Hi all,
work on my dependency checker tool is progressing nicely, and it is
flagging, what I believe is, a harmful addr to ctrl dependency
transformation. For context, see [1] and [2]. I'm using the Clang
compiler.
The dependency in question runs from line 618 into the for loop
increment, i.e. the third expresion in the for loop condition, in line
622 of fs/nfs/delegation.c::nfs_server_return_marked_delegations().
I did my best to reverse-engineer some pseudocode from Clang's IR for
showing what I think is going on.
Clang's unoptimised version:
> restart:
> if(place_holder != NULL)
> delegation = rcu_dereference(place_holder->delegation); /* 618 */
> if(delegation != NULL)
> if(delegation != place_holder_deleg)
> delegation = list_entry_rcu(server->delegations.next, struct nfs_delegation, super_list); /* 620 */
>
> for( ; &(delegation)->super_list != &server->delegations; delegation = list_entry_rcu(delegation->super_list.next, typeof(*(delegation)), super_list)) { /* 622 */
> /*
> * Can continue, "goto restart" or "goto break" (after loop).
> * Can reassign "delegation", "place_holder", "place_holder_deleg".
> * "delegation" might be assigned either a value depending on
> * "delegation" itself, i.e. it is part of the dependency chain,
> * or NULL.
> * Can modifiy fields of the "nfs_delegation" struct "delegation"
> * points to.
> * Assume line 618 has been executed and line 620 hasn't. Then,
> * there exists a path s.t. "delegation" isn't reassigned NULL
> * and the for loop's increment is executed.
> */
> }
Clang's optimised version:
> restart:
> if(place_holder == NULL) {
> delegation = list_entry_rcu(server->delegations.next, struct nfs_delegation, super_list);
> } else {
> cmp = rcu_dereference(place_holder->delegation); /* 618 */
> if(cmp != NULL) { /* Transformation to ctrl dep */
> if(cmp == place_holder_deleg) {
> delegation = place_holder_deleg;
> } else {
> delegation = list_entry_rcu(server->delegations.nex, struct nfs_delegation, super_list);
> }
> } else {
> delegation = list_entry_rcu(server->delegations.next, struct nfs_delegation, super_list);
> }
> }
>
> for( ; &(delegation)->super_list != &server->delegations; delegation = list_entry_rcu(delegation->super_list.next, typeof(*(delegation)), super_list)) {
> /*
> * At this point, "delegation" cannot depend on line 618 anymore
> * since the "rcu_dereference()" was only used for an assignment
> * to "cmp" and a subsequent comparison (ctrl dependency).
> * Therefore, the loop increment cannot depend on the
> * "rcu_dereference()" either. The dependency chain has been
> * broken.
> */
> }
The above is an abstraction of the following control flow path in
"nfs_server_return_marked_delegations()":
1. When "nfs_server_return_marked_delegations()" gets called, it has no
choice but to skip the dependency beginning in line 620, since
"place_holder" is NULL the first time round.
2. Now take a path until "place_holder", the condition for the
dependency beginning, becomes true and "!delegation || delegation !=
place_holder_deleg", the condition for the assignment in line 620,
becomes false. Then, enter the loop again and take a path to one of the
"goto restart" statements without reassigning to "delegation".
3. After going back to "restart", since the condition for line 618
became true, "rcu_dereference()" into "delegation".
4. Enter the for loop again, but avoid the "goto restart" statements in
the first iteration and ensure that "&(delegation)->super_list !=
&server->delegations", the loop condition, remains true and "delegation"
isn't assigned NULL.
5. When the for loop condition is reached for the second time, the loop
increment is executed and there is an address dependency.
Now, why would the compiler decide to assign "place_holder_deleg" to
"delegation" instead of what "rcu_dereference()" returned? Here's my
attempt at explaining.
In the pseudo code above, i.e. in the optimised IR, the assignment of
"place_holder_deleg" is guarded by two conditions. It is executed iff
"place_holder" isn't NULL and the "rcu_dereference()" didn't return
NULL. In other words, iff "place_holder != NULL && rcu_dereference() !=
NULL" holds at line 617, then "delegation == rcu_dereference() ==
place_holder_deleg" must hold at line 622. Otherwise, the optimisation
would be wrong.
Assume control flow has just reached the first if, i.e. line 617, in
source code. Since "place_holder" isn't NULL, it will execute the first
if's body and "rcu_dereference()" into "delegation" (618). Now it has
reached the second if. Per our aussmption, "rcu_dereference()" returned
something that wasn't NULL. Therefore, "!delegation", the first part of
the second if condition's OR, will be false.
However, if we want "rcu_dereference() == delegation" to hold after the
two if's, we can't enter the second if anyway, as it will overwrite
"delegation" with a value that might not be equal to what
"rcu_dereference()" returned. So, we want the second part of the second
if condition's OR, i.e. "delegation != place_holder_deleg" to be false
as well.
When is that the case? It is the case when "delegation ==
place_holder_deleg" holds.
So, if we want "delegation == rcu_dereference() == place_holder_deleg"
to hold after the two if's, "place_holder != NULL && rcu_dereference()
!= NULL" must hold before the two if's, which is what we wanted to show
and what the compiler figured out too.
TL;DR: it appears the compiler optimisation is plausible, yet it still
breaks the address dependency.
For those interested, I have made the unoptimised and optimised IR CFGs
available. In the optimised one, the interesting part is the transition
from "if.end" to "if.end13".
Unoptimised: https://raw.githubusercontent.com/gist/PBHDK/700bf7bdf968fe25c82506de58143bbe/raw/54bf2c1e1a72fb30120f7e812f05ef01ca86b78f/O0-nfs_server_return_marked_delegations.svg
Optimised: https://raw.githubusercontent.com/gist/PBHDK/700bf7bdf968fe25c82506de58143bbe/raw/54bf2c1e1a72fb30120f7e812f05ef01ca86b78f/O2-nfs_server_return_marked_delegations.svg
What do you think?
Many thanks,
Paul
[1]: https://linuxplumbersconf.org/event/7/contributions/821/attachments/598/1075/LPC_2020_--_Dependency_ordering.pdf
[2]: https://lore.kernel.org/llvm/YXknxGFjvaB46d%2Fp@Pauls-MacBook-Pro/T/#u
On Tue, Apr 12, 2022 at 02:48:15PM +0200, Paul Heidekr?ger wrote:
> On Mon, Apr 11, 2022 at 11:21:41AM -0700, Paul E. McKenney wrote:
> > On Thu, Apr 07, 2022 at 05:12:15PM +0200, Paul Heidekr?ger wrote:
> > > Hi all,
> > >
> > > work on my dependency checker tool is progressing nicely, and it is
> > > flagging, what I believe is, a harmful addr to ctrl dependency
> > > transformation. For context, see [1] and [2]. I'm using the Clang
> > > compiler.
> > >
> > > The dependency in question runs from line 618 into the for loop
> > > increment, i.e. the third expresion in the for loop condition, in line
> > > 622 of fs/nfs/delegation.c::nfs_server_return_marked_delegations().
> > >
> > > I did my best to reverse-engineer some pseudocode from Clang's IR for
> > > showing what I think is going on.
> >
> > First, thank you very much for doing this work!
> >
> > > Clang's unoptimised version:
> > >
> > > > restart:
> > > > if(place_holder != NULL)
> > > > delegation = rcu_dereference(place_holder->delegation); /* 618 */
> > > > if(delegation != NULL)
> > > > if(delegation != place_holder_deleg)
> > > > delegation = list_entry_rcu(server->delegations.next, struct nfs_delegation, super_list); /* 620 */
> > > >
> > > > for( ; &(delegation)->super_list != &server->delegations; delegation = list_entry_rcu(delegation->super_list.next, typeof(*(delegation)), super_list)) { /* 622 */
> > > > /*
> > > > * Can continue, "goto restart" or "goto break" (after loop).
> > > > * Can reassign "delegation", "place_holder", "place_holder_deleg".
> > > > * "delegation" might be assigned either a value depending on
> > > > * "delegation" itself, i.e. it is part of the dependency chain,
> > > > * or NULL.
> > > > * Can modifiy fields of the "nfs_delegation" struct "delegation"
> > > > * points to.
> > > > * Assume line 618 has been executed and line 620 hasn't. Then,
> > > > * there exists a path s.t. "delegation" isn't reassigned NULL
> > > > * and the for loop's increment is executed.
> > > > */
> > > > }
> > >
> > > Clang's optimised version:
> > >
> > > > restart:
> > > > if(place_holder == NULL) {
> > > > delegation = list_entry_rcu(server->delegations.next, struct nfs_delegation, super_list);
> > > > } else {
> > > > cmp = rcu_dereference(place_holder->delegation); /* 618 */
> > > > if(cmp != NULL) { /* Transformation to ctrl dep */
> > > > if(cmp == place_holder_deleg) {
> > > > delegation = place_holder_deleg;
> > > > } else {
> > > > delegation = list_entry_rcu(server->delegations.nex, struct nfs_delegation, super_list);
> > > > }
> > > > } else {
> > > > delegation = list_entry_rcu(server->delegations.next, struct nfs_delegation, super_list);
> > > > }
> > > > }
> > > >
> > > > for( ; &(delegation)->super_list != &server->delegations; delegation = list_entry_rcu(delegation->super_list.next, typeof(*(delegation)), super_list)) {
> > > > /*
> > > > * At this point, "delegation" cannot depend on line 618 anymore
> > > > * since the "rcu_dereference()" was only used for an assignment
> > > > * to "cmp" and a subsequent comparison (ctrl dependency).
> > > > * Therefore, the loop increment cannot depend on the
> > > > * "rcu_dereference()" either. The dependency chain has been
> > > > * broken.
> > > > */
> > > > }
> > >
> > > The above is an abstraction of the following control flow path in
> > > "nfs_server_return_marked_delegations()":
> > >
> > > 1. When "nfs_server_return_marked_delegations()" gets called, it has no
> > > choice but to skip the dependency beginning in line 620, since
> > > "place_holder" is NULL the first time round.
> > >
> > > 2. Now take a path until "place_holder", the condition for the
> > > dependency beginning, becomes true and "!delegation || delegation !=
> > > place_holder_deleg", the condition for the assignment in line 620,
> > > becomes false. Then, enter the loop again and take a path to one of the
> > > "goto restart" statements without reassigning to "delegation".
> > >
> > > 3. After going back to "restart", since the condition for line 618
> > > became true, "rcu_dereference()" into "delegation".
> > >
> > > 4. Enter the for loop again, but avoid the "goto restart" statements in
> > > the first iteration and ensure that "&(delegation)->super_list !=
> > > &server->delegations", the loop condition, remains true and "delegation"
> > > isn't assigned NULL.
> > >
> > > 5. When the for loop condition is reached for the second time, the loop
> > > increment is executed and there is an address dependency.
> > >
> > > Now, why would the compiler decide to assign "place_holder_deleg" to
> > > "delegation" instead of what "rcu_dereference()" returned? Here's my
> > > attempt at explaining.
> > >
> > > In the pseudo code above, i.e. in the optimised IR, the assignment of
> > > "place_holder_deleg" is guarded by two conditions. It is executed iff
> > > "place_holder" isn't NULL and the "rcu_dereference()" didn't return
> > > NULL. In other words, iff "place_holder != NULL && rcu_dereference() !=
> > > NULL" holds at line 617, then "delegation == rcu_dereference() ==
> > > place_holder_deleg" must hold at line 622. Otherwise, the optimisation
> > > would be wrong.
> > >
> > > Assume control flow has just reached the first if, i.e. line 617, in
> > > source code. Since "place_holder" isn't NULL, it will execute the first
> > > if's body and "rcu_dereference()" into "delegation" (618). Now it has
> > > reached the second if. Per our aussmption, "rcu_dereference()" returned
> > > something that wasn't NULL. Therefore, "!delegation", the first part of
> > > the second if condition's OR, will be false.
> > >
> > > However, if we want "rcu_dereference() == delegation" to hold after the
> > > two if's, we can't enter the second if anyway, as it will overwrite
> > > "delegation" with a value that might not be equal to what
> > > "rcu_dereference()" returned. So, we want the second part of the second
> > > if condition's OR, i.e. "delegation != place_holder_deleg" to be false
> > > as well.
> > >
> > > When is that the case? It is the case when "delegation ==
> > > place_holder_deleg" holds.
> > >
> > > So, if we want "delegation == rcu_dereference() == place_holder_deleg"
> > > to hold after the two if's, "place_holder != NULL && rcu_dereference()
> > > != NULL" must hold before the two if's, which is what we wanted to show
> > > and what the compiler figured out too.
> > >
> > > TL;DR: it appears the compiler optimisation is plausible, yet it still
> > > breaks the address dependency.
> > >
> > > For those interested, I have made the unoptimised and optimised IR CFGs
> > > available. In the optimised one, the interesting part is the transition
> > > from "if.end" to "if.end13".
> > >
> > > Unoptimised: https://raw.githubusercontent.com/gist/PBHDK/700bf7bdf968fe25c82506de58143bbe/raw/54bf2c1e1a72fb30120f7e812f05ef01ca86b78f/O0-nfs_server_return_marked_delegations.svg
> > >
> > > Optimised: https://raw.githubusercontent.com/gist/PBHDK/700bf7bdf968fe25c82506de58143bbe/raw/54bf2c1e1a72fb30120f7e812f05ef01ca86b78f/O2-nfs_server_return_marked_delegations.svg
> > >
> > > What do you think?
> > >
> > > Many thanks,
> > > Paul
> > >
> > > [1]: https://linuxplumbersconf.org/event/7/contributions/821/attachments/598/1075/LPC_2020_--_Dependency_ordering.pdf
> > > [2]: https://lore.kernel.org/llvm/YXknxGFjvaB46d%2Fp@Pauls-MacBook-Pro/T/#u
> >
> > If I understand this correctly (rather unlikely), this stems from
> > violating the following rule in Documentation/RCU/rcu_dereference.rst:
> >
> > ------------------------------------------------------------------------
> >
> > - Be very careful about comparing pointers obtained from
> > rcu_dereference() against non-NULL values. As Linus Torvalds
> > explained, if the two pointers are equal, the compiler could
> > substitute the pointer you are comparing against for the pointer
> > obtained from rcu_dereference(). For example::
> >
> > p = rcu_dereference(gp);
> > if (p == &default_struct)
> > do_default(p->a);
> >
> > Because the compiler now knows that the value of "p" is exactly
> > the address of the variable "default_struct", it is free to
> > transform this code into the following::
> >
> > p = rcu_dereference(gp);
> > if (p == &default_struct)
> > do_default(default_struct.a);
> >
> > On ARM and Power hardware, the load from "default_struct.a"
> > can now be speculated, such that it might happen before the
> > rcu_dereference(). This could result in bugs due to misordering.
> >
> > However, comparisons are OK in the following cases:
> >
> > - The comparison was against the NULL pointer. If the
> > compiler knows that the pointer is NULL, you had better
> > not be dereferencing it anyway. If the comparison is
> > non-equal, the compiler is none the wiser. Therefore,
> > it is safe to compare pointers from rcu_dereference()
> > against NULL pointers.
> >
> > - The pointer is never dereferenced after being compared.
> > Since there are no subsequent dereferences, the compiler
> > cannot use anything it learned from the comparison
> > to reorder the non-existent subsequent dereferences.
> > This sort of comparison occurs frequently when scanning
> > RCU-protected circular linked lists.
> >
> > Note that if checks for being within an RCU read-side
> > critical section are not required and the pointer is never
> > dereferenced, rcu_access_pointer() should be used in place
> > of rcu_dereference().
> >
> > - The comparison is against a pointer that references memory
> > that was initialized "a long time ago." The reason
> > this is safe is that even if misordering occurs, the
> > misordering will not affect the accesses that follow
> > the comparison. So exactly how long ago is "a long
> > time ago"? Here are some possibilities:
> >
> > - Compile time.
> >
> > - Boot time.
> >
> > - Module-init time for module code.
> >
> > - Prior to kthread creation for kthread code.
> >
> > - During some prior acquisition of the lock that
> > we now hold.
> >
> > - Before mod_timer() time for a timer handler.
> >
> > There are many other possibilities involving the Linux
> > kernel's wide array of primitives that cause code to
> > be invoked at a later time.
> >
> > - The pointer being compared against also came from
> > rcu_dereference(). In this case, both pointers depend
> > on one rcu_dereference() or another, so you get proper
> > ordering either way.
> >
> > That said, this situation can make certain RCU usage
> > bugs more likely to happen. Which can be a good thing,
> > at least if they happen during testing. An example
> > of such an RCU usage bug is shown in the section titled
> > "EXAMPLE OF AMPLIFIED RCU-USAGE BUG".
> >
> > - All of the accesses following the comparison are stores,
> > so that a control dependency preserves the needed ordering.
> > That said, it is easy to get control dependencies wrong.
> > Please see the "CONTROL DEPENDENCIES" section of
> > Documentation/memory-barriers.txt for more details.
> >
> > - The pointers are not equal *and* the compiler does
> > not have enough information to deduce the value of the
> > pointer. Note that the volatile cast in rcu_dereference()
> > will normally prevent the compiler from knowing too much.
> >
> > However, please note that if the compiler knows that the
> > pointer takes on only one of two values, a not-equal
> > comparison will provide exactly the information that the
> > compiler needs to deduce the value of the pointer.
> >
> > ------------------------------------------------------------------------
> >
> > But it would be good to support this use case, for example, by having
> > the compiler provide some way of marking the "delegation" variable as
> > carrying a full dependency.
> >
> > Or did I miss a turn in here somewhere?
> >
> > Thanx, Paul
>
> Actually, I think you're spot on! The original source code has a,
> allbeit nested, comparison of "delegation" against a non-NULL value,
> which is exactly what the documentation discourages as it helps the
> compiler figure out the value of "delegation".
Sometimes I get lucky. ;-)
> I'll try to prepare a patch, using my dependency checker tool to verify
> that this was indeed the issue.
This would be a kernel patch to avoid the comparison? Or a patch to
the compiler to tell it that the "delegation" variable carries a full
dependency? Either would be useful, just curious.
Thanx, Paul
On Thu, Apr 07, 2022 at 05:12:15PM +0200, Paul Heidekr?ger wrote:
> Hi all,
>
> work on my dependency checker tool is progressing nicely, and it is
> flagging, what I believe is, a harmful addr to ctrl dependency
> transformation. For context, see [1] and [2]. I'm using the Clang
> compiler.
>
> The dependency in question runs from line 618 into the for loop
> increment, i.e. the third expresion in the for loop condition, in line
> 622 of fs/nfs/delegation.c::nfs_server_return_marked_delegations().
>
> I did my best to reverse-engineer some pseudocode from Clang's IR for
> showing what I think is going on.
First, thank you very much for doing this work!
> Clang's unoptimised version:
>
> > restart:
> > if(place_holder != NULL)
> > delegation = rcu_dereference(place_holder->delegation); /* 618 */
> > if(delegation != NULL)
> > if(delegation != place_holder_deleg)
> > delegation = list_entry_rcu(server->delegations.next, struct nfs_delegation, super_list); /* 620 */
> >
> > for( ; &(delegation)->super_list != &server->delegations; delegation = list_entry_rcu(delegation->super_list.next, typeof(*(delegation)), super_list)) { /* 622 */
> > /*
> > * Can continue, "goto restart" or "goto break" (after loop).
> > * Can reassign "delegation", "place_holder", "place_holder_deleg".
> > * "delegation" might be assigned either a value depending on
> > * "delegation" itself, i.e. it is part of the dependency chain,
> > * or NULL.
> > * Can modifiy fields of the "nfs_delegation" struct "delegation"
> > * points to.
> > * Assume line 618 has been executed and line 620 hasn't. Then,
> > * there exists a path s.t. "delegation" isn't reassigned NULL
> > * and the for loop's increment is executed.
> > */
> > }
>
> Clang's optimised version:
>
> > restart:
> > if(place_holder == NULL) {
> > delegation = list_entry_rcu(server->delegations.next, struct nfs_delegation, super_list);
> > } else {
> > cmp = rcu_dereference(place_holder->delegation); /* 618 */
> > if(cmp != NULL) { /* Transformation to ctrl dep */
> > if(cmp == place_holder_deleg) {
> > delegation = place_holder_deleg;
> > } else {
> > delegation = list_entry_rcu(server->delegations.nex, struct nfs_delegation, super_list);
> > }
> > } else {
> > delegation = list_entry_rcu(server->delegations.next, struct nfs_delegation, super_list);
> > }
> > }
> >
> > for( ; &(delegation)->super_list != &server->delegations; delegation = list_entry_rcu(delegation->super_list.next, typeof(*(delegation)), super_list)) {
> > /*
> > * At this point, "delegation" cannot depend on line 618 anymore
> > * since the "rcu_dereference()" was only used for an assignment
> > * to "cmp" and a subsequent comparison (ctrl dependency).
> > * Therefore, the loop increment cannot depend on the
> > * "rcu_dereference()" either. The dependency chain has been
> > * broken.
> > */
> > }
>
> The above is an abstraction of the following control flow path in
> "nfs_server_return_marked_delegations()":
>
> 1. When "nfs_server_return_marked_delegations()" gets called, it has no
> choice but to skip the dependency beginning in line 620, since
> "place_holder" is NULL the first time round.
>
> 2. Now take a path until "place_holder", the condition for the
> dependency beginning, becomes true and "!delegation || delegation !=
> place_holder_deleg", the condition for the assignment in line 620,
> becomes false. Then, enter the loop again and take a path to one of the
> "goto restart" statements without reassigning to "delegation".
>
> 3. After going back to "restart", since the condition for line 618
> became true, "rcu_dereference()" into "delegation".
>
> 4. Enter the for loop again, but avoid the "goto restart" statements in
> the first iteration and ensure that "&(delegation)->super_list !=
> &server->delegations", the loop condition, remains true and "delegation"
> isn't assigned NULL.
>
> 5. When the for loop condition is reached for the second time, the loop
> increment is executed and there is an address dependency.
>
> Now, why would the compiler decide to assign "place_holder_deleg" to
> "delegation" instead of what "rcu_dereference()" returned? Here's my
> attempt at explaining.
>
> In the pseudo code above, i.e. in the optimised IR, the assignment of
> "place_holder_deleg" is guarded by two conditions. It is executed iff
> "place_holder" isn't NULL and the "rcu_dereference()" didn't return
> NULL. In other words, iff "place_holder != NULL && rcu_dereference() !=
> NULL" holds at line 617, then "delegation == rcu_dereference() ==
> place_holder_deleg" must hold at line 622. Otherwise, the optimisation
> would be wrong.
>
> Assume control flow has just reached the first if, i.e. line 617, in
> source code. Since "place_holder" isn't NULL, it will execute the first
> if's body and "rcu_dereference()" into "delegation" (618). Now it has
> reached the second if. Per our aussmption, "rcu_dereference()" returned
> something that wasn't NULL. Therefore, "!delegation", the first part of
> the second if condition's OR, will be false.
>
> However, if we want "rcu_dereference() == delegation" to hold after the
> two if's, we can't enter the second if anyway, as it will overwrite
> "delegation" with a value that might not be equal to what
> "rcu_dereference()" returned. So, we want the second part of the second
> if condition's OR, i.e. "delegation != place_holder_deleg" to be false
> as well.
>
> When is that the case? It is the case when "delegation ==
> place_holder_deleg" holds.
>
> So, if we want "delegation == rcu_dereference() == place_holder_deleg"
> to hold after the two if's, "place_holder != NULL && rcu_dereference()
> != NULL" must hold before the two if's, which is what we wanted to show
> and what the compiler figured out too.
>
> TL;DR: it appears the compiler optimisation is plausible, yet it still
> breaks the address dependency.
>
> For those interested, I have made the unoptimised and optimised IR CFGs
> available. In the optimised one, the interesting part is the transition
> from "if.end" to "if.end13".
>
> Unoptimised: https://raw.githubusercontent.com/gist/PBHDK/700bf7bdf968fe25c82506de58143bbe/raw/54bf2c1e1a72fb30120f7e812f05ef01ca86b78f/O0-nfs_server_return_marked_delegations.svg
>
> Optimised: https://raw.githubusercontent.com/gist/PBHDK/700bf7bdf968fe25c82506de58143bbe/raw/54bf2c1e1a72fb30120f7e812f05ef01ca86b78f/O2-nfs_server_return_marked_delegations.svg
>
> What do you think?
>
> Many thanks,
> Paul
>
> [1]: https://linuxplumbersconf.org/event/7/contributions/821/attachments/598/1075/LPC_2020_--_Dependency_ordering.pdf
> [2]: https://lore.kernel.org/llvm/YXknxGFjvaB46d%2Fp@Pauls-MacBook-Pro/T/#u
If I understand this correctly (rather unlikely), this stems from
violating the following rule in Documentation/RCU/rcu_dereference.rst:
------------------------------------------------------------------------
- Be very careful about comparing pointers obtained from
rcu_dereference() against non-NULL values. As Linus Torvalds
explained, if the two pointers are equal, the compiler could
substitute the pointer you are comparing against for the pointer
obtained from rcu_dereference(). For example::
p = rcu_dereference(gp);
if (p == &default_struct)
do_default(p->a);
Because the compiler now knows that the value of "p" is exactly
the address of the variable "default_struct", it is free to
transform this code into the following::
p = rcu_dereference(gp);
if (p == &default_struct)
do_default(default_struct.a);
On ARM and Power hardware, the load from "default_struct.a"
can now be speculated, such that it might happen before the
rcu_dereference(). This could result in bugs due to misordering.
However, comparisons are OK in the following cases:
- The comparison was against the NULL pointer. If the
compiler knows that the pointer is NULL, you had better
not be dereferencing it anyway. If the comparison is
non-equal, the compiler is none the wiser. Therefore,
it is safe to compare pointers from rcu_dereference()
against NULL pointers.
- The pointer is never dereferenced after being compared.
Since there are no subsequent dereferences, the compiler
cannot use anything it learned from the comparison
to reorder the non-existent subsequent dereferences.
This sort of comparison occurs frequently when scanning
RCU-protected circular linked lists.
Note that if checks for being within an RCU read-side
critical section are not required and the pointer is never
dereferenced, rcu_access_pointer() should be used in place
of rcu_dereference().
- The comparison is against a pointer that references memory
that was initialized "a long time ago." The reason
this is safe is that even if misordering occurs, the
misordering will not affect the accesses that follow
the comparison. So exactly how long ago is "a long
time ago"? Here are some possibilities:
- Compile time.
- Boot time.
- Module-init time for module code.
- Prior to kthread creation for kthread code.
- During some prior acquisition of the lock that
we now hold.
- Before mod_timer() time for a timer handler.
There are many other possibilities involving the Linux
kernel's wide array of primitives that cause code to
be invoked at a later time.
- The pointer being compared against also came from
rcu_dereference(). In this case, both pointers depend
on one rcu_dereference() or another, so you get proper
ordering either way.
That said, this situation can make certain RCU usage
bugs more likely to happen. Which can be a good thing,
at least if they happen during testing. An example
of such an RCU usage bug is shown in the section titled
"EXAMPLE OF AMPLIFIED RCU-USAGE BUG".
- All of the accesses following the comparison are stores,
so that a control dependency preserves the needed ordering.
That said, it is easy to get control dependencies wrong.
Please see the "CONTROL DEPENDENCIES" section of
Documentation/memory-barriers.txt for more details.
- The pointers are not equal *and* the compiler does
not have enough information to deduce the value of the
pointer. Note that the volatile cast in rcu_dereference()
will normally prevent the compiler from knowing too much.
However, please note that if the compiler knows that the
pointer takes on only one of two values, a not-equal
comparison will provide exactly the information that the
compiler needs to deduce the value of the pointer.
------------------------------------------------------------------------
But it would be good to support this use case, for example, by having
the compiler provide some way of marking the "delegation" variable as
carrying a full dependency.
Or did I miss a turn in here somewhere?
Thanx, Paul
On Mon, Apr 11, 2022 at 11:21:41AM -0700, Paul E. McKenney wrote:
> On Thu, Apr 07, 2022 at 05:12:15PM +0200, Paul Heidekr?ger wrote:
> > Hi all,
> >
> > work on my dependency checker tool is progressing nicely, and it is
> > flagging, what I believe is, a harmful addr to ctrl dependency
> > transformation. For context, see [1] and [2]. I'm using the Clang
> > compiler.
> >
> > The dependency in question runs from line 618 into the for loop
> > increment, i.e. the third expresion in the for loop condition, in line
> > 622 of fs/nfs/delegation.c::nfs_server_return_marked_delegations().
> >
> > I did my best to reverse-engineer some pseudocode from Clang's IR for
> > showing what I think is going on.
>
> First, thank you very much for doing this work!
>
> > Clang's unoptimised version:
> >
> > > restart:
> > > if(place_holder != NULL)
> > > delegation = rcu_dereference(place_holder->delegation); /* 618 */
> > > if(delegation != NULL)
> > > if(delegation != place_holder_deleg)
> > > delegation = list_entry_rcu(server->delegations.next, struct nfs_delegation, super_list); /* 620 */
> > >
> > > for( ; &(delegation)->super_list != &server->delegations; delegation = list_entry_rcu(delegation->super_list.next, typeof(*(delegation)), super_list)) { /* 622 */
> > > /*
> > > * Can continue, "goto restart" or "goto break" (after loop).
> > > * Can reassign "delegation", "place_holder", "place_holder_deleg".
> > > * "delegation" might be assigned either a value depending on
> > > * "delegation" itself, i.e. it is part of the dependency chain,
> > > * or NULL.
> > > * Can modifiy fields of the "nfs_delegation" struct "delegation"
> > > * points to.
> > > * Assume line 618 has been executed and line 620 hasn't. Then,
> > > * there exists a path s.t. "delegation" isn't reassigned NULL
> > > * and the for loop's increment is executed.
> > > */
> > > }
> >
> > Clang's optimised version:
> >
> > > restart:
> > > if(place_holder == NULL) {
> > > delegation = list_entry_rcu(server->delegations.next, struct nfs_delegation, super_list);
> > > } else {
> > > cmp = rcu_dereference(place_holder->delegation); /* 618 */
> > > if(cmp != NULL) { /* Transformation to ctrl dep */
> > > if(cmp == place_holder_deleg) {
> > > delegation = place_holder_deleg;
> > > } else {
> > > delegation = list_entry_rcu(server->delegations.nex, struct nfs_delegation, super_list);
> > > }
> > > } else {
> > > delegation = list_entry_rcu(server->delegations.next, struct nfs_delegation, super_list);
> > > }
> > > }
> > >
> > > for( ; &(delegation)->super_list != &server->delegations; delegation = list_entry_rcu(delegation->super_list.next, typeof(*(delegation)), super_list)) {
> > > /*
> > > * At this point, "delegation" cannot depend on line 618 anymore
> > > * since the "rcu_dereference()" was only used for an assignment
> > > * to "cmp" and a subsequent comparison (ctrl dependency).
> > > * Therefore, the loop increment cannot depend on the
> > > * "rcu_dereference()" either. The dependency chain has been
> > > * broken.
> > > */
> > > }
> >
> > The above is an abstraction of the following control flow path in
> > "nfs_server_return_marked_delegations()":
> >
> > 1. When "nfs_server_return_marked_delegations()" gets called, it has no
> > choice but to skip the dependency beginning in line 620, since
> > "place_holder" is NULL the first time round.
> >
> > 2. Now take a path until "place_holder", the condition for the
> > dependency beginning, becomes true and "!delegation || delegation !=
> > place_holder_deleg", the condition for the assignment in line 620,
> > becomes false. Then, enter the loop again and take a path to one of the
> > "goto restart" statements without reassigning to "delegation".
> >
> > 3. After going back to "restart", since the condition for line 618
> > became true, "rcu_dereference()" into "delegation".
> >
> > 4. Enter the for loop again, but avoid the "goto restart" statements in
> > the first iteration and ensure that "&(delegation)->super_list !=
> > &server->delegations", the loop condition, remains true and "delegation"
> > isn't assigned NULL.
> >
> > 5. When the for loop condition is reached for the second time, the loop
> > increment is executed and there is an address dependency.
> >
> > Now, why would the compiler decide to assign "place_holder_deleg" to
> > "delegation" instead of what "rcu_dereference()" returned? Here's my
> > attempt at explaining.
> >
> > In the pseudo code above, i.e. in the optimised IR, the assignment of
> > "place_holder_deleg" is guarded by two conditions. It is executed iff
> > "place_holder" isn't NULL and the "rcu_dereference()" didn't return
> > NULL. In other words, iff "place_holder != NULL && rcu_dereference() !=
> > NULL" holds at line 617, then "delegation == rcu_dereference() ==
> > place_holder_deleg" must hold at line 622. Otherwise, the optimisation
> > would be wrong.
> >
> > Assume control flow has just reached the first if, i.e. line 617, in
> > source code. Since "place_holder" isn't NULL, it will execute the first
> > if's body and "rcu_dereference()" into "delegation" (618). Now it has
> > reached the second if. Per our aussmption, "rcu_dereference()" returned
> > something that wasn't NULL. Therefore, "!delegation", the first part of
> > the second if condition's OR, will be false.
> >
> > However, if we want "rcu_dereference() == delegation" to hold after the
> > two if's, we can't enter the second if anyway, as it will overwrite
> > "delegation" with a value that might not be equal to what
> > "rcu_dereference()" returned. So, we want the second part of the second
> > if condition's OR, i.e. "delegation != place_holder_deleg" to be false
> > as well.
> >
> > When is that the case? It is the case when "delegation ==
> > place_holder_deleg" holds.
> >
> > So, if we want "delegation == rcu_dereference() == place_holder_deleg"
> > to hold after the two if's, "place_holder != NULL && rcu_dereference()
> > != NULL" must hold before the two if's, which is what we wanted to show
> > and what the compiler figured out too.
> >
> > TL;DR: it appears the compiler optimisation is plausible, yet it still
> > breaks the address dependency.
> >
> > For those interested, I have made the unoptimised and optimised IR CFGs
> > available. In the optimised one, the interesting part is the transition
> > from "if.end" to "if.end13".
> >
> > Unoptimised: https://raw.githubusercontent.com/gist/PBHDK/700bf7bdf968fe25c82506de58143bbe/raw/54bf2c1e1a72fb30120f7e812f05ef01ca86b78f/O0-nfs_server_return_marked_delegations.svg
> >
> > Optimised: https://raw.githubusercontent.com/gist/PBHDK/700bf7bdf968fe25c82506de58143bbe/raw/54bf2c1e1a72fb30120f7e812f05ef01ca86b78f/O2-nfs_server_return_marked_delegations.svg
> >
> > What do you think?
> >
> > Many thanks,
> > Paul
> >
> > [1]: https://linuxplumbersconf.org/event/7/contributions/821/attachments/598/1075/LPC_2020_--_Dependency_ordering.pdf
> > [2]: https://lore.kernel.org/llvm/YXknxGFjvaB46d%2Fp@Pauls-MacBook-Pro/T/#u
>
> If I understand this correctly (rather unlikely), this stems from
> violating the following rule in Documentation/RCU/rcu_dereference.rst:
>
> ------------------------------------------------------------------------
>
> - Be very careful about comparing pointers obtained from
> rcu_dereference() against non-NULL values. As Linus Torvalds
> explained, if the two pointers are equal, the compiler could
> substitute the pointer you are comparing against for the pointer
> obtained from rcu_dereference(). For example::
>
> p = rcu_dereference(gp);
> if (p == &default_struct)
> do_default(p->a);
>
> Because the compiler now knows that the value of "p" is exactly
> the address of the variable "default_struct", it is free to
> transform this code into the following::
>
> p = rcu_dereference(gp);
> if (p == &default_struct)
> do_default(default_struct.a);
>
> On ARM and Power hardware, the load from "default_struct.a"
> can now be speculated, such that it might happen before the
> rcu_dereference(). This could result in bugs due to misordering.
>
> However, comparisons are OK in the following cases:
>
> - The comparison was against the NULL pointer. If the
> compiler knows that the pointer is NULL, you had better
> not be dereferencing it anyway. If the comparison is
> non-equal, the compiler is none the wiser. Therefore,
> it is safe to compare pointers from rcu_dereference()
> against NULL pointers.
>
> - The pointer is never dereferenced after being compared.
> Since there are no subsequent dereferences, the compiler
> cannot use anything it learned from the comparison
> to reorder the non-existent subsequent dereferences.
> This sort of comparison occurs frequently when scanning
> RCU-protected circular linked lists.
>
> Note that if checks for being within an RCU read-side
> critical section are not required and the pointer is never
> dereferenced, rcu_access_pointer() should be used in place
> of rcu_dereference().
>
> - The comparison is against a pointer that references memory
> that was initialized "a long time ago." The reason
> this is safe is that even if misordering occurs, the
> misordering will not affect the accesses that follow
> the comparison. So exactly how long ago is "a long
> time ago"? Here are some possibilities:
>
> - Compile time.
>
> - Boot time.
>
> - Module-init time for module code.
>
> - Prior to kthread creation for kthread code.
>
> - During some prior acquisition of the lock that
> we now hold.
>
> - Before mod_timer() time for a timer handler.
>
> There are many other possibilities involving the Linux
> kernel's wide array of primitives that cause code to
> be invoked at a later time.
>
> - The pointer being compared against also came from
> rcu_dereference(). In this case, both pointers depend
> on one rcu_dereference() or another, so you get proper
> ordering either way.
>
> That said, this situation can make certain RCU usage
> bugs more likely to happen. Which can be a good thing,
> at least if they happen during testing. An example
> of such an RCU usage bug is shown in the section titled
> "EXAMPLE OF AMPLIFIED RCU-USAGE BUG".
>
> - All of the accesses following the comparison are stores,
> so that a control dependency preserves the needed ordering.
> That said, it is easy to get control dependencies wrong.
> Please see the "CONTROL DEPENDENCIES" section of
> Documentation/memory-barriers.txt for more details.
>
> - The pointers are not equal *and* the compiler does
> not have enough information to deduce the value of the
> pointer. Note that the volatile cast in rcu_dereference()
> will normally prevent the compiler from knowing too much.
>
> However, please note that if the compiler knows that the
> pointer takes on only one of two values, a not-equal
> comparison will provide exactly the information that the
> compiler needs to deduce the value of the pointer.
>
> ------------------------------------------------------------------------
>
> But it would be good to support this use case, for example, by having
> the compiler provide some way of marking the "delegation" variable as
> carrying a full dependency.
>
> Or did I miss a turn in here somewhere?
>
> Thanx, Paul
Actually, I think you're spot on! The original source code has a,
allbeit nested, comparison of "delegation" against a non-NULL value,
which is exactly what the documentation discourages as it helps the
compiler figure out the value of "delegation".
I'll try to prepare a patch, using my dependency checker tool to verify
that this was indeed the issue.
Many thanks,
Paul
On Thu, Apr 7, 2022 at 8:22 AM Paul Heidekrüger
<[email protected]> wrote:
>
> Hi all,
>
> work on my dependency checker tool is progressing nicely, and it is
> flagging, what I believe is, a harmful addr to ctrl dependency
> transformation. For context, see [1] and [2]. I'm using the Clang
> compiler.
> [1]: https://linuxplumbersconf.org/event/7/contributions/821/attachments/598/1075/LPC_2020_--_Dependency_ordering.pdf
> [2]: https://lore.kernel.org/llvm/YXknxGFjvaB46d%2Fp@Pauls-MacBook-Pro/T/#u
Hi Paul,
Thanks for the report and your (and your team's) work on this tool.
Orthogonal to your report, Jose (cc'ed) and I are currently in the
planning process to put together a Kernel+Toolchain microconference
track at Linux Plumbers Conference [0] this year (Sept 12-14) in
Dublin, Ireland. Would you or someone from your group be able and
interested in presenting more information about your work to an
audience of kernel and toolchain developers at such an event?
Would others be interested in such a topic? (What do they say in
Starship Troopers...?...Would you like to know more?)
[0] https://lpc.events/event/16/
--
Thanks,
~Nick Desaulniers
On Tue, Apr 12, 2022 at 08:26:49AM -0700, Paul E. McKenney wrote:
> On Tue, Apr 12, 2022 at 02:48:15PM +0200, Paul Heidekr?ger wrote:
> > On Mon, Apr 11, 2022 at 11:21:41AM -0700, Paul E. McKenney wrote:
> > > On Thu, Apr 07, 2022 at 05:12:15PM +0200, Paul Heidekr?ger wrote:
> > > > Hi all,
> > > >
> > > > work on my dependency checker tool is progressing nicely, and it is
> > > > flagging, what I believe is, a harmful addr to ctrl dependency
> > > > transformation. For context, see [1] and [2]. I'm using the Clang
> > > > compiler.
> > > >
> > > > The dependency in question runs from line 618 into the for loop
> > > > increment, i.e. the third expresion in the for loop condition, in line
> > > > 622 of fs/nfs/delegation.c::nfs_server_return_marked_delegations().
> > > >
> > > > I did my best to reverse-engineer some pseudocode from Clang's IR for
> > > > showing what I think is going on.
> > >
> > > First, thank you very much for doing this work!
> > >
> > > > Clang's unoptimised version:
> > > >
> > > > > restart:
> > > > > if(place_holder != NULL)
> > > > > delegation = rcu_dereference(place_holder->delegation); /* 618 */
> > > > > if(delegation != NULL)
> > > > > if(delegation != place_holder_deleg)
> > > > > delegation = list_entry_rcu(server->delegations.next, struct nfs_delegation, super_list); /* 620 */
> > > > >
> > > > > for( ; &(delegation)->super_list != &server->delegations; delegation = list_entry_rcu(delegation->super_list.next, typeof(*(delegation)), super_list)) { /* 622 */
> > > > > /*
> > > > > * Can continue, "goto restart" or "goto break" (after loop).
> > > > > * Can reassign "delegation", "place_holder", "place_holder_deleg".
> > > > > * "delegation" might be assigned either a value depending on
> > > > > * "delegation" itself, i.e. it is part of the dependency chain,
> > > > > * or NULL.
> > > > > * Can modifiy fields of the "nfs_delegation" struct "delegation"
> > > > > * points to.
> > > > > * Assume line 618 has been executed and line 620 hasn't. Then,
> > > > > * there exists a path s.t. "delegation" isn't reassigned NULL
> > > > > * and the for loop's increment is executed.
> > > > > */
> > > > > }
> > > >
> > > > Clang's optimised version:
> > > >
> > > > > restart:
> > > > > if(place_holder == NULL) {
> > > > > delegation = list_entry_rcu(server->delegations.next, struct nfs_delegation, super_list);
> > > > > } else {
> > > > > cmp = rcu_dereference(place_holder->delegation); /* 618 */
> > > > > if(cmp != NULL) { /* Transformation to ctrl dep */
> > > > > if(cmp == place_holder_deleg) {
> > > > > delegation = place_holder_deleg;
> > > > > } else {
> > > > > delegation = list_entry_rcu(server->delegations.nex, struct nfs_delegation, super_list);
> > > > > }
> > > > > } else {
> > > > > delegation = list_entry_rcu(server->delegations.next, struct nfs_delegation, super_list);
> > > > > }
> > > > > }
> > > > >
> > > > > for( ; &(delegation)->super_list != &server->delegations; delegation = list_entry_rcu(delegation->super_list.next, typeof(*(delegation)), super_list)) {
> > > > > /*
> > > > > * At this point, "delegation" cannot depend on line 618 anymore
> > > > > * since the "rcu_dereference()" was only used for an assignment
> > > > > * to "cmp" and a subsequent comparison (ctrl dependency).
> > > > > * Therefore, the loop increment cannot depend on the
> > > > > * "rcu_dereference()" either. The dependency chain has been
> > > > > * broken.
> > > > > */
> > > > > }
> > > >
> > > > The above is an abstraction of the following control flow path in
> > > > "nfs_server_return_marked_delegations()":
> > > >
> > > > 1. When "nfs_server_return_marked_delegations()" gets called, it has no
> > > > choice but to skip the dependency beginning in line 620, since
> > > > "place_holder" is NULL the first time round.
> > > >
> > > > 2. Now take a path until "place_holder", the condition for the
> > > > dependency beginning, becomes true and "!delegation || delegation !=
> > > > place_holder_deleg", the condition for the assignment in line 620,
> > > > becomes false. Then, enter the loop again and take a path to one of the
> > > > "goto restart" statements without reassigning to "delegation".
> > > >
> > > > 3. After going back to "restart", since the condition for line 618
> > > > became true, "rcu_dereference()" into "delegation".
> > > >
> > > > 4. Enter the for loop again, but avoid the "goto restart" statements in
> > > > the first iteration and ensure that "&(delegation)->super_list !=
> > > > &server->delegations", the loop condition, remains true and "delegation"
> > > > isn't assigned NULL.
> > > >
> > > > 5. When the for loop condition is reached for the second time, the loop
> > > > increment is executed and there is an address dependency.
> > > >
> > > > Now, why would the compiler decide to assign "place_holder_deleg" to
> > > > "delegation" instead of what "rcu_dereference()" returned? Here's my
> > > > attempt at explaining.
> > > >
> > > > In the pseudo code above, i.e. in the optimised IR, the assignment of
> > > > "place_holder_deleg" is guarded by two conditions. It is executed iff
> > > > "place_holder" isn't NULL and the "rcu_dereference()" didn't return
> > > > NULL. In other words, iff "place_holder != NULL && rcu_dereference() !=
> > > > NULL" holds at line 617, then "delegation == rcu_dereference() ==
> > > > place_holder_deleg" must hold at line 622. Otherwise, the optimisation
> > > > would be wrong.
> > > >
> > > > Assume control flow has just reached the first if, i.e. line 617, in
> > > > source code. Since "place_holder" isn't NULL, it will execute the first
> > > > if's body and "rcu_dereference()" into "delegation" (618). Now it has
> > > > reached the second if. Per our aussmption, "rcu_dereference()" returned
> > > > something that wasn't NULL. Therefore, "!delegation", the first part of
> > > > the second if condition's OR, will be false.
> > > >
> > > > However, if we want "rcu_dereference() == delegation" to hold after the
> > > > two if's, we can't enter the second if anyway, as it will overwrite
> > > > "delegation" with a value that might not be equal to what
> > > > "rcu_dereference()" returned. So, we want the second part of the second
> > > > if condition's OR, i.e. "delegation != place_holder_deleg" to be false
> > > > as well.
> > > >
> > > > When is that the case? It is the case when "delegation ==
> > > > place_holder_deleg" holds.
> > > >
> > > > So, if we want "delegation == rcu_dereference() == place_holder_deleg"
> > > > to hold after the two if's, "place_holder != NULL && rcu_dereference()
> > > > != NULL" must hold before the two if's, which is what we wanted to show
> > > > and what the compiler figured out too.
> > > >
> > > > TL;DR: it appears the compiler optimisation is plausible, yet it still
> > > > breaks the address dependency.
> > > >
> > > > For those interested, I have made the unoptimised and optimised IR CFGs
> > > > available. In the optimised one, the interesting part is the transition
> > > > from "if.end" to "if.end13".
> > > >
> > > > Unoptimised: https://raw.githubusercontent.com/gist/PBHDK/700bf7bdf968fe25c82506de58143bbe/raw/54bf2c1e1a72fb30120f7e812f05ef01ca86b78f/O0-nfs_server_return_marked_delegations.svg
> > > >
> > > > Optimised: https://raw.githubusercontent.com/gist/PBHDK/700bf7bdf968fe25c82506de58143bbe/raw/54bf2c1e1a72fb30120f7e812f05ef01ca86b78f/O2-nfs_server_return_marked_delegations.svg
> > > >
> > > > What do you think?
> > > >
> > > > Many thanks,
> > > > Paul
> > > >
> > > > [1]: https://linuxplumbersconf.org/event/7/contributions/821/attachments/598/1075/LPC_2020_--_Dependency_ordering.pdf
> > > > [2]: https://lore.kernel.org/llvm/YXknxGFjvaB46d%2Fp@Pauls-MacBook-Pro/T/#u
> > >
> > > If I understand this correctly (rather unlikely), this stems from
> > > violating the following rule in Documentation/RCU/rcu_dereference.rst:
> > >
> > > ------------------------------------------------------------------------
> > >
> > > - Be very careful about comparing pointers obtained from
> > > rcu_dereference() against non-NULL values. As Linus Torvalds
> > > explained, if the two pointers are equal, the compiler could
> > > substitute the pointer you are comparing against for the pointer
> > > obtained from rcu_dereference(). For example::
> > >
> > > p = rcu_dereference(gp);
> > > if (p == &default_struct)
> > > do_default(p->a);
> > >
> > > Because the compiler now knows that the value of "p" is exactly
> > > the address of the variable "default_struct", it is free to
> > > transform this code into the following::
> > >
> > > p = rcu_dereference(gp);
> > > if (p == &default_struct)
> > > do_default(default_struct.a);
> > >
> > > On ARM and Power hardware, the load from "default_struct.a"
> > > can now be speculated, such that it might happen before the
> > > rcu_dereference(). This could result in bugs due to misordering.
> > >
> > > However, comparisons are OK in the following cases:
> > >
> > > - The comparison was against the NULL pointer. If the
> > > compiler knows that the pointer is NULL, you had better
> > > not be dereferencing it anyway. If the comparison is
> > > non-equal, the compiler is none the wiser. Therefore,
> > > it is safe to compare pointers from rcu_dereference()
> > > against NULL pointers.
> > >
> > > - The pointer is never dereferenced after being compared.
> > > Since there are no subsequent dereferences, the compiler
> > > cannot use anything it learned from the comparison
> > > to reorder the non-existent subsequent dereferences.
> > > This sort of comparison occurs frequently when scanning
> > > RCU-protected circular linked lists.
> > >
> > > Note that if checks for being within an RCU read-side
> > > critical section are not required and the pointer is never
> > > dereferenced, rcu_access_pointer() should be used in place
> > > of rcu_dereference().
> > >
> > > - The comparison is against a pointer that references memory
> > > that was initialized "a long time ago." The reason
> > > this is safe is that even if misordering occurs, the
> > > misordering will not affect the accesses that follow
> > > the comparison. So exactly how long ago is "a long
> > > time ago"? Here are some possibilities:
> > >
> > > - Compile time.
> > >
> > > - Boot time.
> > >
> > > - Module-init time for module code.
> > >
> > > - Prior to kthread creation for kthread code.
> > >
> > > - During some prior acquisition of the lock that
> > > we now hold.
> > >
> > > - Before mod_timer() time for a timer handler.
> > >
> > > There are many other possibilities involving the Linux
> > > kernel's wide array of primitives that cause code to
> > > be invoked at a later time.
> > >
> > > - The pointer being compared against also came from
> > > rcu_dereference(). In this case, both pointers depend
> > > on one rcu_dereference() or another, so you get proper
> > > ordering either way.
> > >
> > > That said, this situation can make certain RCU usage
> > > bugs more likely to happen. Which can be a good thing,
> > > at least if they happen during testing. An example
> > > of such an RCU usage bug is shown in the section titled
> > > "EXAMPLE OF AMPLIFIED RCU-USAGE BUG".
> > >
> > > - All of the accesses following the comparison are stores,
> > > so that a control dependency preserves the needed ordering.
> > > That said, it is easy to get control dependencies wrong.
> > > Please see the "CONTROL DEPENDENCIES" section of
> > > Documentation/memory-barriers.txt for more details.
> > >
> > > - The pointers are not equal *and* the compiler does
> > > not have enough information to deduce the value of the
> > > pointer. Note that the volatile cast in rcu_dereference()
> > > will normally prevent the compiler from knowing too much.
> > >
> > > However, please note that if the compiler knows that the
> > > pointer takes on only one of two values, a not-equal
> > > comparison will provide exactly the information that the
> > > compiler needs to deduce the value of the pointer.
> > >
> > > ------------------------------------------------------------------------
> > >
> > > But it would be good to support this use case, for example, by having
> > > the compiler provide some way of marking the "delegation" variable as
> > > carrying a full dependency.
> > >
> > > Or did I miss a turn in here somewhere?
> > >
> > > Thanx, Paul
> >
> > Actually, I think you're spot on! The original source code has a,
> > allbeit nested, comparison of "delegation" against a non-NULL value,
> > which is exactly what the documentation discourages as it helps the
> > compiler figure out the value of "delegation".
>
> Sometimes I get lucky. ;-)
>
> > I'll try to prepare a patch, using my dependency checker tool to verify
> > that this was indeed the issue.
>
> This would be a kernel patch to avoid the comparison? Or a patch to
> the compiler to tell it that the "delegation" variable carries a full
> dependency? Either would be useful, just curious.
Ah, sorry, ambiguous. I was referring to a kernel patch which avoids the
comparions. But we, i.e. everyone involved with the dependency checker
tool, are also aware of the value of a dependency-preserving compiler
and have had some discussions around the topic. Not making any promises
in that regard though :-)
Many thanks,
Paul
On Thu, Apr 14, 2022 at 11:04:42AM +0200, Paul Heidekr?ger wrote:
> On Tue, Apr 12, 2022 at 08:26:49AM -0700, Paul E. McKenney wrote:
> > On Tue, Apr 12, 2022 at 02:48:15PM +0200, Paul Heidekr?ger wrote:
> > > On Mon, Apr 11, 2022 at 11:21:41AM -0700, Paul E. McKenney wrote:
> > > > On Thu, Apr 07, 2022 at 05:12:15PM +0200, Paul Heidekr?ger wrote:
> > > > > Hi all,
> > > > >
> > > > > work on my dependency checker tool is progressing nicely, and it is
> > > > > flagging, what I believe is, a harmful addr to ctrl dependency
> > > > > transformation. For context, see [1] and [2]. I'm using the Clang
> > > > > compiler.
> > > > >
> > > > > The dependency in question runs from line 618 into the for loop
> > > > > increment, i.e. the third expresion in the for loop condition, in line
> > > > > 622 of fs/nfs/delegation.c::nfs_server_return_marked_delegations().
> > > > >
> > > > > I did my best to reverse-engineer some pseudocode from Clang's IR for
> > > > > showing what I think is going on.
> > > >
> > > > First, thank you very much for doing this work!
> > > >
> > > > > Clang's unoptimised version:
> > > > >
> > > > > > restart:
> > > > > > if(place_holder != NULL)
> > > > > > delegation = rcu_dereference(place_holder->delegation); /* 618 */
> > > > > > if(delegation != NULL)
> > > > > > if(delegation != place_holder_deleg)
> > > > > > delegation = list_entry_rcu(server->delegations.next, struct nfs_delegation, super_list); /* 620 */
> > > > > >
> > > > > > for( ; &(delegation)->super_list != &server->delegations; delegation = list_entry_rcu(delegation->super_list.next, typeof(*(delegation)), super_list)) { /* 622 */
> > > > > > /*
> > > > > > * Can continue, "goto restart" or "goto break" (after loop).
> > > > > > * Can reassign "delegation", "place_holder", "place_holder_deleg".
> > > > > > * "delegation" might be assigned either a value depending on
> > > > > > * "delegation" itself, i.e. it is part of the dependency chain,
> > > > > > * or NULL.
> > > > > > * Can modifiy fields of the "nfs_delegation" struct "delegation"
> > > > > > * points to.
> > > > > > * Assume line 618 has been executed and line 620 hasn't. Then,
> > > > > > * there exists a path s.t. "delegation" isn't reassigned NULL
> > > > > > * and the for loop's increment is executed.
> > > > > > */
> > > > > > }
> > > > >
> > > > > Clang's optimised version:
> > > > >
> > > > > > restart:
> > > > > > if(place_holder == NULL) {
> > > > > > delegation = list_entry_rcu(server->delegations.next, struct nfs_delegation, super_list);
> > > > > > } else {
> > > > > > cmp = rcu_dereference(place_holder->delegation); /* 618 */
> > > > > > if(cmp != NULL) { /* Transformation to ctrl dep */
> > > > > > if(cmp == place_holder_deleg) {
> > > > > > delegation = place_holder_deleg;
> > > > > > } else {
> > > > > > delegation = list_entry_rcu(server->delegations.nex, struct nfs_delegation, super_list);
> > > > > > }
> > > > > > } else {
> > > > > > delegation = list_entry_rcu(server->delegations.next, struct nfs_delegation, super_list);
> > > > > > }
> > > > > > }
> > > > > >
> > > > > > for( ; &(delegation)->super_list != &server->delegations; delegation = list_entry_rcu(delegation->super_list.next, typeof(*(delegation)), super_list)) {
> > > > > > /*
> > > > > > * At this point, "delegation" cannot depend on line 618 anymore
> > > > > > * since the "rcu_dereference()" was only used for an assignment
> > > > > > * to "cmp" and a subsequent comparison (ctrl dependency).
> > > > > > * Therefore, the loop increment cannot depend on the
> > > > > > * "rcu_dereference()" either. The dependency chain has been
> > > > > > * broken.
> > > > > > */
> > > > > > }
> > > > >
> > > > > The above is an abstraction of the following control flow path in
> > > > > "nfs_server_return_marked_delegations()":
> > > > >
> > > > > 1. When "nfs_server_return_marked_delegations()" gets called, it has no
> > > > > choice but to skip the dependency beginning in line 620, since
> > > > > "place_holder" is NULL the first time round.
> > > > >
> > > > > 2. Now take a path until "place_holder", the condition for the
> > > > > dependency beginning, becomes true and "!delegation || delegation !=
> > > > > place_holder_deleg", the condition for the assignment in line 620,
> > > > > becomes false. Then, enter the loop again and take a path to one of the
> > > > > "goto restart" statements without reassigning to "delegation".
> > > > >
> > > > > 3. After going back to "restart", since the condition for line 618
> > > > > became true, "rcu_dereference()" into "delegation".
> > > > >
> > > > > 4. Enter the for loop again, but avoid the "goto restart" statements in
> > > > > the first iteration and ensure that "&(delegation)->super_list !=
> > > > > &server->delegations", the loop condition, remains true and "delegation"
> > > > > isn't assigned NULL.
> > > > >
> > > > > 5. When the for loop condition is reached for the second time, the loop
> > > > > increment is executed and there is an address dependency.
> > > > >
> > > > > Now, why would the compiler decide to assign "place_holder_deleg" to
> > > > > "delegation" instead of what "rcu_dereference()" returned? Here's my
> > > > > attempt at explaining.
> > > > >
> > > > > In the pseudo code above, i.e. in the optimised IR, the assignment of
> > > > > "place_holder_deleg" is guarded by two conditions. It is executed iff
> > > > > "place_holder" isn't NULL and the "rcu_dereference()" didn't return
> > > > > NULL. In other words, iff "place_holder != NULL && rcu_dereference() !=
> > > > > NULL" holds at line 617, then "delegation == rcu_dereference() ==
> > > > > place_holder_deleg" must hold at line 622. Otherwise, the optimisation
> > > > > would be wrong.
> > > > >
> > > > > Assume control flow has just reached the first if, i.e. line 617, in
> > > > > source code. Since "place_holder" isn't NULL, it will execute the first
> > > > > if's body and "rcu_dereference()" into "delegation" (618). Now it has
> > > > > reached the second if. Per our aussmption, "rcu_dereference()" returned
> > > > > something that wasn't NULL. Therefore, "!delegation", the first part of
> > > > > the second if condition's OR, will be false.
> > > > >
> > > > > However, if we want "rcu_dereference() == delegation" to hold after the
> > > > > two if's, we can't enter the second if anyway, as it will overwrite
> > > > > "delegation" with a value that might not be equal to what
> > > > > "rcu_dereference()" returned. So, we want the second part of the second
> > > > > if condition's OR, i.e. "delegation != place_holder_deleg" to be false
> > > > > as well.
> > > > >
> > > > > When is that the case? It is the case when "delegation ==
> > > > > place_holder_deleg" holds.
> > > > >
> > > > > So, if we want "delegation == rcu_dereference() == place_holder_deleg"
> > > > > to hold after the two if's, "place_holder != NULL && rcu_dereference()
> > > > > != NULL" must hold before the two if's, which is what we wanted to show
> > > > > and what the compiler figured out too.
> > > > >
> > > > > TL;DR: it appears the compiler optimisation is plausible, yet it still
> > > > > breaks the address dependency.
> > > > >
> > > > > For those interested, I have made the unoptimised and optimised IR CFGs
> > > > > available. In the optimised one, the interesting part is the transition
> > > > > from "if.end" to "if.end13".
> > > > >
> > > > > Unoptimised: https://raw.githubusercontent.com/gist/PBHDK/700bf7bdf968fe25c82506de58143bbe/raw/54bf2c1e1a72fb30120f7e812f05ef01ca86b78f/O0-nfs_server_return_marked_delegations.svg
> > > > >
> > > > > Optimised: https://raw.githubusercontent.com/gist/PBHDK/700bf7bdf968fe25c82506de58143bbe/raw/54bf2c1e1a72fb30120f7e812f05ef01ca86b78f/O2-nfs_server_return_marked_delegations.svg
> > > > >
> > > > > What do you think?
> > > > >
> > > > > Many thanks,
> > > > > Paul
> > > > >
> > > > > [1]: https://linuxplumbersconf.org/event/7/contributions/821/attachments/598/1075/LPC_2020_--_Dependency_ordering.pdf
> > > > > [2]: https://lore.kernel.org/llvm/YXknxGFjvaB46d%2Fp@Pauls-MacBook-Pro/T/#u
> > > >
> > > > If I understand this correctly (rather unlikely), this stems from
> > > > violating the following rule in Documentation/RCU/rcu_dereference.rst:
> > > >
> > > > ------------------------------------------------------------------------
> > > >
> > > > - Be very careful about comparing pointers obtained from
> > > > rcu_dereference() against non-NULL values. As Linus Torvalds
> > > > explained, if the two pointers are equal, the compiler could
> > > > substitute the pointer you are comparing against for the pointer
> > > > obtained from rcu_dereference(). For example::
> > > >
> > > > p = rcu_dereference(gp);
> > > > if (p == &default_struct)
> > > > do_default(p->a);
> > > >
> > > > Because the compiler now knows that the value of "p" is exactly
> > > > the address of the variable "default_struct", it is free to
> > > > transform this code into the following::
> > > >
> > > > p = rcu_dereference(gp);
> > > > if (p == &default_struct)
> > > > do_default(default_struct.a);
> > > >
> > > > On ARM and Power hardware, the load from "default_struct.a"
> > > > can now be speculated, such that it might happen before the
> > > > rcu_dereference(). This could result in bugs due to misordering.
> > > >
> > > > However, comparisons are OK in the following cases:
> > > >
> > > > - The comparison was against the NULL pointer. If the
> > > > compiler knows that the pointer is NULL, you had better
> > > > not be dereferencing it anyway. If the comparison is
> > > > non-equal, the compiler is none the wiser. Therefore,
> > > > it is safe to compare pointers from rcu_dereference()
> > > > against NULL pointers.
> > > >
> > > > - The pointer is never dereferenced after being compared.
> > > > Since there are no subsequent dereferences, the compiler
> > > > cannot use anything it learned from the comparison
> > > > to reorder the non-existent subsequent dereferences.
> > > > This sort of comparison occurs frequently when scanning
> > > > RCU-protected circular linked lists.
> > > >
> > > > Note that if checks for being within an RCU read-side
> > > > critical section are not required and the pointer is never
> > > > dereferenced, rcu_access_pointer() should be used in place
> > > > of rcu_dereference().
> > > >
> > > > - The comparison is against a pointer that references memory
> > > > that was initialized "a long time ago." The reason
> > > > this is safe is that even if misordering occurs, the
> > > > misordering will not affect the accesses that follow
> > > > the comparison. So exactly how long ago is "a long
> > > > time ago"? Here are some possibilities:
> > > >
> > > > - Compile time.
> > > >
> > > > - Boot time.
> > > >
> > > > - Module-init time for module code.
> > > >
> > > > - Prior to kthread creation for kthread code.
> > > >
> > > > - During some prior acquisition of the lock that
> > > > we now hold.
> > > >
> > > > - Before mod_timer() time for a timer handler.
> > > >
> > > > There are many other possibilities involving the Linux
> > > > kernel's wide array of primitives that cause code to
> > > > be invoked at a later time.
> > > >
> > > > - The pointer being compared against also came from
> > > > rcu_dereference(). In this case, both pointers depend
> > > > on one rcu_dereference() or another, so you get proper
> > > > ordering either way.
> > > >
> > > > That said, this situation can make certain RCU usage
> > > > bugs more likely to happen. Which can be a good thing,
> > > > at least if they happen during testing. An example
> > > > of such an RCU usage bug is shown in the section titled
> > > > "EXAMPLE OF AMPLIFIED RCU-USAGE BUG".
> > > >
> > > > - All of the accesses following the comparison are stores,
> > > > so that a control dependency preserves the needed ordering.
> > > > That said, it is easy to get control dependencies wrong.
> > > > Please see the "CONTROL DEPENDENCIES" section of
> > > > Documentation/memory-barriers.txt for more details.
> > > >
> > > > - The pointers are not equal *and* the compiler does
> > > > not have enough information to deduce the value of the
> > > > pointer. Note that the volatile cast in rcu_dereference()
> > > > will normally prevent the compiler from knowing too much.
> > > >
> > > > However, please note that if the compiler knows that the
> > > > pointer takes on only one of two values, a not-equal
> > > > comparison will provide exactly the information that the
> > > > compiler needs to deduce the value of the pointer.
> > > >
> > > > ------------------------------------------------------------------------
> > > >
> > > > But it would be good to support this use case, for example, by having
> > > > the compiler provide some way of marking the "delegation" variable as
> > > > carrying a full dependency.
> > > >
> > > > Or did I miss a turn in here somewhere?
> > > >
> > > > Thanx, Paul
> > >
> > > Actually, I think you're spot on! The original source code has a,
> > > allbeit nested, comparison of "delegation" against a non-NULL value,
> > > which is exactly what the documentation discourages as it helps the
> > > compiler figure out the value of "delegation".
> >
> > Sometimes I get lucky. ;-)
> >
> > > I'll try to prepare a patch, using my dependency checker tool to verify
> > > that this was indeed the issue.
> >
> > This would be a kernel patch to avoid the comparison? Or a patch to
> > the compiler to tell it that the "delegation" variable carries a full
> > dependency? Either would be useful, just curious.
>
> Ah, sorry, ambiguous. I was referring to a kernel patch which avoids the
> comparions. But we, i.e. everyone involved with the dependency checker
> tool, are also aware of the value of a dependency-preserving compiler
> and have had some discussions around the topic. Not making any promises
> in that regard though :-)
Thank you for the information!
Is there a reasonable way to hide the comparison from the compiler?
Two unreasonable ways are to implement an asm or an external function
that does the comparison. ;-)
Thanx, Paul
On Thu, Apr 14, 2022 at 02:21:25PM -0700, Nick Desaulniers wrote:
> On Thu, Apr 7, 2022 at 8:22 AM Paul Heidekr?ger
> <[email protected]> wrote:
> >
> > Hi all,
> >
> > work on my dependency checker tool is progressing nicely, and it is
> > flagging, what I believe is, a harmful addr to ctrl dependency
> > transformation. For context, see [1] and [2]. I'm using the Clang
> > compiler.
> > [1]: https://linuxplumbersconf.org/event/7/contributions/821/attachments/598/1075/LPC_2020_--_Dependency_ordering.pdf
> > [2]: https://lore.kernel.org/llvm/YXknxGFjvaB46d%2Fp@Pauls-MacBook-Pro/T/#u
>
> Hi Paul,
> Thanks for the report and your (and your team's) work on this tool.
> Orthogonal to your report, Jose (cc'ed) and I are currently in the
> planning process to put together a Kernel+Toolchain microconference
> track at Linux Plumbers Conference [0] this year (Sept 12-14) in
> Dublin, Ireland. Would you or someone from your group be able and
> interested in presenting more information about your work to an
> audience of kernel and toolchain developers at such an event?
+1 on presenting at Plumbers!
Thanx, Paul
> Would others be interested in such a topic? (What do they say in
> Starship Troopers...?...Would you like to know more?)
>
> [0] https://lpc.events/event/16/
> --
> Thanks,
> ~Nick Desaulniers
On Thu, Apr 14, 2022 at 02:21:25PM -0700, Nick Desaulniers wrote:
> On Thu, Apr 7, 2022 at 8:22 AM Paul Heidekr?ger
> <[email protected]> wrote:
> >
> > Hi all,
> >
> > work on my dependency checker tool is progressing nicely, and it is
> > flagging, what I believe is, a harmful addr to ctrl dependency
> > transformation. For context, see [1] and [2]. I'm using the Clang
> > compiler.
> > [1]: https://linuxplumbersconf.org/event/7/contributions/821/attachments/598/1075/LPC_2020_--_Dependency_ordering.pdf
> > [2]: https://lore.kernel.org/llvm/YXknxGFjvaB46d%2Fp@Pauls-MacBook-Pro/T/#u
>
> Hi Paul,
> Thanks for the report and your (and your team's) work on this tool.
> Orthogonal to your report, Jose (cc'ed) and I are currently in the
> planning process to put together a Kernel+Toolchain microconference
> track at Linux Plumbers Conference [0] this year (Sept 12-14) in
> Dublin, Ireland. Would you or someone from your group be able and
> interested in presenting more information about your work to an
> audience of kernel and toolchain developers at such an event?
>
> Would others be interested in such a topic? (What do they say in
> Starship Troopers...?...Would you like to know more?)
>
> [0] https://lpc.events/event/16/
> --
> Thanks,
> ~Nick Desaulniers
Hi Nick and Jose,
Many thanks for inviting us! I would love to do a talk at LPC! Hopefully
in person too.
Given that there have been several talks around this topic at LPC
already, it seems very fitting, and we'll hopefully have more to share
by then. Actually we have more to share already :-)
https://lore.kernel.org/all/YmKE%[email protected]/T/#u
I assume we will have to submit an abstract soon?
Many thanks,
Paul
On Fri, Apr 22, 2022 at 3:39 AM Paul Heidekrüger
<[email protected]> wrote:
>
> On Thu, Apr 14, 2022 at 02:21:25PM -0700, Nick Desaulniers wrote:
> > On Thu, Apr 7, 2022 at 8:22 AM Paul Heidekrüger
> > <[email protected]> wrote:
> > >
> > > Hi all,
> > >
> > > work on my dependency checker tool is progressing nicely, and it is
> > > flagging, what I believe is, a harmful addr to ctrl dependency
> > > transformation. For context, see [1] and [2]. I'm using the Clang
> > > compiler.
> > > [1]: https://linuxplumbersconf.org/event/7/contributions/821/attachments/598/1075/LPC_2020_--_Dependency_ordering.pdf
> > > [2]: https://lore.kernel.org/llvm/YXknxGFjvaB46d%2Fp@Pauls-MacBook-Pro/T/#u
> >
> > Hi Paul,
> > Thanks for the report and your (and your team's) work on this tool.
> > Orthogonal to your report, Jose (cc'ed) and I are currently in the
> > planning process to put together a Kernel+Toolchain microconference
> > track at Linux Plumbers Conference [0] this year (Sept 12-14) in
> > Dublin, Ireland. Would you or someone from your group be able and
> > interested in presenting more information about your work to an
> > audience of kernel and toolchain developers at such an event?
> >
> > Would others be interested in such a topic? (What do they say in
> > Starship Troopers...?...Would you like to know more?)
> >
> > [0] https://lpc.events/event/16/
> > --
> > Thanks,
> > ~Nick Desaulniers
>
> Hi Nick and Jose,
>
> Many thanks for inviting us! I would love to do a talk at LPC! Hopefully
> in person too.
>
> Given that there have been several talks around this topic at LPC
> already, it seems very fitting, and we'll hopefully have more to share
> by then. Actually we have more to share already :-)
>
> https://lore.kernel.org/all/YmKE%[email protected]/T/#u
>
> I assume we will have to submit an abstract soon?
Yes, if you go to: https://lpc.events/event/16/abstracts/
click "Submit new abstract" in the bottom right.
Under the "Track" dropdown, please select "Toolchains Track."
--
Thanks,
~Nick Desaulniers
On Tue, May 17, 2022 at 3:29 PM Nick Desaulniers
<[email protected]> wrote:
>
> On Fri, Apr 22, 2022 at 3:39 AM Paul Heidekrüger
> <[email protected]> wrote:
> >
> > On Thu, Apr 14, 2022 at 02:21:25PM -0700, Nick Desaulniers wrote:
> > > On Thu, Apr 7, 2022 at 8:22 AM Paul Heidekrüger
> > > <[email protected]> wrote:
> > > >
> > > > Hi all,
> > > >
> > > > work on my dependency checker tool is progressing nicely, and it is
> > > > flagging, what I believe is, a harmful addr to ctrl dependency
> > > > transformation. For context, see [1] and [2]. I'm using the Clang
> > > > compiler.
> > > > [1]: https://linuxplumbersconf.org/event/7/contributions/821/attachments/598/1075/LPC_2020_--_Dependency_ordering.pdf
> > > > [2]: https://lore.kernel.org/llvm/YXknxGFjvaB46d%2Fp@Pauls-MacBook-Pro/T/#u
> > >
> > > Hi Paul,
> > > Thanks for the report and your (and your team's) work on this tool.
> > > Orthogonal to your report, Jose (cc'ed) and I are currently in the
> > > planning process to put together a Kernel+Toolchain microconference
> > > track at Linux Plumbers Conference [0] this year (Sept 12-14) in
> > > Dublin, Ireland. Would you or someone from your group be able and
> > > interested in presenting more information about your work to an
> > > audience of kernel and toolchain developers at such an event?
> > >
> > > Would others be interested in such a topic? (What do they say in
> > > Starship Troopers...?...Would you like to know more?)
> > >
> > > [0] https://lpc.events/event/16/
> > > --
> > > Thanks,
> > > ~Nick Desaulniers
> >
> > Hi Nick and Jose,
> >
> > Many thanks for inviting us! I would love to do a talk at LPC! Hopefully
> > in person too.
> >
> > Given that there have been several talks around this topic at LPC
> > already, it seems very fitting, and we'll hopefully have more to share
> > by then. Actually we have more to share already :-)
> >
> > https://lore.kernel.org/all/YmKE%[email protected]/T/#u
> >
> > I assume we will have to submit an abstract soon?
>
> Yes, if you go to: https://lpc.events/event/16/abstracts/
>
> click "Submit new abstract" in the bottom right.
>
> Under the "Track" dropdown, please select "Toolchains Track."
Hi Paul, we'll need all proposals soon.
If you're still considering attending Linux Plumbers conf, please
submit a proposal:
https://lpc.events/event/16/abstracts/
Please make sure to select "Toolchains Track" as the "Track" after
clicking on "Submit new abstract."
> --
> Thanks,
> ~Nick Desaulniers
--
Thanks,
~Nick Desaulniers
> On 22. Jun 2022, at 00:05, Nick Desaulniers <[email protected]> wrote:
>
> On Tue, May 17, 2022 at 3:29 PM Nick Desaulniers
> <[email protected]> wrote:
>> On Fri, Apr 22, 2022 at 3:39 AM Paul Heidekrüger
>> <[email protected]> wrote:
>>> On Thu, Apr 14, 2022 at 02:21:25PM -0700, Nick Desaulniers wrote:
>>>> On Thu, Apr 7, 2022 at 8:22 AM Paul Heidekrüger
>>>> <[email protected]> wrote:
>>>>> Hi all,
>>>>>
>>>>> work on my dependency checker tool is progressing nicely, and it is
>>>>> flagging, what I believe is, a harmful addr to ctrl dependency
>>>>> transformation. For context, see [1] and [2]. I'm using the Clang
>>>>> compiler.
>>>>> [1]: https://linuxplumbersconf.org/event/7/contributions/821/attachments/598/1075/LPC_2020_--_Dependency_ordering.pdf
>>>>> [2]: https://lore.kernel.org/llvm/YXknxGFjvaB46d%2Fp@Pauls-MacBook-Pro/T/#u
>>>>
>>>> Hi Paul,
>>>> Thanks for the report and your (and your team's) work on this tool.
>>>> Orthogonal to your report, Jose (cc'ed) and I are currently in the
>>>> planning process to put together a Kernel+Toolchain microconference
>>>> track at Linux Plumbers Conference [0] this year (Sept 12-14) in
>>>> Dublin, Ireland. Would you or someone from your group be able and
>>>> interested in presenting more information about your work to an
>>>> audience of kernel and toolchain developers at such an event?
>>>>
>>>> Would others be interested in such a topic? (What do they say in
>>>> Starship Troopers...?...Would you like to know more?)
>>>>
>>>> [0] https://lpc.events/event/16/
>>>> --
>>>> Thanks,
>>>> ~Nick Desaulniers
>>>
>>> Hi Nick and Jose,
>>>
>>> Many thanks for inviting us! I would love to do a talk at LPC! Hopefully
>>> in person too.
>>>
>>> Given that there have been several talks around this topic at LPC
>>> already, it seems very fitting, and we'll hopefully have more to share
>>> by then. Actually we have more to share already :-)
>>>
>>> https://lore.kernel.org/all/YmKE%[email protected]/T/#u
>>>
>>> I assume we will have to submit an abstract soon?
>>
>> Yes, if you go to: https://lpc.events/event/16/abstracts/
>>
>> click "Submit new abstract" in the bottom right.
>>
>> Under the "Track" dropdown, please select "Toolchains Track."
>
> Hi Paul, we'll need all proposals soon.
> If you're still considering attending Linux Plumbers conf, please
> submit a proposal:
> https://lpc.events/event/16/abstracts/
> Please make sure to select "Toolchains Track" as the "Track" after
> clicking on "Submit new abstract."
Hi Nick,
Of course! :-)
Sorry for taking so long, and thanks for reminding! I was under the
impression the deadline was sometime mid-August and therefore hadn't
prioritised it high enough - sorry!
You'll have the proposal ASAP.
Many thanks,
Paul
>> --
>> Thanks,
>> ~Nick Desaulniers
>
>
>
> --
> Thanks,
> ~Nick Desaulniers
