools/memory-model/Documentation/references.txt | 2
tools/memory-model/Documentation/simple.txt | 271 ++++
tools/memory-model/README | 182 ---
tools/memory-model/control-dependencies.txt | 256 ++++
15 files changed, 2730 insertions(+), 159 deletions(-)
From: "Alexander A. Klimov" <[email protected]>
Rationale:
Reduces attack surface on kernel devs opening the links for MITM
as HTTPS traffic is much harder to manipulate.
Deterministic algorithm:
For each file:
If not .svg:
For each line:
If doesn't contain `\bxmlns\b`:
For each link, `\bhttp://[^# \t\r\n]*(?:\w|/)`:
If both the HTTP and HTTPS versions
return 200 OK and serve the same content:
Replace HTTP with HTTPS.
Signed-off-by: Alexander A. Klimov <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>
---
tools/memory-model/Documentation/references.txt | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/tools/memory-model/Documentation/references.txt b/tools/memory-model/Documentation/references.txt
index ecbbaa5..c5fdfd1 100644
--- a/tools/memory-model/Documentation/references.txt
+++ b/tools/memory-model/Documentation/references.txt
@@ -120,7 +120,7 @@ o Jade Alglave, Luc Maranget, and Michael Tautschnig. 2014. "Herding
o Jade Alglave, Patrick Cousot, and Luc Maranget. 2016. "Syntax and
semantics of the weak consistency model specification language
- cat". CoRR abs/1608.07531 (2016). http://arxiv.org/abs/1608.07531
+ cat". CoRR abs/1608.07531 (2016). https://arxiv.org/abs/1608.07531
Memory-model comparisons
--
2.9.5
From: "Paul E. McKenney" <[email protected]>
The current LKMM documentation says very little about litmus tests, and
worse yet directs people to the herd7 documentation for more information.
Now, the herd7 documentation is quite voluminous and educational,
but it is intended for people creating and modifying memory models,
not those attempting to use them.
This commit therefore updates README and creates a litmus-tests.txt
file that gives an overview of litmus-test format and describes ways of
modeling various special cases, illustrated with numerous examples.
[ paulmck: Add Alan Stern feedback. ]
[ paulmck: Apply Dave Chinner feedback. ]
[ paulmck: Apply Andrii Nakryiko feedback. ]
[ paulmck: Apply Johannes Weiner feedback. ]
Link: https://lwn.net/Articles/827180/
Reported-by: Dave Chinner <[email protected]>
Acked-by: Peter Zijlstra (Intel) <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>
---
tools/memory-model/Documentation/litmus-tests.txt | 1070 +++++++++++++++++++++
tools/memory-model/README | 155 +--
2 files changed, 1108 insertions(+), 117 deletions(-)
create mode 100644 tools/memory-model/Documentation/litmus-tests.txt
diff --git a/tools/memory-model/Documentation/litmus-tests.txt b/tools/memory-model/Documentation/litmus-tests.txt
new file mode 100644
index 0000000..289a38d
--- /dev/null
+++ b/tools/memory-model/Documentation/litmus-tests.txt
@@ -0,0 +1,1070 @@
+Linux-Kernel Memory Model Litmus Tests
+======================================
+
+This file describes the LKMM litmus-test format by example, describes
+some tricks and traps, and finally outlines LKMM's limitations. Earlier
+versions of this material appeared in a number of LWN articles, including:
+
+https://lwn.net/Articles/720550/
+ A formal kernel memory-ordering model (part 2)
+https://lwn.net/Articles/608550/
+ Axiomatic validation of memory barriers and atomic instructions
+https://lwn.net/Articles/470681/
+ Validating Memory Barriers and Atomic Instructions
+
+This document presents information in decreasing order of applicability,
+so that, where possible, the information that has proven more commonly
+useful is shown near the beginning.
+
+For information on installing LKMM, including the underlying "herd7"
+tool, please see tools/memory-model/README.
+
+
+Copy-Pasta
+==========
+
+As with other software, it is often better (if less macho) to adapt an
+existing litmus test than it is to create one from scratch. A number
+of litmus tests may be found in the kernel source tree:
+
+ tools/memory-model/litmus-tests/
+ Documentation/litmus-tests/
+
+Several thousand more example litmus tests are available on github
+and kernel.org:
+
+ https://github.com/paulmckrcu/litmus
+ https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/perfbook.git/tree/CodeSamples/formal/herd
+ https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/perfbook.git/tree/CodeSamples/formal/litmus
+
+The -l and -L arguments to "git grep" can be quite helpful in identifying
+existing litmus tests that are similar to the one you need. But even if
+you start with an existing litmus test, it is still helpful to have a
+good understanding of the litmus-test format.
+
+
+Examples and Format
+===================
+
+This section describes the overall format of litmus tests, starting
+with a small example of the message-passing pattern and moving on to
+more complex examples that illustrate explicit initialization and LKMM's
+minimalistic set of flow-control statements.
+
+
+Message-Passing Example
+-----------------------
+
+This section gives an overview of the format of a litmus test using an
+example based on the common message-passing use case. This use case
+appears often in the Linux kernel. For example, a flag (modeled by "y"
+below) indicates that a buffer (modeled by "x" below) is now completely
+filled in and ready for use. It would be very bad if the consumer saw the
+flag set, but, due to memory misordering, saw old values in the buffer.
+
+This example asks whether smp_store_release() and smp_load_acquire()
+suffices to avoid this bad outcome:
+
+ 1 C MP+pooncerelease+poacquireonce
+ 2
+ 3 {}
+ 4
+ 5 P0(int *x, int *y)
+ 6 {
+ 7 WRITE_ONCE(*x, 1);
+ 8 smp_store_release(y, 1);
+ 9 }
+10
+11 P1(int *x, int *y)
+12 {
+13 int r0;
+14 int r1;
+15
+16 r0 = smp_load_acquire(y);
+17 r1 = READ_ONCE(*x);
+18 }
+19
+20 exists (1:r0=1 /\ 1:r1=0)
+
+Line 1 starts with "C", which identifies this file as being in the
+LKMM C-language format (which, as we will see, is a small fragment
+of the full C language). The remainder of line 1 is the name of
+the test, which by convention is the filename with the ".litmus"
+suffix stripped. In this case, the actual test may be found in
+tools/memory-model/litmus-tests/MP+pooncerelease+poacquireonce.litmus
+in the Linux-kernel source tree.
+
+Mechanically generated litmus tests will often have an optional
+double-quoted comment string on the second line. Such strings are ignored
+when running the test. Yes, you can add your own comments to litmus
+tests, but this is a bit involved due to the use of multiple parsers.
+For now, you can use C-language comments in the C code, and these comments
+may be in either the "/* */" or the "//" style. A later section will
+cover the full litmus-test commenting story.
+
+Line 3 is the initialization section. Because the default initialization
+to zero suffices for this test, the "{}" syntax is used, which mean the
+initialization section is empty. Litmus tests requiring non-default
+initialization must have non-empty initialization sections, as in the
+example that will be presented later in this document.
+
+Lines 5-9 show the first process and lines 11-18 the second process. Each
+process corresponds to a Linux-kernel task (or kthread, workqueue, thread,
+and so on; LKMM discussions often use these terms interchangeably).
+The name of the first process is "P0" and that of the second "P1".
+You can name your processes anything you like as long as the names consist
+of a single "P" followed by a number, and as long as the numbers are
+consecutive starting with zero. This can actually be quite helpful,
+for example, a .litmus file matching "^P1(" but not matching "^P2("
+must contain a two-process litmus test.
+
+The argument list for each function are pointers to the global variables
+used by that function. Unlike normal C-language function parameters, the
+names are significant. The fact that both P0() and P1() have a formal
+parameter named "x" means that these two processes are working with the
+same global variable, also named "x". So the "int *x, int *y" on P0()
+and P1() mean that both processes are working with two shared global
+variables, "x" and "y". Global variables are always passed to processes
+by reference, hence "P0(int *x, int *y)", but *never* "P0(int x, int y)".
+
+P0() has no local variables, but P1() has two of them named "r0" and "r1".
+These names may be freely chosen, but for historical reasons stemming from
+other litmus-test formats, it is conventional to use names consisting of
+"r" followed by a number as shown here. A common bug in litmus tests
+is forgetting to add a global variable to a process's parameter list.
+This will sometimes result in an error message, but can also cause the
+intended global to instead be silently treated as an undeclared local
+variable.
+
+Each process's code is similar to Linux-kernel C, as can be seen on lines
+7-8 and 13-17. This code may use many of the Linux kernel's atomic
+operations, some of its exclusive-lock functions, and some of its RCU
+and SRCU functions. An approximate list of the currently supported
+functions may be found in the linux-kernel.def file.
+
+The P0() process does "WRITE_ONCE(*x, 1)" on line 7. Because "x" is a
+pointer in P0()'s parameter list, this does an unordered store to global
+variable "x". Line 8 does "smp_store_release(y, 1)", and because "y"
+is also in P0()'s parameter list, this does a release store to global
+variable "y".
+
+The P1() process declares two local variables on lines 13 and 14.
+Line 16 does "r0 = smp_load_acquire(y)" which does an acquire load
+from global variable "y" into local variable "r0". Line 17 does a
+"r1 = READ_ONCE(*x)", which does an unordered load from "*x" into local
+variable "r1". Both "x" and "y" are in P1()'s parameter list, so both
+reference the same global variables that are used by P0().
+
+Line 20 is the "exists" assertion expression to evaluate the final state.
+This final state is evaluated after the dust has settled: both processes
+have completed and all of their memory references and memory barriers
+have propagated to all parts of the system. The references to the local
+variables "r0" and "r1" in line 24 must be prefixed with "1:" to specify
+which process they are local to.
+
+Note that the assertion expression is written in the litmus-test
+language rather than in C. For example, single "=" is an equality
+operator rather than an assignment. The "/\" character combination means
+"and". Similarly, "\/" stands for "or". Both of these are ASCII-art
+representations of the corresponding mathematical symbols. Finally,
+"~" stands for "logical not", which is "!" in C, and not to be confused
+with the C-language "~" operator which instead stands for "bitwise not".
+Parentheses may be used to override precedence.
+
+The "exists" assertion on line 20 is satisfied if the consumer sees the
+flag ("y") set but the buffer ("x") as not yet filled in, that is, if P1()
+loaded a value from "x" that was equal to 1 but loaded a value from "y"
+that was still equal to zero.
+
+This example can be checked by running the following command, which
+absolutely must be run from the tools/memory-model directory and from
+this directory only:
+
+herd7 -conf linux-kernel.cfg litmus-tests/MP+pooncerelease+poacquireonce.litmus
+
+The output is the result of something similar to a full state-space
+search, and is as follows:
+
+ 1 Test MP+pooncerelease+poacquireonce Allowed
+ 2 States 3
+ 3 1:r0=0; 1:r1=0;
+ 4 1:r0=0; 1:r1=1;
+ 5 1:r0=1; 1:r1=1;
+ 6 No
+ 7 Witnesses
+ 8 Positive: 0 Negative: 3
+ 9 Condition exists (1:r0=1 /\ 1:r1=0)
+10 Observation MP+pooncerelease+poacquireonce Never 0 3
+11 Time MP+pooncerelease+poacquireonce 0.00
+12 Hash=579aaa14d8c35a39429b02e698241d09
+
+The most pertinent line is line 10, which contains "Never 0 3", which
+indicates that the bad result flagged by the "exists" clause never
+happens. This line might instead say "Sometimes" to indicate that the
+bad result happened in some but not all executions, or it might say
+"Always" to indicate that the bad result happened in all executions.
+(The herd7 tool doesn't judge, so it is only an LKMM convention that the
+"exists" clause indicates a bad result. To see this, invert the "exists"
+clause's condition and run the test.) The numbers ("0 3") at the end
+of this line indicate the number of end states satisfying the "exists"
+clause (0) and the number not not satisfying that clause (3).
+
+Another important part of this output is shown in lines 2-5, repeated here:
+
+ 2 States 3
+ 3 1:r0=0; 1:r1=0;
+ 4 1:r0=0; 1:r1=1;
+ 5 1:r0=1; 1:r1=1;
+
+Line 2 gives the total number of end states, and each of lines 3-5 list
+one of these states, with the first ("1:r0=0; 1:r1=0;") indicating that
+both of P1()'s loads returned the value "0". As expected, given the
+"Never" on line 10, the state flagged by the "exists" clause is not
+listed. This full list of states can be helpful when debugging a new
+litmus test.
+
+The rest of the output is not normally needed, either due to irrelevance
+or due to being redundant with the lines discussed above. However, the
+following paragraph lists them for the benefit of readers possessed of
+an insatiable curiosity. Other readers should feel free to skip ahead.
+
+Line 1 echos the test name, along with the "Test" and "Allowed". Line 6's
+"No" says that the "exists" clause was not satisfied by any execution,
+and as such it has the same meaning as line 10's "Never". Line 7 is a
+lead-in to line 8's "Positive: 0 Negative: 3", which lists the number
+of end states satisfying and not satisfying the "exists" clause, just
+like the two numbers at the end of line 10. Line 9 repeats the "exists"
+clause so that you don't have to look it up in the litmus-test file.
+The number at the end of line 11 (which begins with "Time") gives the
+time in seconds required to analyze the litmus test. Small tests such
+as this one complete in a few milliseconds, so "0.00" is quite common.
+Line 12 gives a hash of the contents for the litmus-test file, and is used
+by tooling that manages litmus tests and their output. This tooling is
+used by people modifying LKMM itself, and among other things lets such
+people know which of the several thousand relevant litmus tests were
+affected by a given change to LKMM.
+
+
+Initialization
+--------------
+
+The previous example relied on the default zero initialization for
+"x" and "y", but a similar litmus test could instead initialize them
+to some other value:
+
+ 1 C MP+pooncerelease+poacquireonce
+ 2
+ 3 {
+ 4 x=42;
+ 5 y=42;
+ 6 }
+ 7
+ 8 P0(int *x, int *y)
+ 9 {
+10 WRITE_ONCE(*x, 1);
+11 smp_store_release(y, 1);
+12 }
+13
+14 P1(int *x, int *y)
+15 {
+16 int r0;
+17 int r1;
+18
+19 r0 = smp_load_acquire(y);
+20 r1 = READ_ONCE(*x);
+21 }
+22
+23 exists (1:r0=1 /\ 1:r1=42)
+
+Lines 3-6 now initialize both "x" and "y" to the value 42. This also
+means that the "exists" clause on line 23 must change "1:r1=0" to
+"1:r1=42".
+
+Running the test gives the same overall result as before, but with the
+value 42 appearing in place of the value zero:
+
+ 1 Test MP+pooncerelease+poacquireonce Allowed
+ 2 States 3
+ 3 1:r0=1; 1:r1=1;
+ 4 1:r0=42; 1:r1=1;
+ 5 1:r0=42; 1:r1=42;
+ 6 No
+ 7 Witnesses
+ 8 Positive: 0 Negative: 3
+ 9 Condition exists (1:r0=1 /\ 1:r1=42)
+10 Observation MP+pooncerelease+poacquireonce Never 0 3
+11 Time MP+pooncerelease+poacquireonce 0.02
+12 Hash=ab9a9b7940a75a792266be279a980156
+
+It is tempting to avoid the open-coded repetitions of the value "42"
+by defining another global variable "initval=42" and replacing all
+occurrences of "42" with "initval". This will not, repeat *not*,
+initialize "x" and "y" to 42, but instead to the address of "initval"
+(try it!). See the section below on linked lists to learn more about
+why this approach to initialization can be useful.
+
+
+Control Structures
+------------------
+
+LKMM supports the C-language "if" statement, which allows modeling of
+conditional branches. In LKMM, conditional branches can affect ordering,
+but only if you are *very* careful (compilers are surprisingly able
+to optimize away conditional branches). The following example shows
+the "load buffering" (LB) use case that is used in the Linux kernel to
+synchronize between ring-buffer producers and consumers. In the example
+below, P0() is one side checking to see if an operation may proceed and
+P1() is the other side completing its update.
+
+ 1 C LB+fencembonceonce+ctrlonceonce
+ 2
+ 3 {}
+ 4
+ 5 P0(int *x, int *y)
+ 6 {
+ 7 int r0;
+ 8
+ 9 r0 = READ_ONCE(*x);
+10 if (r0)
+11 WRITE_ONCE(*y, 1);
+12 }
+13
+14 P1(int *x, int *y)
+15 {
+16 int r0;
+17
+18 r0 = READ_ONCE(*y);
+19 smp_mb();
+20 WRITE_ONCE(*x, 1);
+21 }
+22
+23 exists (0:r0=1 /\ 1:r0=1)
+
+P1()'s "if" statement on line 10 works as expected, so that line 11 is
+executed only if line 9 loads a non-zero value from "x". Because P1()'s
+write of "1" to "x" happens only after P1()'s read from "y", one would
+hope that the "exists" clause cannot be satisfied. LKMM agrees:
+
+ 1 Test LB+fencembonceonce+ctrlonceonce Allowed
+ 2 States 2
+ 3 0:r0=0; 1:r0=0;
+ 4 0:r0=1; 1:r0=0;
+ 5 No
+ 6 Witnesses
+ 7 Positive: 0 Negative: 2
+ 8 Condition exists (0:r0=1 /\ 1:r0=1)
+ 9 Observation LB+fencembonceonce+ctrlonceonce Never 0 2
+10 Time LB+fencembonceonce+ctrlonceonce 0.00
+11 Hash=e5260556f6de495fd39b556d1b831c3b
+
+However, there is no "while" statement due to the fact that full
+state-space search has some difficulty with iteration. However, there
+are tricks that may be used to handle some special cases, which are
+discussed below. In addition, loop-unrolling tricks may be applied,
+albeit sparingly.
+
+
+Tricks and Traps
+================
+
+This section covers extracting debug output from herd7, emulating
+spin loops, handling trivial linked lists, adding comments to litmus tests,
+emulating call_rcu(), and finally tricks to improve herd7 performance
+in order to better handle large litmus tests.
+
+
+Debug Output
+------------
+
+By default, the herd7 state output includes all variables mentioned
+in the "exists" clause. But sometimes debugging efforts are greatly
+aided by the values of other variables. Consider this litmus test
+(tools/memory-order/litmus-tests/SB+rfionceonce-poonceonces.litmus but
+slightly modified), which probes an obscure corner of hardware memory
+ordering:
+
+ 1 C SB+rfionceonce-poonceonces
+ 2
+ 3 {}
+ 4
+ 5 P0(int *x, int *y)
+ 6 {
+ 7 int r1;
+ 8 int r2;
+ 9
+10 WRITE_ONCE(*x, 1);
+11 r1 = READ_ONCE(*x);
+12 r2 = READ_ONCE(*y);
+13 }
+14
+15 P1(int *x, int *y)
+16 {
+17 int r3;
+18 int r4;
+19
+20 WRITE_ONCE(*y, 1);
+21 r3 = READ_ONCE(*y);
+22 r4 = READ_ONCE(*x);
+23 }
+24
+25 exists (0:r2=0 /\ 1:r4=0)
+
+The herd7 output is as follows:
+
+ 1 Test SB+rfionceonce-poonceonces Allowed
+ 2 States 4
+ 3 0:r2=0; 1:r4=0;
+ 4 0:r2=0; 1:r4=1;
+ 5 0:r2=1; 1:r4=0;
+ 6 0:r2=1; 1:r4=1;
+ 7 Ok
+ 8 Witnesses
+ 9 Positive: 1 Negative: 3
+10 Condition exists (0:r2=0 /\ 1:r4=0)
+11 Observation SB+rfionceonce-poonceonces Sometimes 1 3
+12 Time SB+rfionceonce-poonceonces 0.01
+13 Hash=c7f30fe0faebb7d565405d55b7318ada
+
+(This output indicates that CPUs are permitted to "snoop their own
+store buffers", which all of Linux's CPU families other than s390 will
+happily do. Such snooping results in disagreement among CPUs on the
+order of stores from different CPUs, which is rarely an issue.)
+
+But the herd7 output shows only the two variables mentioned in the
+"exists" clause. Someone modifying this test might wish to know the
+values of "x", "y", "0:r1", and "0:r3" as well. The "locations"
+statement on line 25 shows how to cause herd7 to display additional
+variables:
+
+ 1 C SB+rfionceonce-poonceonces
+ 2
+ 3 {}
+ 4
+ 5 P0(int *x, int *y)
+ 6 {
+ 7 int r1;
+ 8 int r2;
+ 9
+10 WRITE_ONCE(*x, 1);
+11 r1 = READ_ONCE(*x);
+12 r2 = READ_ONCE(*y);
+13 }
+14
+15 P1(int *x, int *y)
+16 {
+17 int r3;
+18 int r4;
+19
+20 WRITE_ONCE(*y, 1);
+21 r3 = READ_ONCE(*y);
+22 r4 = READ_ONCE(*x);
+23 }
+24
+25 locations [0:r1; 1:r3; x; y]
+26 exists (0:r2=0 /\ 1:r4=0)
+
+The herd7 output then displays the values of all the variables:
+
+ 1 Test SB+rfionceonce-poonceonces Allowed
+ 2 States 4
+ 3 0:r1=1; 0:r2=0; 1:r3=1; 1:r4=0; x=1; y=1;
+ 4 0:r1=1; 0:r2=0; 1:r3=1; 1:r4=1; x=1; y=1;
+ 5 0:r1=1; 0:r2=1; 1:r3=1; 1:r4=0; x=1; y=1;
+ 6 0:r1=1; 0:r2=1; 1:r3=1; 1:r4=1; x=1; y=1;
+ 7 Ok
+ 8 Witnesses
+ 9 Positive: 1 Negative: 3
+10 Condition exists (0:r2=0 /\ 1:r4=0)
+11 Observation SB+rfionceonce-poonceonces Sometimes 1 3
+12 Time SB+rfionceonce-poonceonces 0.01
+13 Hash=40de8418c4b395388f6501cafd1ed38d
+
+What if you would like to know the value of a particular global variable
+at some particular point in a given process's execution? One approach
+is to use a READ_ONCE() to load that global variable into a new local
+variable, then add that local variable to the "locations" clause.
+But be careful: In some litmus tests, adding a READ_ONCE() will change
+the outcome! For one example, please see the C-READ_ONCE.litmus and
+C-READ_ONCE-omitted.litmus tests located here:
+
+ https://github.com/paulmckrcu/litmus/blob/master/manual/kernel/
+
+
+Spin Loops
+----------
+
+The analysis carried out by herd7 explores full state space, which is
+at best of exponential time complexity. Adding processes and increasing
+the amount of code in a give process can greatly increase execution time.
+Potentially infinite loops, such as those used to wait for locks to
+become available, are clearly problematic.
+
+Fortunately, it is possible to avoid state-space explosion by specially
+modeling such loops. For example, the following litmus tests emulates
+locking using xchg_acquire(), but instead of enclosing xchg_acquire()
+in a spin loop, it instead excludes executions that fail to acquire the
+lock using a herd7 "filter" clause. Note that for exclusive locking, you
+are better off using the spin_lock() and spin_unlock() that LKMM directly
+models, if for no other reason that these are much faster. However, the
+techniques illustrated in this section can be used for other purposes,
+such as emulating reader-writer locking, which LKMM does not yet model.
+
+ 1 C C-SB+l-o-o-u+l-o-o-u-X
+ 2
+ 3 {
+ 4 }
+ 5
+ 6 P0(int *sl, int *x0, int *x1)
+ 7 {
+ 8 int r2;
+ 9 int r1;
+10
+11 r2 = xchg_acquire(sl, 1);
+12 WRITE_ONCE(*x0, 1);
+13 r1 = READ_ONCE(*x1);
+14 smp_store_release(sl, 0);
+15 }
+16
+17 P1(int *sl, int *x0, int *x1)
+18 {
+19 int r2;
+20 int r1;
+21
+22 r2 = xchg_acquire(sl, 1);
+23 WRITE_ONCE(*x1, 1);
+24 r1 = READ_ONCE(*x0);
+25 smp_store_release(sl, 0);
+26 }
+27
+28 filter (0:r2=0 /\ 1:r2=0)
+29 exists (0:r1=0 /\ 1:r1=0)
+
+This litmus test may be found here:
+
+https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/perfbook.git/tree/CodeSamples/formal/herd/C-SB+l-o-o-u+l-o-o-u-X.litmus
+
+This test uses two global variables, "x1" and "x2", and also emulates a
+single global spinlock named "sl". This spinlock is held by whichever
+process changes the value of "sl" from "0" to "1", and is released when
+that process sets "sl" back to "0". P0()'s lock acquisition is emulated
+on line 11 using xchg_acquire(), which unconditionally stores the value
+"1" to "sl" and stores either "0" or "1" to "r2", depending on whether
+the lock acquisition was successful or unsuccessful (due to "sl" already
+having the value "1"), respectively. P1() operates in a similar manner.
+
+Rather unconventionally, execution appears to proceed to the critical
+section on lines 12 and 13 in either case. Line 14 then uses an
+smp_store_release() to store zero to "sl", thus emulating lock release.
+
+The case where xchg_acquire() fails to acquire the lock is handled by
+the "filter" clause on line 28, which tells herd7 to keep only those
+executions in which both "0:r2" and "1:r2" are zero, that is to pay
+attention only to those executions in which both locks are actually
+acquired. Thus, the bogus executions that would execute the critical
+sections are discarded and any effects that they might have had are
+ignored. Note well that the "filter" clause keeps those executions
+for which its expression is satisfied, that is, for which the expression
+evaluates to true. In other words, the "filter" clause says what to
+keep, not what to discard.
+
+The result of running this test is as follows:
+
+ 1 Test C-SB+l-o-o-u+l-o-o-u-X Allowed
+ 2 States 2
+ 3 0:r1=0; 1:r1=1;
+ 4 0:r1=1; 1:r1=0;
+ 5 No
+ 6 Witnesses
+ 7 Positive: 0 Negative: 2
+ 8 Condition exists (0:r1=0 /\ 1:r1=0)
+ 9 Observation C-SB+l-o-o-u+l-o-o-u-X Never 0 2
+10 Time C-SB+l-o-o-u+l-o-o-u-X 0.03
+
+The "Never" on line 9 indicates that this use of xchg_acquire() and
+smp_store_release() really does correctly emulate locking.
+
+Why doesn't the litmus test take the simpler approach of using a spin loop
+to handle failed spinlock acquisitions, like the kernel does? The key
+insight behind this litmus test is that spin loops have no effect on the
+possible "exists"-clause outcomes of program execution in the absence
+of deadlock. In other words, given a high-quality lock-acquisition
+primitive in a deadlock-free program running on high-quality hardware,
+each lock acquisition will eventually succeed. Because herd7 already
+explores the full state space, the length of time required to actually
+acquire the lock does not matter. After all, herd7 already models all
+possible durations of the xchg_acquire() statements.
+
+Why not just add the "filter" clause to the "exists" clause, thus
+avoiding the "filter" clause entirely? This does work, but is slower.
+The reason that the "filter" clause is faster is that (in the common case)
+herd7 knows to abandon an execution as soon as the "filter" expression
+fails to be satisfied. In contrast, the "exists" clause is evaluated
+only at the end of time, thus requiring herd7 to waste time on bogus
+executions in which both critical sections proceed concurrently. In
+addition, some LKMM users like the separation of concerns provided by
+using the both the "filter" and "exists" clauses.
+
+Readers lacking a pathological interest in odd corner cases should feel
+free to skip the remainder of this section.
+
+But what if the litmus test were to temporarily set "0:r2" to a non-zero
+value? Wouldn't that cause herd7 to abandon the execution prematurely
+due to an early mismatch of the "filter" clause?
+
+Why not just try it? Line 4 of the following modified litmus test
+introduces a new global variable "x2" that is initialized to "1". Line 23
+of P1() reads that variable into "1:r2" to force an early mismatch with
+the "filter" clause. Line 24 does a known-true "if" condition to avoid
+and static analysis that herd7 might do. Finally the "exists" clause
+on line 32 is updated to a condition that is alway satisfied at the end
+of the test.
+
+ 1 C C-SB+l-o-o-u+l-o-o-u-X
+ 2
+ 3 {
+ 4 x2=1;
+ 5 }
+ 6
+ 7 P0(int *sl, int *x0, int *x1)
+ 8 {
+ 9 int r2;
+10 int r1;
+11
+12 r2 = xchg_acquire(sl, 1);
+13 WRITE_ONCE(*x0, 1);
+14 r1 = READ_ONCE(*x1);
+15 smp_store_release(sl, 0);
+16 }
+17
+18 P1(int *sl, int *x0, int *x1, int *x2)
+19 {
+20 int r2;
+21 int r1;
+22
+23 r2 = READ_ONCE(*x2);
+24 if (r2)
+25 r2 = xchg_acquire(sl, 1);
+26 WRITE_ONCE(*x1, 1);
+27 r1 = READ_ONCE(*x0);
+28 smp_store_release(sl, 0);
+29 }
+30
+31 filter (0:r2=0 /\ 1:r2=0)
+32 exists (x1=1)
+
+If the "filter" clause were to check each variable at each point in the
+execution, running this litmus test would display no executions because
+all executions would be filtered out at line 23. However, the output
+is instead as follows:
+
+ 1 Test C-SB+l-o-o-u+l-o-o-u-X Allowed
+ 2 States 1
+ 3 x1=1;
+ 4 Ok
+ 5 Witnesses
+ 6 Positive: 2 Negative: 0
+ 7 Condition exists (x1=1)
+ 8 Observation C-SB+l-o-o-u+l-o-o-u-X Always 2 0
+ 9 Time C-SB+l-o-o-u+l-o-o-u-X 0.04
+10 Hash=080bc508da7f291e122c6de76c0088e3
+
+Line 3 shows that there is one execution that did not get filtered out,
+so the "filter" clause is evaluated only on the last assignment to
+the variables that it checks. In this case, the "filter" clause is a
+disjunction, so it might be evaluated twice, once at the final (and only)
+assignment to "0:r2" and once at the final assignment to "1:r2".
+
+
+Linked Lists
+------------
+
+LKMM can handle linked lists, but only linked lists in which each node
+contains nothing except a pointer to the next node in the list. This is
+of course quite restrictive, but there is nevertheless quite a bit that
+can be done within these confines, as can be seen in the litmus test
+at tools/memory-model/litmus-tests/MP+onceassign+derefonce.litmus:
+
+ 1 C MP+onceassign+derefonce
+ 2
+ 3 {
+ 4 y=z;
+ 5 z=0;
+ 6 }
+ 7
+ 8 P0(int *x, int **y)
+ 9 {
+10 WRITE_ONCE(*x, 1);
+11 rcu_assign_pointer(*y, x);
+12 }
+13
+14 P1(int *x, int **y)
+15 {
+16 int *r0;
+17 int r1;
+18
+19 rcu_read_lock();
+20 r0 = rcu_dereference(*y);
+21 r1 = READ_ONCE(*r0);
+22 rcu_read_unlock();
+23 }
+24
+25 exists (1:r0=x /\ 1:r1=0)
+
+Line 4's "y=z" may seem odd, given that "z" has not yet been initialized.
+But "y=z" does not set the value of "y" to that of "z", but instead
+sets the value of "y" to the *address* of "z". Lines 4 and 5 therefore
+create a simple linked list, with "y" pointing to "z" and "z" having a
+NULL pointer. A much longer linked list could be created if desired,
+and circular singly linked lists can also be created and manipulated.
+
+The "exists" clause works the same way, with the "1:r0=x" comparing P1()'s
+"r0" not to the value of "x", but again to its address. This term of the
+"exists" clause therefore tests whether line 20's load from "y" saw the
+value stored by line 11, which is in fact what is required in this case.
+
+P0()'s line 10 initializes "x" to the value 1 then line 11 links to "x"
+from "y", replacing "z".
+
+P1()'s line 20 loads a pointer from "y", and line 21 dereferences that
+pointer. The RCU read-side critical section spanning lines 19-22 is
+just for show in this example.
+
+Running this test results in the following:
+
+ 1 Test MP+onceassign+derefonce Allowed
+ 2 States 2
+ 3 1:r0=x; 1:r1=1;
+ 4 1:r0=z; 1:r1=0;
+ 5 No
+ 6 Witnesses
+ 7 Positive: 0 Negative: 2
+ 8 Condition exists (1:r0=x /\ 1:r1=0)
+ 9 Observation MP+onceassign+derefonce Never 0 2
+10 Time MP+onceassign+derefonce 0.00
+11 Hash=49ef7a741563570102448a256a0c8568
+
+The only possible outcomes feature P1() loading a pointer to "z"
+(which contains zero) on the one hand and P1() loading a pointer to "x"
+(which contains the value one) on the other. This should be reassuring
+because it says that RCU readers cannot see the old preinitialization
+values when accessing a newly inserted list node. This undesirable
+scenario is flagged by the "exists" clause, and would occur if P1()
+loaded a pointer to "x", but obtained the pre-initialization value of
+zero after dereferencing that pointer.
+
+
+Comments
+--------
+
+Different portions of a litmus test are processed by different parsers,
+which has the charming effect of requiring different comment syntax in
+different portions of the litmus test. The C-syntax portions use
+C-language comments (either "/* */" or "//"), while the other portions
+use Ocaml comments "(* *)".
+
+The following litmus test illustrates the comment style corresponding
+to each syntactic unit of the test:
+
+ 1 C MP+onceassign+derefonce (* A *)
+ 2
+ 3 (* B *)
+ 4
+ 5 {
+ 6 y=z; (* C *)
+ 7 z=0;
+ 8 } // D
+ 9
+10 // E
+11
+12 P0(int *x, int **y) // F
+13 {
+14 WRITE_ONCE(*x, 1); // G
+15 rcu_assign_pointer(*y, x);
+16 }
+17
+18 // H
+19
+20 P1(int *x, int **y)
+21 {
+22 int *r0;
+23 int r1;
+24
+25 rcu_read_lock();
+26 r0 = rcu_dereference(*y);
+27 r1 = READ_ONCE(*r0);
+28 rcu_read_unlock();
+29 }
+30
+31 // I
+32
+33 exists (* J *) (1:r0=x /\ (* K *) 1:r1=0) (* L *)
+
+In short, use C-language comments in the C code and Ocaml comments in
+the rest of the litmus test.
+
+On the other hand, if you prefer C-style comments everywhere, the
+C preprocessor is your friend.
+
+
+Asynchronous RCU Grace Periods
+------------------------------
+
+The following litmus test is derived from the example show in
+Documentation/litmus-tests/rcu/RCU+sync+free.litmus, but converted to
+emulate call_rcu():
+
+ 1 C RCU+sync+free
+ 2
+ 3 {
+ 4 int x = 1;
+ 5 int *y = &x;
+ 6 int z = 1;
+ 7 }
+ 8
+ 9 P0(int *x, int *z, int **y)
+10 {
+11 int *r0;
+12 int r1;
+13
+14 rcu_read_lock();
+15 r0 = rcu_dereference(*y);
+16 r1 = READ_ONCE(*r0);
+17 rcu_read_unlock();
+18 }
+19
+20 P1(int *z, int **y, int *c)
+21 {
+22 rcu_assign_pointer(*y, z);
+23 smp_store_release(*c, 1); // Emulate call_rcu().
+24 }
+25
+26 P2(int *x, int *z, int **y, int *c)
+27 {
+28 int r0;
+29
+30 r0 = smp_load_acquire(*c); // Note call_rcu() request.
+31 synchronize_rcu(); // Wait one grace period.
+32 WRITE_ONCE(*x, 0); // Emulate the RCU callback.
+33 }
+34
+35 filter (2:r0=1) (* Reject too-early starts. *)
+36 exists (0:r0=x /\ 0:r1=0)
+
+Lines 4-6 initialize a linked list headed by "y" that initially contains
+"x". In addition, "z" is pre-initialized to prepare for P1(), which
+will replace "x" with "z" in this list.
+
+P0() on lines 9-18 enters an RCU read-side critical section, loads the
+list header "y" and dereferences it, leaving the node in "0:r0" and
+the node's value in "0:r1".
+
+P1() on lines 20-24 updates the list header to instead reference "z",
+then emulates call_rcu() by doing a release store into "c".
+
+P2() on lines 27-33 emulates the behind-the-scenes effect of doing a
+call_rcu(). Line 30 first does an acquire load from "c", then line 31
+waits for an RCU grace period to elapse, and finally line 32 emulates
+the RCU callback, which in turn emulates a call to kfree().
+
+Of course, it is possible for P2() to start too soon, so that the
+value of "2:r0" is zero rather than the required value of "1".
+The "filter" clause on line 35 handles this possibility, rejecting
+all executions in which "2:r0" is not equal to the value "1".
+
+
+Performance
+-----------
+
+LKMM's exploration of the full state-space can be extremely helpful,
+but it does not come for free. The price is exponential computational
+complexity in terms of the number of processes, the average number
+of statements in each process, and the total number of stores in the
+litmus test.
+
+So it is best to start small and then work up. Where possible, break
+your code down into small pieces each representing a core concurrency
+requirement.
+
+That said, herd7 is quite fast. On an unprepossessing x86 laptop, it
+was able to analyze the following 10-process RCU litmus test in about
+six seconds.
+
+https://github.com/paulmckrcu/litmus/blob/master/auto/C-RW-R+RW-R+RW-G+RW-G+RW-G+RW-G+RW-R+RW-R+RW-R+RW-R.litmus
+
+One way to make herd7 run faster is to use the "-speedcheck true" option.
+This option prevents herd7 from generating all possible end states,
+instead causing it to focus solely on whether or not the "exists"
+clause can be satisfied. With this option, herd7 evaluates the above
+litmus test in about 300 milliseconds, for more than an order of magnitude
+improvement in performance.
+
+Larger 16-process litmus tests that would normally consume 15 minutes
+of time complete in about 40 seconds with this option. To be fair,
+you do get an extra 65,535 states when you leave off the "-speedcheck
+true" option.
+
+https://github.com/paulmckrcu/litmus/blob/master/auto/C-RW-R+RW-R+RW-G+RW-G+RW-G+RW-G+RW-R+RW-R+RW-R+RW-R+RW-G+RW-G+RW-G+RW-G+RW-R+RW-R.litmus
+
+Nevertheless, litmus-test analysis really is of exponential complexity,
+whether with or without "-speedcheck true". Increasing by just three
+processes to a 19-process litmus test requires 2 hours and 40 minutes
+without, and about 8 minutes with "-speedcheck true". Each of these
+results represent roughly an order of magnitude slowdown compared to the
+16-process litmus test. Again, to be fair, the multi-hour run explores
+no fewer than 524,287 additional states compared to the shorter one.
+
+https://github.com/paulmckrcu/litmus/blob/master/auto/C-RW-R+RW-R+RW-G+RW-G+RW-G+RW-G+RW-R+RW-R+RW-R+RW-R+RW-R+RW-R+RW-G+RW-G+RW-G+RW-G+RW-R+RW-R+RW-R.litmus
+
+If you don't like command-line arguments, you can obtain a similar speedup
+by adding a "filter" clause with exactly the same expression as your
+"exists" clause.
+
+However, please note that seeing the full set of states can be extremely
+helpful when developing and debugging litmus tests.
+
+
+LIMITATIONS
+===========
+
+Limitations of the Linux-kernel memory model (LKMM) include:
+
+1. Compiler optimizations are not accurately modeled. Of course,
+ the use of READ_ONCE() and WRITE_ONCE() limits the compiler's
+ ability to optimize, but under some circumstances it is possible
+ for the compiler to undermine the memory model. For more
+ information, see Documentation/explanation.txt (in particular,
+ the "THE PROGRAM ORDER RELATION: po AND po-loc" and "A WARNING"
+ sections).
+
+ Note that this limitation in turn limits LKMM's ability to
+ accurately model address, control, and data dependencies.
+ For example, if the compiler can deduce the value of some variable
+ carrying a dependency, then the compiler can break that dependency
+ by substituting a constant of that value.
+
+2. Multiple access sizes for a single variable are not supported,
+ and neither are misaligned or partially overlapping accesses.
+
+3. Exceptions and interrupts are not modeled. In some cases,
+ this limitation can be overcome by modeling the interrupt or
+ exception with an additional process.
+
+4. I/O such as MMIO or DMA is not supported.
+
+5. Self-modifying code (such as that found in the kernel's
+ alternatives mechanism, function tracer, Berkeley Packet Filter
+ JIT compiler, and module loader) is not supported.
+
+6. Complete modeling of all variants of atomic read-modify-write
+ operations, locking primitives, and RCU is not provided.
+ For example, call_rcu() and rcu_barrier() are not supported.
+ However, a substantial amount of support is provided for these
+ operations, as shown in the linux-kernel.def file.
+
+ Here are specific limitations:
+
+ a. When rcu_assign_pointer() is passed NULL, the Linux
+ kernel provides no ordering, but LKMM models this
+ case as a store release.
+
+ b. The "unless" RMW operations are not currently modeled:
+ atomic_long_add_unless(), atomic_inc_unless_negative(),
+ and atomic_dec_unless_positive(). These can be emulated
+ in litmus tests, for example, by using atomic_cmpxchg().
+
+ One exception of this limitation is atomic_add_unless(),
+ which is provided directly by herd7 (so no corresponding
+ definition in linux-kernel.def). atomic_add_unless() is
+ modeled by herd7 therefore it can be used in litmus tests.
+
+ c. The call_rcu() function is not modeled. As was shown above,
+ it can be emulated in litmus tests by adding another
+ process that invokes synchronize_rcu() and the body of the
+ callback function, with (for example) a release-acquire
+ from the site of the emulated call_rcu() to the beginning
+ of the additional process.
+
+ d. The rcu_barrier() function is not modeled. It can be
+ emulated in litmus tests emulating call_rcu() via
+ (for example) a release-acquire from the end of each
+ additional call_rcu() process to the site of the
+ emulated rcu-barrier().
+
+ e. Although sleepable RCU (SRCU) is now modeled, there
+ are some subtle differences between its semantics and
+ those in the Linux kernel. For example, the kernel
+ might interpret the following sequence as two partially
+ overlapping SRCU read-side critical sections:
+
+ 1 r1 = srcu_read_lock(&my_srcu);
+ 2 do_something_1();
+ 3 r2 = srcu_read_lock(&my_srcu);
+ 4 do_something_2();
+ 5 srcu_read_unlock(&my_srcu, r1);
+ 6 do_something_3();
+ 7 srcu_read_unlock(&my_srcu, r2);
+
+ In contrast, LKMM will interpret this as a nested pair of
+ SRCU read-side critical sections, with the outer critical
+ section spanning lines 1-7 and the inner critical section
+ spanning lines 3-5.
+
+ This difference would be more of a concern had anyone
+ identified a reasonable use case for partially overlapping
+ SRCU read-side critical sections. For more information
+ on the trickiness of such overlapping, please see:
+ https://paulmck.livejournal.com/40593.html
+
+ f. Reader-writer locking is not modeled. It can be
+ emulated in litmus tests using atomic read-modify-write
+ operations.
+
+The fragment of the C language supported by these litmus tests is quite
+limited and in some ways non-standard:
+
+1. There is no automatic C-preprocessor pass. You can of course
+ run it manually, if you choose.
+
+2. There is no way to create functions other than the Pn() functions
+ that model the concurrent processes.
+
+3. The Pn() functions' formal parameters must be pointers to the
+ global shared variables. Nothing can be passed by value into
+ these functions.
+
+4. The only functions that can be invoked are those built directly
+ into herd7 or that are defined in the linux-kernel.def file.
+
+5. The "switch", "do", "for", "while", and "goto" C statements are
+ not supported. The "switch" statement can be emulated by the
+ "if" statement. The "do", "for", and "while" statements can
+ often be emulated by manually unrolling the loop, or perhaps by
+ enlisting the aid of the C preprocessor to minimize the resulting
+ code duplication. Some uses of "goto" can be emulated by "if",
+ and some others by unrolling.
+
+6. Although you can use a wide variety of types in litmus-test
+ variable declarations, and especially in global-variable
+ declarations, the "herd7" tool understands only int and
+ pointer types. There is no support for floating-point types,
+ enumerations, characters, strings, arrays, or structures.
+
+7. Parsing of variable declarations is very loose, with almost no
+ type checking.
+
+8. Initializers differ from their C-language counterparts.
+ For example, when an initializer contains the name of a shared
+ variable, that name denotes a pointer to that variable, not
+ the current value of that variable. For example, "int x = y"
+ is interpreted the way "int x = &y" would be in C.
+
+9. Dynamic memory allocation is not supported, although this can
+ be worked around in some cases by supplying multiple statically
+ allocated variables.
+
+Some of these limitations may be overcome in the future, but others are
+more likely to be addressed by incorporating the Linux-kernel memory model
+into other tools.
+
+Finally, please note that LKMM is subject to change as hardware, use cases,
+and compilers evolve.
diff --git a/tools/memory-model/README b/tools/memory-model/README
index ecb7385..d2e03c4 100644
--- a/tools/memory-model/README
+++ b/tools/memory-model/README
@@ -63,10 +63,32 @@ BASIC USAGE: HERD7
==================
The memory model is used, in conjunction with "herd7", to exhaustively
-explore the state space of small litmus tests.
+explore the state space of small litmus tests. Documentation describing
+the format, features, capabilities and limitations of these litmus
+tests is available in tools/memory-model/Documentation/litmus-tests.txt.
-For example, to run SB+fencembonceonces.litmus against the memory model:
+Example litmus tests may be found in the Linux-kernel source tree:
+ tools/memory-model/litmus-tests/
+ Documentation/litmus-tests/
+
+Several thousand more example litmus tests are available here:
+
+ https://github.com/paulmckrcu/litmus
+ https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/perfbook.git/tree/CodeSamples/formal/herd
+ https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/perfbook.git/tree/CodeSamples/formal/litmus
+
+Documentation describing litmus tests and now to use them may be found
+here:
+
+ tools/memory-model/Documentation/litmus-tests.txt
+
+The remainder of this section uses the SB+fencembonceonces.litmus test
+located in the tools/memory-model directory.
+
+To run SB+fencembonceonces.litmus against the memory model:
+
+ $ cd $LINUX_SOURCE_TREE/tools/memory-model
$ herd7 -conf linux-kernel.cfg litmus-tests/SB+fencembonceonces.litmus
Here is the corresponding output:
@@ -87,7 +109,11 @@ Here is the corresponding output:
The "Positive: 0 Negative: 3" and the "Never 0 3" each indicate that
this litmus test's "exists" clause can not be satisfied.
-See "herd7 -help" or "herdtools7/doc/" for more information.
+See "herd7 -help" or "herdtools7/doc/" for more information on running the
+tool itself, but please be aware that this documentation is intended for
+people who work on the memory model itself, that is, people making changes
+to the tools/memory-model/linux-kernel.* files. It is not intended for
+people focusing on writing, understanding, and running LKMM litmus tests.
=====================
@@ -124,7 +150,11 @@ that during two million trials, the state specified in this litmus
test's "exists" clause was not reached.
And, as with "herd7", please see "klitmus7 -help" or "herdtools7/doc/"
-for more information.
+for more information. And again, please be aware that this documentation
+is intended for people who work on the memory model itself, that is,
+people making changes to the tools/memory-model/linux-kernel.* files.
+It is not intended for people focusing on writing, understanding, and
+running LKMM litmus tests.
====================
@@ -137,6 +167,10 @@ Documentation/cheatsheet.txt
Documentation/explanation.txt
Describes the memory model in detail.
+Documentation/litmus-tests.txt
+ Describes the format, features, capabilities, and limitations
+ of the litmus tests that LKMM can evaluate.
+
Documentation/recipes.txt
Lists common memory-ordering patterns.
@@ -187,116 +221,3 @@ README
This file.
scripts Various scripts, see scripts/README.
-
-
-===========
-LIMITATIONS
-===========
-
-The Linux-kernel memory model (LKMM) has the following limitations:
-
-1. Compiler optimizations are not accurately modeled. Of course,
- the use of READ_ONCE() and WRITE_ONCE() limits the compiler's
- ability to optimize, but under some circumstances it is possible
- for the compiler to undermine the memory model. For more
- information, see Documentation/explanation.txt (in particular,
- the "THE PROGRAM ORDER RELATION: po AND po-loc" and "A WARNING"
- sections).
-
- Note that this limitation in turn limits LKMM's ability to
- accurately model address, control, and data dependencies.
- For example, if the compiler can deduce the value of some variable
- carrying a dependency, then the compiler can break that dependency
- by substituting a constant of that value.
-
-2. Multiple access sizes for a single variable are not supported,
- and neither are misaligned or partially overlapping accesses.
-
-3. Exceptions and interrupts are not modeled. In some cases,
- this limitation can be overcome by modeling the interrupt or
- exception with an additional process.
-
-4. I/O such as MMIO or DMA is not supported.
-
-5. Self-modifying code (such as that found in the kernel's
- alternatives mechanism, function tracer, Berkeley Packet Filter
- JIT compiler, and module loader) is not supported.
-
-6. Complete modeling of all variants of atomic read-modify-write
- operations, locking primitives, and RCU is not provided.
- For example, call_rcu() and rcu_barrier() are not supported.
- However, a substantial amount of support is provided for these
- operations, as shown in the linux-kernel.def file.
-
- a. When rcu_assign_pointer() is passed NULL, the Linux
- kernel provides no ordering, but LKMM models this
- case as a store release.
-
- b. The "unless" RMW operations are not currently modeled:
- atomic_long_add_unless(), atomic_inc_unless_negative(),
- and atomic_dec_unless_positive(). These can be emulated
- in litmus tests, for example, by using atomic_cmpxchg().
-
- One exception of this limitation is atomic_add_unless(),
- which is provided directly by herd7 (so no corresponding
- definition in linux-kernel.def). atomic_add_unless() is
- modeled by herd7 therefore it can be used in litmus tests.
-
- c. The call_rcu() function is not modeled. It can be
- emulated in litmus tests by adding another process that
- invokes synchronize_rcu() and the body of the callback
- function, with (for example) a release-acquire from
- the site of the emulated call_rcu() to the beginning
- of the additional process.
-
- d. The rcu_barrier() function is not modeled. It can be
- emulated in litmus tests emulating call_rcu() via
- (for example) a release-acquire from the end of each
- additional call_rcu() process to the site of the
- emulated rcu-barrier().
-
- e. Although sleepable RCU (SRCU) is now modeled, there
- are some subtle differences between its semantics and
- those in the Linux kernel. For example, the kernel
- might interpret the following sequence as two partially
- overlapping SRCU read-side critical sections:
-
- 1 r1 = srcu_read_lock(&my_srcu);
- 2 do_something_1();
- 3 r2 = srcu_read_lock(&my_srcu);
- 4 do_something_2();
- 5 srcu_read_unlock(&my_srcu, r1);
- 6 do_something_3();
- 7 srcu_read_unlock(&my_srcu, r2);
-
- In contrast, LKMM will interpret this as a nested pair of
- SRCU read-side critical sections, with the outer critical
- section spanning lines 1-7 and the inner critical section
- spanning lines 3-5.
-
- This difference would be more of a concern had anyone
- identified a reasonable use case for partially overlapping
- SRCU read-side critical sections. For more information,
- please see: https://paulmck.livejournal.com/40593.html
-
- f. Reader-writer locking is not modeled. It can be
- emulated in litmus tests using atomic read-modify-write
- operations.
-
-The "herd7" tool has some additional limitations of its own, apart from
-the memory model:
-
-1. Non-trivial data structures such as arrays or structures are
- not supported. However, pointers are supported, allowing trivial
- linked lists to be constructed.
-
-2. Dynamic memory allocation is not supported, although this can
- be worked around in some cases by supplying multiple statically
- allocated variables.
-
-Some of these limitations may be overcome in the future, but others are
-more likely to be addressed by incorporating the Linux-kernel memory model
-into other tools.
-
-Finally, please note that LKMM is subject to change as hardware, use cases,
-and compilers evolve.
--
2.9.5
From: "Paul E. McKenney" <[email protected]>
The Linux kernel has a number of categories of ordering primitives, which
are recorded in the LKMM implementation and hinted at by cheatsheet.txt.
But there is no overview of these categories, and such an overview
is needed in order to understand multithreaded LKMM litmus tests.
This commit therefore adds an ordering.txt as well as extracting a
control-dependencies.txt from memory-barriers.txt. It also updates the
README file.
Signed-off-by: Paul E. McKenney <[email protected]>
---
tools/memory-model/Documentation/README | 24 +-
tools/memory-model/Documentation/ordering.txt | 462 ++++++++++++++++++++++++++
tools/memory-model/control-dependencies.txt | 256 ++++++++++++++
3 files changed, 740 insertions(+), 2 deletions(-)
create mode 100644 tools/memory-model/Documentation/ordering.txt
create mode 100644 tools/memory-model/control-dependencies.txt
diff --git a/tools/memory-model/Documentation/README b/tools/memory-model/Documentation/README
index 4326603..16177aa 100644
--- a/tools/memory-model/Documentation/README
+++ b/tools/memory-model/Documentation/README
@@ -8,10 +8,19 @@ number of places.
This document therefore describes a number of places to start reading
the documentation in this directory, depending on what you know and what
-you would like to learn:
+you would like to learn. These are cumulative, that is, understanding
+of the documents earlier in this list is required by the documents later
+in this list.
o You are new to Linux-kernel concurrency: simple.txt
+o You have some background in Linux-kernel concurrency, and would
+ like an overview of the types of low-level concurrency primitives
+ that are provided: ordering.txt
+
+ Here, "low level" means atomic operations to single locations in
+ memory.
+
o You are familiar with the concurrency facilities that you
need, and just want to get started with LKMM litmus tests:
litmus-tests.txt
@@ -20,6 +29,9 @@ o You are familiar with Linux-kernel concurrency, and would
like a detailed intuitive understanding of LKMM, including
situations involving more than two threads: recipes.txt
+o You would like a detailed understanding of what your compiler can
+ and cannot do to control dependencies: control-dependencies.txt
+
o You are familiar with Linux-kernel concurrency and the
use of LKMM, and would like a cheat sheet to remind you
of LKMM's guarantees: cheatsheet.txt
@@ -37,12 +49,16 @@ o You are interested in the publications related to LKMM, including
DESCRIPTION OF FILES
====================
-Documentation/README
+README
This file.
Documentation/cheatsheet.txt
Quick-reference guide to the Linux-kernel memory model.
+Documentation/control-dependencies.txt
+ A guide to preventing compiler optimizations from destroying
+ your control dependencies.
+
Documentation/explanation.txt
Describes the memory model in detail.
@@ -50,6 +66,10 @@ Documentation/litmus-tests.txt
Describes the format, features, capabilities, and limitations
of the litmus tests that LKMM can evaluate.
+Documentation/ordering.txt
+ Describes the Linux kernel's low-level memory-ordering primitives
+ by category.
+
Documentation/recipes.txt
Lists common memory-ordering patterns.
diff --git a/tools/memory-model/Documentation/ordering.txt b/tools/memory-model/Documentation/ordering.txt
new file mode 100644
index 0000000..4b2cc55
--- /dev/null
+++ b/tools/memory-model/Documentation/ordering.txt
@@ -0,0 +1,462 @@
+This document expands on the types of ordering that are summarized in
+cheatsheet.txt and used in in various other files.
+
+
+Types of Ordering
+=================
+
+This section describes the types of ordering in roughly decreasing order
+of strength on the theory that stronger ordering is more heavily used
+and easier to understand. Each of the following types of ordering has
+its own subsection below:
+
+1. Barriers (also known as "fences"). A barrier orders some or all
+ of the CPU's prior operations against some or all of its subsequent
+ operations.
+
+ a. Full memory barriers: More famously, smp_mb(), but this
+ category also includes those non-void (value returning)
+ read-modify-write (RMW) atomic operations whose
+ names do not end in _acquire, _release, or _relaxed.
+ It also includes RCU grace-period operations such as
+ synchronize_rcu(), but at a very high cost, especially
+ in terms of latency. These operations order all prior
+ memory accesses against all subsequent memory accesses.
+
+ b. RMW ordering augmentation. The smp_mb__before_atomic()
+ and smp_mb__after_atomic() are by far the most heavily
+ used of these. They provide smp_mb()-style full ordering
+ to a later (or earlier, respectively) non-value-returning
+ RMW atomic operations such as atomic_inc().
+
+ c. Write memory barrier. This is smp_wmb(), which orders
+ prior marked stores against later marked stores.
+
+ d. Read memory barrier. This is smp_rmb(), which orders
+ prior loads against later loads.
+
+2. Ordered memory accesses. These operations order themselves
+ against some or all of the CPUs prior or subsequent accesses,
+ depending on the category of operation.
+
+ a. Release operations. This category includes
+ smp_store_release(), atomic_set_release(),
+ rcu_assign_pointer(), and value-returning RMW operations
+ whose names end in _release. These operations order
+ their own store against all of the CPU's subsequent
+ memory accesses.
+
+ b. Acquire operations. This category includes
+ smp_load_acquire(), atomic_read_acquire(), and
+ value-returning RMW operations whose names end in
+ _acquire. These operations order their own load against
+ all of the CPU's prior memory accesses.
+
+ c. RCU read-side ordering. This category includes
+ rcu_dereference() and srcu_dereference(). These
+ operations order their load (which must be a pointer)
+ against any of the CPU's subsequent memory accesses
+ whose address has been calculated from the value loaded,
+ that is against any subsequent memory access having
+ an *address dependency* on the value returned by the
+ rcu_dereference() or srcu_dereference().
+
+ d. Control dependencies. A control dependency extends
+ from a marked load (READ_ONCE() or stronger) through
+ an "if" condition to a marked store (WRITE_ONCE() or
+ stronger) that is executed only one of the legs of that
+ "if" statement. Control dependencies are fragile and
+ easily destroyed by compiler optimizers.
+
+ Control dependencies are so named because they are
+ mediated by control-flow instructions such as comparisons
+ and conditional branches.
+
+3. Unordered accesses, as the name indicates, have no ordering
+ properties except to the extent that they interact with one of
+ the ordering mechanisms called out above.
+
+ a. Unordered marked operations. This category includes
+ READ_ONCE(), WRITE_ONCE(), atomic_read(), atomic_set(),
+ volatile variables (such as the "jiffies" counter),
+ value-returning RMW operations whose names end in
+ _relaxed, and non-value-returning RMW operations
+ whose names do not end in either _acquire or _release.
+ These operations provide no ordering guarantees.
+
+ b. Unmarked C-language accesses. This category includes
+ accesses to normal variables, that is, variables that are
+ not marked "volatile" and are not C11 atomic variables.
+ These operations provide no ordering guarantees, and
+ further do not guarantee "atomic" access. For example,
+ the compiler might (and sometimes does) split a plain
+ C-language store into multiple smaller stores. A load
+ from that same variable running on some other CPU while
+ such a store is executing might see a value that is a
+ mashup of the old value and the new value.
+
+Each of the above categories is covered in more detail by one of the
+following section.
+
+Note well that none of these primitives generate any code in kernels
+built with CONFIG_SMP=n. Therefore, if you are attempting to order
+accesses to a physical device within a device driver, please use the
+ordering primitives provided for that purpose, for example, mb() instead
+of smp_mb(). See "Linux Kernel Device Drivers" for more information.
+
+
+Full Memory Barriers
+--------------------
+
+A number of Linux-kernel primitives provide full-memory-barrier semantics.
+Suppose that a given CPU invokes such a primitive. Then all CPUs will
+agree that any earlier action taken by that CPU happened before any
+later action taken by that same CPU. For example, consider the following:
+
+ WRITE_ONCE(x, 1);
+ smp_mb(); // Order store to x before load from y.
+ r1 = READ_ONCE(y);
+
+All CPUs will agree that the store to "x" happened before the load from "y",
+as indicated by the comment. And yes, please comment your memory-ordering
+primitives. It is surprisingly hard to remember what they were for even
+a few months after the fact.
+
+Linux-kernel primitives providing full ordering include the following:
+
+o The smp_mb() full memory barrier, as shown above.
+
+o Value-returning read-modify-write (RMW) atomic operations
+ whose names do not end in _acquire, _release, or _relaxed.
+ Value-returning operations can be recognized by their
+ non-void return types. Examples include atomic_add_return(),
+ atomic_dec_and_test(), cmpxchg(), and xchg(). Note that
+ conditional operations such as cmpxchg() are only guaranteed
+ to provide ordering when they succeed.
+
+ In contrast, non-value-returning RMW atomic operations, that is,
+ those with void return types, do not guarantee any ordering
+ whatsoever. Nor do value-returning RMW atomic operations
+ whose names end in _relaxed. Examples of the former include
+ atomic_inc() and atomic_dec(), while examples of the latter
+ include atomic_cmpxchg_relaxed() and atomic_xchg_relaxed().
+
+ Value-returning RMW atomic operations whose names end in _acquire
+ or _release provide limited ordering, and will be described
+ later in this document.
+
+o RCU's grace-period primitives, including synchronize_rcu(),
+ synchronize_rcu_expedited(), synchronize_srcu() and so on.
+ However, these primitives have orders of magnitude greater
+ overhead than smp_mb(), atomic_xchg(), and so on. Therefore,
+ RCU's grace-period primitives are typically instead used to
+ provide ordering against RCU read-side critical sections, as
+ documented in their comment headers. But of course if you need a
+ synchronize_rcu() to interact with readers, it costs you nothing
+ to also rely on its additional semantics as a full memory barrier.
+ Just please carefully comment this, otherwise your future self
+ will hate you.
+
+
+RMW Ordering Augmentation
+-------------------------
+
+As noted in the previous section, non-value-returning RMW operations
+such as atomic_inc() and atomic_dec() guarantee no ordering whatsoever.
+One way to get full ordering is through use of smp_mb(), for example,
+as follows:
+
+Nevertheless, a number of popular CPU families, including x86,
+nevertheless provide full ordering for these primitives. One way to
+obtain full ordering is to use smp_mb(), like this:
+
+ WRITE_ONCE(x, 1);
+ atomic_inc(&my_counter);
+ smp_mb(); // Inefficient on x86!!!
+ r1 = READ_ONCE(y);
+
+Except that this is inefficient on x86, on which atomic_inc() provides
+full ordering all by itself. The smp_mb__after_atomic() primitive
+can be used instead:
+
+ WRITE_ONCE(x, 1);
+ atomic_inc(&my_counter);
+ smp_mb__after_atomic(); // Order store to x before load from y.
+ r1 = READ_ONCE(y);
+
+The smp_mb__after_atomic() primitive emits code only on CPUs whose
+atomic_inc() implementations do not guarantee full ordering. There
+are a number of variations on the smp_mb__*() theme:
+
+o smp_mb__before_atomic(), which provides full ordering prior
+ to an unordered RMW atomic operation.
+
+o smp_mb__after_atomic(), which, as shown above, provides full
+ ordering subsequent to an unordered RMW atomic operation.
+
+o smp_mb__after_spinlock(), which provides full ordering subsequent
+ to a successful spinlock acquisition. Note that spin_lock() is
+ always successful but spin_trylock() might not be.
+
+o smp_mb__after_srcu_read_unlock(), which provides full ordering
+ subsequent to an srcu_read_unlock().
+
+Placing code between the smp__*() primitive and the thing whose ordering
+that it is augmenting is generally bad practice because the ordering of
+the intervening code will differ from one CPU architecture to another.
+
+
+Write Memory Barrier
+--------------------
+
+The Linux kernel's write memory barrier is smp_wmb(). If a CPU executes
+the following code:
+
+ WRITE_ONCE(x, 1);
+ smp_wmb();
+ WRITE_ONCE(y, 1);
+
+Then any given CPU will see the write to "x" has having preceded the write
+to "y". However, you are usually better off using a release store, as
+described in the "Release Operations" section below.
+
+Note that smp_wmb() might fail to provide ordering for unmarked C-language
+stores because profile-driven optimization could determine that the value
+being overwritten is almost always the value being written. Such a compiler
+might then reasonably decide to transform "x = 1" and "y = 1" as follows:
+
+ if (x != 1)
+ x = 1;
+ smp_wmb(); // BUG: does not order the reads!!!
+ if (y != 1)
+ y = 1;
+
+Therefore, if you need to use smp_wmb() with unmarked C-language
+writes, please make sure that your compiler will not make this sort
+of transformation.
+
+
+Read Memory Barrier
+-------------------
+
+The Linux kernel's read memory barrier is smp_rmb(). If a CPU executes
+the following code:
+
+ r0 = READ_ONCE(y);
+ smp_rmb();
+ r1 = READ_ONCE(x);
+
+Then any given CPU will see the read from "y" as having preceded the read from
+"x". However, you are usually better off using an acquire load, as described
+in the "Acquire Operations" section below.
+
+
+Release Operations
+------------------
+
+The smp_wmb() example shown above is usually improved by instead using
+a release store:
+
+ WRITE_ONCE(x, 1);
+ smp_store_release(&y, 1);
+
+This saves a line of code, and more important makes it easier to connect
+up the different pieces of the concurrent algorithm. The variable stored
+to by the smp_store_release(), in this case "y", will normally be used
+in an acquire operation in the other piece of the concurrent algorithm.
+
+There is a wide variety of release operations:
+
+o Store operations, including smp_store_release(),
+ atomic_set_release(), and atomic_long_set_release().
+
+o RCU's rcu_assign_pointer() operation. This is the same as
+ smp_store_release() except that: (1) It takes the pointer
+ to be assigned to instead of a pointer to that pointer,
+ as smp_store_release() would, (2) It is intended to be used
+ in conjunction with rcu_dereference() and similar, and
+ (3) It checks for an RCU-protected pointer.
+
+o Value-returning RMW operations whose names end in _release,
+ such as atomic_fetch_add_release() and cmpxchg_release().
+ Note that release ordering is provided only against the
+ memory-store portion of the RMW operation. Note also that
+ conditional operations such as cmpxchg_release() are
+ only guaranteed to provide ordering when they succeed.
+
+As mentioned earlier, release operations are often paired with
+acquire operations, which are the subject of the next section.
+
+
+Acquire Operations
+------------------
+
+The smp_rmb() example shown above is usually improved by instead using
+an acquire load:
+
+ r0 = smp_load_acquire(&y);
+ r1 = READ_ONCE(x);
+
+As with smp_store_release(), this saves a line of code and makes it easier
+to connect the different pieces of the concurrent algorithm by looking for
+the smp_store_release() that stores to "y".
+
+There are a couple of categories of acquire operations:
+
+o Load operations, including smp_load_acquire(),
+ atomic_read_acquire(), and atomic64_read_acquire().
+
+o Value-returning RMW operations whose names end in _acquire, such
+ as atomic_xchg_acquire() and atomic_cmpxchg_acquire(). Note that
+ release ordering is provided only against the memory-load portion
+ of the RMW operation. Note also that conditional operations
+ such as atomic_cmpxchg_acquire() are only guaranteed to provide
+ ordering when they succeed.
+
+Symmetry being what it is, acquire operations are often paired with
+release operations.
+
+
+RCU Read-Side Ordering
+----------------------
+
+There are two major types of RCU read-side ordering:
+
+o Marking of RCU read-side critical sections, for example,
+ via rcu_read_lock() and rcu_read_unlock(). These operations
+ incur very low overhead because they interact only with
+ the corresponding grace-period primitives, in this case,
+ synchronize_rcu() and friends. The way this works is that
+ if a given call to synchronize_rcu() cannot prove that it
+ started before a given call to rcu_read_lock(), then that
+ synchronize_rcu() is not permitted to return until the matching
+ rcu_read_unlock() is reached.
+
+ For more information, please see the synchronize_rcu() docbook
+ header comment and the material in Documentation/RCU.
+
+o Accessing RCU-protected pointers via rcu_dereference()
+ and friends. A call to rcu_dereference() is usually paired
+ with a call to rcu_assign_pointer() in much the same way
+ that a call to smp_load_acquire() could be paired with a
+ call to smp_store_release(). Calls to rcu_dereference() and
+ rcu_assign_pointer are often buried in other APIs, for example,
+ the RCU list API members defined in include/linux/rculist.h.
+ For more information, please see the docbook headers in that
+ file and again the material in Documentation/RCU.
+
+ If there is any significant processing of the pointer value
+ between the rcu_dereference() that returned it and a later
+ dereference(), please read Documentation/RCU/rcu_dereference.txt.
+
+It can also be quite helpful to review uses in the Linux kernel.
+
+
+Control Dependencies
+--------------------
+
+A control dependency can enforce ordering between an READ_ONCE() and
+a WRITE_ONCE() when there is an "if" condition between them. The
+classic example is as follows:
+
+ q = READ_ONCE(a);
+ if (q) {
+ WRITE_ONCE(b, 1);
+ }
+
+In this case, all CPUs would see the read from "a" as happening before
+the write to "b".
+
+However, control dependencies are easily destroyed by compiler
+optimizations. Please see the "control-dependencies.txt" file for
+more information.
+
+
+Unordered Marked Operations
+---------------------------
+
+Unordered operations to different variables are just that, unordered.
+However, if a group of CPUs apply these operations to a single variable,
+all the CPUs will agree on the operation order. Of course, it is also
+possible to constrain reordering of unordered operations to different
+variables using the various mechanisms described earlier in this document.
+
+These operations come in three categories:
+
+o Marked writes, such as WRITE_ONCE() and atomic_set(). These
+ primitives prevent the compiler from a number of destructive
+ optimizations such as omitting an early write to a variable
+ in favor of a later write to that same variable. They provide
+ no ordering guarantees, and in fact many CPUs will happily
+ reorder marked writes with each other or with other unordered
+ operations, unless these operations are on the same variable.
+
+o Marked reads, such as READ_ONCE() and atomic_read(). These
+ primitives prevent the compiler from a number of destructive
+ optimizations such as fusing a pair of successive reads from
+ the same variable into a single read. They provide no ordering
+ guarantees, and in fact many CPUs will happily reorder marked
+ reads with each other or with other unordered operations, unless
+ these operations are on the same variable.
+
+o Unordered RMW atomic operations. These are non-value-returning
+ RMW atomic operations whose names do not end in _acquire or
+ _release, and also value-returning RMW operations whose names
+ end in _relaxed. Examples include atomic_add(), atomic_or(),
+ and atomic64_fetch_xor_relaxed(). These operations do carry
+ out the specified RMW operation atomically, for example, five
+ concurrent atomic_add() operations applied to a given variable
+ will reliably increase the value of that variable by five.
+ However, many CPUs will happily reorder these operations with
+ each other or with other unordered operations.
+
+ This category of operations can be efficiently ordered using
+ smp_mb__before_atomic() and smp_mb__after_atomic(). as was
+ discussed in the "RMW Ordering Augmentation" section
+
+In short, these operations can be freely reordered unless they are all
+operating on a single variable or unless they are constrained by one of
+the operations called out earlier in this document.
+
+
+Unmarked C-Language Accesses
+----------------------------
+
+Unmarked C-language accesses are unordered, and are also subject to
+any number of compiler optimizations, many of which can break your
+concurrent code. It is possible to used unmarked C-language accesses for
+shared variables that are subject to concurrent access, but great care
+is required on an ongoing basis. The compiler-constraining barrier()
+primitive can be helpful, as can the various ordering primitives discussed
+in this document. It nevertheless bears repeating that use of unmarked
+C-language accesses requires careful attention to not just your code,
+but to all the compilers that might be used to build it.
+
+Here are some ways of using unmarked C-language accesses for shared
+variables without such worries:
+
+o Guard all accesses to a given variable by a particular lock,
+ so that there are never concurrent conflicting accesses to that
+ variable. (There are "conflicting accesses" when at least one of
+ the concurrent accesses to a variable is an unmarked C-language
+ access and when at least one of those accesses is a write.)
+
+o As above, but using other synchronization primitives such
+ as reader-writer locks or sequence locks as designed.
+
+o Restrict use of a given variable to statistics or heuristics
+ where the occasional bogus value can be tolerated.
+
+If you need to live more dangerously, please do take the time to
+understand the compilers. One place to start is these two LWN
+articles:
+
+Who's afraid of a big bad optimizing compiler?
+ https://lwn.net/Articles/793253
+Calibrating your fear of big bad optimizing compilers
+ https://lwn.net/Articles/799218
+
+Used properly, unmarked C-language accesses can reduce overhead on
+fastpaths. However, the price is great care and continual attention
+to your compiler as new versions come out and as new optimizations
+are enabled.
diff --git a/tools/memory-model/control-dependencies.txt b/tools/memory-model/control-dependencies.txt
new file mode 100644
index 0000000..366520c
--- /dev/null
+++ b/tools/memory-model/control-dependencies.txt
@@ -0,0 +1,256 @@
+CONTROL DEPENDENCIES
+====================
+
+Control dependencies can be a bit tricky because current compilers do
+not understand them. The purpose of this section is to help you prevent
+the compiler's ignorance from breaking your code.
+
+A load-load control dependency requires a full read memory barrier, not
+simply a data dependency barrier to make it work correctly. Consider the
+following bit of code:
+
+ q = READ_ONCE(a);
+ if (q) {
+ <data dependency barrier> /* BUG: No data dependency!!! */
+ p = READ_ONCE(b);
+ }
+
+This will not have the desired effect because there is no actual data
+dependency, but rather a control dependency that the CPU may short-circuit
+by attempting to predict the outcome in advance, so that other CPUs see
+the load from b as having happened before the load from a. In such a
+case what's actually required is:
+
+ q = READ_ONCE(a);
+ if (q) {
+ <read barrier>
+ p = READ_ONCE(b);
+ }
+
+However, stores are not speculated. This means that ordering -is- provided
+for load-store control dependencies, as in the following example:
+
+ q = READ_ONCE(a);
+ if (q) {
+ WRITE_ONCE(b, 1);
+ }
+
+Control dependencies pair normally with other types of barriers.
+That said, please note that neither READ_ONCE() nor WRITE_ONCE()
+are optional! Without the READ_ONCE(), the compiler might combine the
+load from "a" with other loads from "a". Without the WRITE_ONCE(),
+the compiler might combine the store to "b" with other stores to "b",
+or, worse yet, convert the store into a check followed by a store.
+
+Worse yet, if the compiler is able to prove (say) that the value of
+variable "a" is always non-zero, it would be well within its rights
+to optimize the original example by eliminating the "if" statement
+as follows:
+
+ q = a;
+ b = 1; /* BUG: Compiler and CPU can both reorder!!! */
+
+So don't leave out either the READ_ONCE() or the WRITE_ONCE().
+
+It is tempting to try to enforce ordering on identical stores on both
+branches of the "if" statement as follows:
+
+ q = READ_ONCE(a);
+ if (q) {
+ barrier();
+ WRITE_ONCE(b, 1);
+ do_something();
+ } else {
+ barrier();
+ WRITE_ONCE(b, 1);
+ do_something_else();
+ }
+
+Unfortunately, current compilers will transform this as follows at high
+optimization levels:
+
+ q = READ_ONCE(a);
+ barrier();
+ WRITE_ONCE(b, 1); /* BUG: No ordering vs. load from a!!! */
+ if (q) {
+ /* WRITE_ONCE(b, 1); -- moved up, BUG!!! */
+ do_something();
+ } else {
+ /* WRITE_ONCE(b, 1); -- moved up, BUG!!! */
+ do_something_else();
+ }
+
+Now there is no conditional between the load from "a" and the store to
+"b", which means that the CPU is within its rights to reorder them:
+The conditional is absolutely required, and must be present in the
+assembly code even after all compiler optimizations have been applied.
+Therefore, if you need ordering in this example, you need explicit
+memory barriers, for example, smp_store_release():
+
+ q = READ_ONCE(a);
+ if (q) {
+ smp_store_release(&b, 1);
+ do_something();
+ } else {
+ smp_store_release(&b, 1);
+ do_something_else();
+ }
+
+In contrast, without explicit memory barriers, two-legged-if control
+ordering is guaranteed only when the stores differ, for example:
+
+ q = READ_ONCE(a);
+ if (q) {
+ WRITE_ONCE(b, 1);
+ do_something();
+ } else {
+ WRITE_ONCE(b, 2);
+ do_something_else();
+ }
+
+The initial READ_ONCE() is still required to prevent the compiler from
+proving the value of "a".
+
+In addition, you need to be careful what you do with the local variable "q",
+otherwise the compiler might be able to guess the value and again remove
+the needed conditional. For example:
+
+ q = READ_ONCE(a);
+ if (q % MAX) {
+ WRITE_ONCE(b, 1);
+ do_something();
+ } else {
+ WRITE_ONCE(b, 2);
+ do_something_else();
+ }
+
+If MAX is defined to be 1, then the compiler knows that (q % MAX) is
+equal to zero, in which case the compiler is within its rights to
+transform the above code into the following:
+
+ q = READ_ONCE(a);
+ WRITE_ONCE(b, 2);
+ do_something_else();
+
+Given this transformation, the CPU is not required to respect the ordering
+between the load from variable "a" and the store to variable "b". It is
+tempting to add a barrier(), but this does not help. The conditional
+is gone, and the barrier won't bring it back. Therefore, if you are
+relying on this ordering, you should make sure that MAX is greater than
+one, perhaps as follows:
+
+ q = READ_ONCE(a);
+ BUILD_BUG_ON(MAX <= 1); /* Order load from a with store to b. */
+ if (q % MAX) {
+ WRITE_ONCE(b, 1);
+ do_something();
+ } else {
+ WRITE_ONCE(b, 2);
+ do_something_else();
+ }
+
+Please note once again that the stores to "b" differ. If they were
+identical, as noted earlier, the compiler could pull this store outside
+of the 'if' statement.
+
+You must also be careful not to rely too much on boolean short-circuit
+evaluation. Consider this example:
+
+ q = READ_ONCE(a);
+ if (q || 1 > 0)
+ WRITE_ONCE(b, 1);
+
+Because the first condition cannot fault and the second condition is
+always true, the compiler can transform this example as following,
+defeating control dependency:
+
+ q = READ_ONCE(a);
+ WRITE_ONCE(b, 1);
+
+This example underscores the need to ensure that the compiler cannot
+out-guess your code. More generally, although READ_ONCE() does force
+the compiler to actually emit code for a given load, it does not force
+the compiler to use the results.
+
+In addition, control dependencies apply only to the then-clause and
+else-clause of the if-statement in question. In particular, it does
+not necessarily apply to code following the if-statement:
+
+ q = READ_ONCE(a);
+ if (q) {
+ WRITE_ONCE(b, 1);
+ } else {
+ WRITE_ONCE(b, 2);
+ }
+ WRITE_ONCE(c, 1); /* BUG: No ordering against the read from "a". */
+
+It is tempting to argue that there in fact is ordering because the
+compiler cannot reorder volatile accesses and also cannot reorder
+the writes to "b" with the condition. Unfortunately for this line
+of reasoning, the compiler might compile the two writes to "b" as
+conditional-move instructions, as in this fanciful pseudo-assembly
+language:
+
+ ld r1,a
+ cmp r1,$0
+ cmov,ne r4,$1
+ cmov,eq r4,$2
+ st r4,b
+ st $1,c
+
+A weakly ordered CPU would have no dependency of any sort between the load
+from "a" and the store to "c". The control dependencies would extend
+only to the pair of cmov instructions and the store depending on them.
+In short, control dependencies apply only to the stores in the then-clause
+and else-clause of the if-statement in question (including functions
+invoked by those two clauses), not to code following that if-statement.
+
+
+Note well that the ordering provided by a control dependency is local
+to the CPU containing it. See the section on "Multicopy atomicity"
+for more information.
+
+
+In summary:
+
+ (*) Control dependencies can order prior loads against later stores.
+ However, they do -not- guarantee any other sort of ordering:
+ Not prior loads against later loads, nor prior stores against
+ later anything. If you need these other forms of ordering,
+ use smp_rmb(), smp_wmb(), or, in the case of prior stores and
+ later loads, smp_mb().
+
+ (*) If both legs of the "if" statement begin with identical stores to
+ the same variable, then those stores must be ordered, either by
+ preceding both of them with smp_mb() or by using smp_store_release()
+ to carry out the stores. Please note that it is -not- sufficient
+ to use barrier() at beginning of each leg of the "if" statement
+ because, as shown by the example above, optimizing compilers can
+ destroy the control dependency while respecting the letter of the
+ barrier() law.
+
+ (*) Control dependencies require at least one run-time conditional
+ between the prior load and the subsequent store, and this
+ conditional must involve the prior load. If the compiler is able
+ to optimize the conditional away, it will have also optimized
+ away the ordering. Careful use of READ_ONCE() and WRITE_ONCE()
+ can help to preserve the needed conditional.
+
+ (*) Control dependencies require that the compiler avoid reordering the
+ dependency into nonexistence. Careful use of READ_ONCE() or
+ atomic{,64}_read() can help to preserve your control dependency.
+ Please see the COMPILER BARRIER section for more information.
+
+ (*) Control dependencies apply only to the then-clause and else-clause
+ of the if-statement containing the control dependency, including
+ any functions that these two clauses call. Control dependencies
+ do -not- apply to code following the if-statement containing the
+ control dependency.
+
+ (*) Control dependencies pair normally with other types of barriers.
+
+ (*) Control dependencies do -not- provide multicopy atomicity. If you
+ need all the CPUs to see a given store at the same time, use smp_mb().
+
+ (*) Compilers do not understand control dependencies. It is therefore
+ your job to ensure that they do not break your code.
--
2.9.5
On Mon, 31 Aug 2020 11:20:36 -0700, [email protected] wrote:
> From: "Paul E. McKenney" <[email protected]>
>
> The Linux kernel has a number of categories of ordering primitives, which
> are recorded in the LKMM implementation and hinted at by cheatsheet.txt.
> But there is no overview of these categories, and such an overview
> is needed in order to understand multithreaded LKMM litmus tests.
> This commit therefore adds an ordering.txt as well as extracting a
> control-dependencies.txt from memory-barriers.txt. It also updates the
> README file.
>
> Signed-off-by: Paul E. McKenney <[email protected]>
> ---
> tools/memory-model/Documentation/README | 24 +-
> tools/memory-model/Documentation/ordering.txt | 462 ++++++++++++++++++++++++++
> tools/memory-model/control-dependencies.txt | 256 ++++++++++++++
> 3 files changed, 740 insertions(+), 2 deletions(-)
> create mode 100644 tools/memory-model/Documentation/ordering.txt
> create mode 100644 tools/memory-model/control-dependencies.txt
Hi Paul,
Didn't you mean to put control-dependencies.txt under tools/memory-model/Documentation/ ?
Thanks, Akira
>
> diff --git a/tools/memory-model/Documentation/README b/tools/memory-model/Documentation/README
> index 4326603..16177aa 100644
> --- a/tools/memory-model/Documentation/README
> +++ b/tools/memory-model/Documentation/README
> @@ -8,10 +8,19 @@ number of places.
>
> This document therefore describes a number of places to start reading
> the documentation in this directory, depending on what you know and what
> -you would like to learn:
> +you would like to learn. These are cumulative, that is, understanding
> +of the documents earlier in this list is required by the documents later
> +in this list.
>
> o You are new to Linux-kernel concurrency: simple.txt
>
> +o You have some background in Linux-kernel concurrency, and would
> + like an overview of the types of low-level concurrency primitives
> + that are provided: ordering.txt
> +
> + Here, "low level" means atomic operations to single locations in
> + memory.
> +
> o You are familiar with the concurrency facilities that you
> need, and just want to get started with LKMM litmus tests:
> litmus-tests.txt
> @@ -20,6 +29,9 @@ o You are familiar with Linux-kernel concurrency, and would
> like a detailed intuitive understanding of LKMM, including
> situations involving more than two threads: recipes.txt
>
> +o You would like a detailed understanding of what your compiler can
> + and cannot do to control dependencies: control-dependencies.txt
> +
> o You are familiar with Linux-kernel concurrency and the
> use of LKMM, and would like a cheat sheet to remind you
> of LKMM's guarantees: cheatsheet.txt
> @@ -37,12 +49,16 @@ o You are interested in the publications related to LKMM, including
> DESCRIPTION OF FILES
> ====================
>
> -Documentation/README
> +README
> This file.
>
> Documentation/cheatsheet.txt
> Quick-reference guide to the Linux-kernel memory model.
>
> +Documentation/control-dependencies.txt
> + A guide to preventing compiler optimizations from destroying
> + your control dependencies.
> +
> Documentation/explanation.txt
> Describes the memory model in detail.
[...]
On Tue, Sep 01, 2020 at 07:34:20AM +0900, Akira Yokosawa wrote:
> On Mon, 31 Aug 2020 11:20:36 -0700, [email protected] wrote:
> > From: "Paul E. McKenney" <[email protected]>
> >
> > The Linux kernel has a number of categories of ordering primitives, which
> > are recorded in the LKMM implementation and hinted at by cheatsheet.txt.
> > But there is no overview of these categories, and such an overview
> > is needed in order to understand multithreaded LKMM litmus tests.
> > This commit therefore adds an ordering.txt as well as extracting a
> > control-dependencies.txt from memory-barriers.txt. It also updates the
> > README file.
> >
> > Signed-off-by: Paul E. McKenney <[email protected]>
> > ---
> > tools/memory-model/Documentation/README | 24 +-
> > tools/memory-model/Documentation/ordering.txt | 462 ++++++++++++++++++++++++++
> > tools/memory-model/control-dependencies.txt | 256 ++++++++++++++
> > 3 files changed, 740 insertions(+), 2 deletions(-)
> > create mode 100644 tools/memory-model/Documentation/ordering.txt
> > create mode 100644 tools/memory-model/control-dependencies.txt
>
> Hi Paul,
>
> Didn't you mean to put control-dependencies.txt under tools/memory-model/Documentation/ ?
Indeed I did, good catch, thank you!
Thanx, Paul
> Thanks, Akira
>
> >
> > diff --git a/tools/memory-model/Documentation/README b/tools/memory-model/Documentation/README
> > index 4326603..16177aa 100644
> > --- a/tools/memory-model/Documentation/README
> > +++ b/tools/memory-model/Documentation/README
> > @@ -8,10 +8,19 @@ number of places.
> >
> > This document therefore describes a number of places to start reading
> > the documentation in this directory, depending on what you know and what
> > -you would like to learn:
> > +you would like to learn. These are cumulative, that is, understanding
> > +of the documents earlier in this list is required by the documents later
> > +in this list.
> >
> > o You are new to Linux-kernel concurrency: simple.txt
> >
> > +o You have some background in Linux-kernel concurrency, and would
> > + like an overview of the types of low-level concurrency primitives
> > + that are provided: ordering.txt
> > +
> > + Here, "low level" means atomic operations to single locations in
> > + memory.
> > +
> > o You are familiar with the concurrency facilities that you
> > need, and just want to get started with LKMM litmus tests:
> > litmus-tests.txt
> > @@ -20,6 +29,9 @@ o You are familiar with Linux-kernel concurrency, and would
> > like a detailed intuitive understanding of LKMM, including
> > situations involving more than two threads: recipes.txt
> >
> > +o You would like a detailed understanding of what your compiler can
> > + and cannot do to control dependencies: control-dependencies.txt
> > +
> > o You are familiar with Linux-kernel concurrency and the
> > use of LKMM, and would like a cheat sheet to remind you
> > of LKMM's guarantees: cheatsheet.txt
> > @@ -37,12 +49,16 @@ o You are interested in the publications related to LKMM, including
> > DESCRIPTION OF FILES
> > ====================
> >
> > -Documentation/README
> > +README
> > This file.
> >
> > Documentation/cheatsheet.txt
> > Quick-reference guide to the Linux-kernel memory model.
> >
> > +Documentation/control-dependencies.txt
> > + A guide to preventing compiler optimizations from destroying
> > + your control dependencies.
> > +
> > Documentation/explanation.txt
> > Describes the memory model in detail.
> [...]
On Mon, Aug 31, 2020 at 11:20:36AM -0700, [email protected] wrote:
> From: "Paul E. McKenney" <[email protected]>
>
> The Linux kernel has a number of categories of ordering primitives, which
> are recorded in the LKMM implementation and hinted at by cheatsheet.txt.
> But there is no overview of these categories, and such an overview
> is needed in order to understand multithreaded LKMM litmus tests.
> This commit therefore adds an ordering.txt as well as extracting a
> control-dependencies.txt from memory-barriers.txt. It also updates the
> README file.
>
> Signed-off-by: Paul E. McKenney <[email protected]>
> ---
This document could use some careful editing. But one pair of errors
stands out in particular:
> diff --git a/tools/memory-model/Documentation/ordering.txt b/tools/memory-model/Documentation/ordering.txt
> new file mode 100644
> index 0000000..4b2cc55
> --- /dev/null
> +++ b/tools/memory-model/Documentation/ordering.txt
> +2. Ordered memory accesses. These operations order themselves
> + against some or all of the CPUs prior or subsequent accesses,
> + depending on the category of operation.
> +
> + a. Release operations. This category includes
> + smp_store_release(), atomic_set_release(),
> + rcu_assign_pointer(), and value-returning RMW operations
> + whose names end in _release. These operations order
> + their own store against all of the CPU's subsequent
---------------------------------------------------------^^^^^^^^^^
> + memory accesses.
> +
> + b. Acquire operations. This category includes
> + smp_load_acquire(), atomic_read_acquire(), and
> + value-returning RMW operations whose names end in
> + _acquire. These operations order their own load against
> + all of the CPU's prior memory accesses.
---------------------------------^^^^^
Double-oops!
Alan
On Mon, Aug 31, 2020 at 09:23:09PM -0400, Alan Stern wrote:
> On Mon, Aug 31, 2020 at 11:20:36AM -0700, [email protected] wrote:
> > From: "Paul E. McKenney" <[email protected]>
> >
> > The Linux kernel has a number of categories of ordering primitives, which
> > are recorded in the LKMM implementation and hinted at by cheatsheet.txt.
> > But there is no overview of these categories, and such an overview
> > is needed in order to understand multithreaded LKMM litmus tests.
> > This commit therefore adds an ordering.txt as well as extracting a
> > control-dependencies.txt from memory-barriers.txt. It also updates the
> > README file.
> >
> > Signed-off-by: Paul E. McKenney <[email protected]>
> > ---
>
> This document could use some careful editing. But one pair of errors
> stands out in particular:
>
> > diff --git a/tools/memory-model/Documentation/ordering.txt b/tools/memory-model/Documentation/ordering.txt
> > new file mode 100644
> > index 0000000..4b2cc55
> > --- /dev/null
> > +++ b/tools/memory-model/Documentation/ordering.txt
>
> > +2. Ordered memory accesses. These operations order themselves
> > + against some or all of the CPUs prior or subsequent accesses,
> > + depending on the category of operation.
> > +
> > + a. Release operations. This category includes
> > + smp_store_release(), atomic_set_release(),
> > + rcu_assign_pointer(), and value-returning RMW operations
> > + whose names end in _release. These operations order
> > + their own store against all of the CPU's subsequent
> ---------------------------------------------------------^^^^^^^^^^
> > + memory accesses.
> > +
> > + b. Acquire operations. This category includes
> > + smp_load_acquire(), atomic_read_acquire(), and
> > + value-returning RMW operations whose names end in
> > + _acquire. These operations order their own load against
> > + all of the CPU's prior memory accesses.
> ---------------------------------^^^^^
>
> Double-oops!
Hey, at least I am consistently wrong! ;-)
Fixed, thank you!
Thanx, Paul