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Date:   Wed, 28 Feb 2018 16:30:33 -0800
From:   "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To:     Alan Stern <stern@rowland.harvard.edu>
Cc:     LKMM Maintainers -- Akira Yokosawa <akiyks@gmail.com>,
        Andrea Parri <parri.andrea@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        David Howells <dhowells@redhat.com>,
        Jade Alglave <j.alglave@ucl.ac.uk>,
        Luc Maranget <luc.maranget@inria.fr>,
        Nicholas Piggin <npiggin@gmail.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Will Deacon <will.deacon@arm.com>,
        Kernel development list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/2 RFC] tools/memory-model: rename link and rcu-path to
 rcu-link and rb
Reply-To: paulmck@linux.vnet.ibm.com
References: <Pine.LNX.4.44L0.1802281508550.1421-100000@iolanthe.rowland.org>
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On Wed, Feb 28, 2018 at 03:13:45PM -0500, Alan Stern wrote:
> This patch makes a simple non-functional change to the RCU portion of
> the Linux Kernel Memory Consistency Model by renaming the "link" and
> "rcu-path" relations to "rcu-link" and "rb".
> 
> The name "link" was an unfortunate choice, because it was too generic
> and subject to confusion with other meanings of the same word, which
> occur quite often in LKMM documentation.  The name "rcu-path" is not
> very appropriate, because the relation is analogous to the
> happens-before (hb) and propagates-before (pb) relations -- although
> that fact won't become apparent until the second patch in this series.
> 
> Signed-off-by: Alan Stern <stern@rowland.harvard.edu>

Tested-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

> ---
> 
> Index: usb-4.x/tools/memory-model/linux-kernel.cat
> ===================================================================
> --- usb-4.x.orig/tools/memory-model/linux-kernel.cat
> +++ usb-4.x/tools/memory-model/linux-kernel.cat
> @@ -100,22 +100,22 @@ let rscs = po ; crit^-1 ; po?
>   * one but two non-rf relations, but only in conjunction with an RCU
>   * read-side critical section.
>   *)
> -let link = hb* ; pb* ; prop
> +let rcu-link = hb* ; pb* ; prop
> 
>  (* Chains that affect the RCU grace-period guarantee *)
> -let gp-link = gp ; link
> -let rscs-link = rscs ; link
> +let gp-link = gp ; rcu-link
> +let rscs-link = rscs ; rcu-link
> 
>  (*
>   * A cycle containing at least as many grace periods as RCU read-side
>   * critical sections is forbidden.
>   *)
> -let rec rcu-path =
> +let rec rb =
>  	gp-link |
>  	(gp-link ; rscs-link) |
>  	(rscs-link ; gp-link) |
> -	(rcu-path ; rcu-path) |
> -	(gp-link ; rcu-path ; rscs-link) |
> -	(rscs-link ; rcu-path ; gp-link)
> +	(rb ; rb) |
> +	(gp-link ; rb ; rscs-link) |
> +	(rscs-link ; rb ; gp-link)
> 
> -irreflexive rcu-path as rcu
> +irreflexive rb as rcu
> Index: usb-4.x/tools/memory-model/Documentation/explanation.txt
> ===================================================================
> --- usb-4.x.orig/tools/memory-model/Documentation/explanation.txt
> +++ usb-4.x/tools/memory-model/Documentation/explanation.txt
> @@ -27,7 +27,7 @@ Explanation of the Linux-Kernel Memory C
>    19. AND THEN THERE WAS ALPHA
>    20. THE HAPPENS-BEFORE RELATION: hb
>    21. THE PROPAGATES-BEFORE RELATION: pb
> -  22. RCU RELATIONS: link, gp-link, rscs-link, and rcu-path
> +  22. RCU RELATIONS: rcu-link, gp-link, rscs-link, and rb
>    23. ODDS AND ENDS
> 
> 
> @@ -1451,8 +1451,8 @@ they execute means that it cannot have c
>  the content of the LKMM's "propagation" axiom.
> 
> 
> -RCU RELATIONS: link, gp-link, rscs-link, and rcu-path
> ------------------------------------------------------
> +RCU RELATIONS: rcu-link, gp-link, rscs-link, and rb
> +---------------------------------------------------
> 
>  RCU (Read-Copy-Update) is a powerful synchronization mechanism.  It
>  rests on two concepts: grace periods and read-side critical sections.
> @@ -1509,8 +1509,8 @@ y, which occurs before the end of the cr
>  propagate to P1 before the end of the grace period, violating the
>  Guarantee.
> 
> -In the kernel's implementations of RCU, the business about stores
> -propagating to every CPU is realized by placing strong fences at
> +In the kernel's implementations of RCU, the requirements for stores
> +to propagate to every CPU are fulfilled by placing strong fences at
>  suitable places in the RCU-related code.  Thus, if a critical section
>  starts before a grace period does then the critical section's CPU will
>  execute an smp_mb() fence after the end of the critical section and
> @@ -1523,19 +1523,19 @@ executes.
>  What exactly do we mean by saying that a critical section "starts
>  before" or "ends after" a grace period?  Some aspects of the meaning
>  are pretty obvious, as in the example above, but the details aren't
> -entirely clear.  The LKMM formalizes this notion by means of a
> -relation with the unfortunately generic name "link".  It is a very
> -general relation; among other things, X ->link Z includes cases where
> -X happens-before or is equal to some event Y which is equal to or
> -comes before Z in the coherence order.  Taking Y = Z, this says that
> -X ->rfe Z implies X ->link Z, and taking Y = X, it says that X ->fr Z
> -and X ->co Z each imply X ->link Z.
> +entirely clear.  The LKMM formalizes this notion by means of the
> +rcu-link relation.  rcu-link encompasses a very general notion of
> +"before": Among other things, X ->rcu-link Z includes cases where X
> +happens-before or is equal to some event Y which is equal to or comes
> +before Z in the coherence order.  When Y = Z this says that X ->rfe Z
> +implies X ->rcu-link Z.  In addition, when Y = X it says that X ->fr Z
> +and X ->co Z each imply X ->rcu-link Z.
> 
> -The formal definition of the link relation is more than a little
> +The formal definition of the rcu-link relation is more than a little
>  obscure, and we won't give it here.  It is closely related to the pb
>  relation, and the details don't matter unless you want to comb through
>  a somewhat lengthy formal proof.  Pretty much all you need to know
> -about link is the information in the preceding paragraph.
> +about rcu-link is the information in the preceding paragraph.
> 
>  The LKMM goes on to define the gp-link and rscs-link relations.  They
>  bring grace periods and read-side critical sections into the picture,
> @@ -1543,32 +1543,33 @@ in the following way:
> 
>  	E ->gp-link F means there is a synchronize_rcu() fence event S
>  	and an event X such that E ->po S, either S ->po X or S = X,
> -	and X ->link F.  In other words, E and F are connected by a
> -	grace period followed by an instance of link.
> +	and X ->rcu-link F.  In other words, E and F are linked by a
> +	grace period followed by an instance of rcu-link.
> 
>  	E ->rscs-link F means there is a critical section delimited by
>  	an rcu_read_lock() fence L and an rcu_read_unlock() fence U,
>  	and an event X such that E ->po U, either L ->po X or L = X,
> -	and X ->link F.  Roughly speaking, this says that some event
> -	in the same critical section as E is connected by link to F.
> -
> -If we think of the link relation as standing for an extended "before",
> -then E ->gp-link F says that E executes before a grace period which
> -ends before F executes.  (In fact it says more than this, because it
> -includes cases where E executes before a grace period and some store
> -propagates to F's CPU before F executes and doesn't propagate to some
> -other CPU until after the grace period ends.)  Similarly,
> -E ->rscs-link F says that E is part of (or before the start of) a
> -critical section which starts before F executes.
> +	and X ->rcu-link F.  Roughly speaking, this says that some
> +	event in the same critical section as E is linked by rcu-link
> +	to F.
> +
> +If we think of the rcu-link relation as standing for an extended
> +"before", then E ->gp-link F says that E executes before a grace
> +period which ends before F executes.  (In fact it covers more than
> +this, because it also includes cases where E executes before a grace
> +period and some store propagates to F's CPU before F executes and
> +doesn't propagate to some other CPU until after the grace period
> +ends.)  Similarly, E ->rscs-link F says that E is part of (or before
> +the start of) a critical section which starts before F executes.
> 
>  Putting this all together, the LKMM expresses the Grace Period
>  Guarantee by requiring that there are no cycles consisting of gp-link
> -and rscs-link connections in which the number of gp-link instances is
> ->= the number of rscs-link instances.  It does this by defining the
> -rcu-path relation to link events E and F whenever it is possible to
> -pass from E to F by a sequence of gp-link and rscs-link connections
> -with at least as many of the former as the latter.  The LKMM's "rcu"
> -axiom then says that there are no events E such that E ->rcu-path E.
> +and rscs-link links in which the number of gp-link instances is >= the
> +number of rscs-link instances.  It does this by defining the rb
> +relation to link events E and F whenever it is possible to pass from E
> +to F by a sequence of gp-link and rscs-link links with at least as
> +many of the former as the latter.  The LKMM's "rcu" axiom then says
> +that there are no events E with E ->rb E.
> 
>  Justifying this axiom takes some intellectual effort, but it is in
>  fact a valid formalization of the Grace Period Guarantee.  We won't
> @@ -1585,10 +1586,10 @@ rcu_read_unlock() fence events delimitin
>  question, and let S be the synchronize_rcu() fence event for the grace
>  period.  Saying that the critical section starts before S means there
>  are events E and F where E is po-after L (which marks the start of the
> -critical section), E is "before" F in the sense of the link relation,
> -and F is po-before the grace period S:
> +critical section), E is "before" F in the sense of the rcu-link
> +relation, and F is po-before the grace period S:
> 
> -	L ->po E ->link F ->po S.
> +	L ->po E ->rcu-link F ->po S.
> 
>  Let W be the store mentioned above, let Z come before the end of the
>  critical section and witness that W propagates to the critical
> @@ -1600,12 +1601,12 @@ some event X which is po-after S.  Symbo
> 
>  The fr link from Y to W indicates that W has not propagated to Y's CPU
>  at the time that Y executes.  From this, it can be shown (see the
> -discussion of the link relation earlier) that X and Z are connected by
> -link, yielding:
> +discussion of the rcu-link relation earlier) that X and Z are related
> +by rcu-link, yielding:
> 
> -	S ->po X ->link Z ->po U.
> +	S ->po X ->rcu-link Z ->po U.
> 
> -These formulas say that S is po-between F and X, hence F ->gp-link Z
> +The formulas say that S is po-between F and X, hence F ->gp-link Z
>  via X.  They also say that Z comes before the end of the critical
>  section and E comes after its start, hence Z ->rscs-link F via E.  But
>  now we have a forbidden cycle: F ->gp-link Z ->rscs-link F.  Thus the
> @@ -1635,13 +1636,13 @@ time with statement labels added to the
>  	}
> 
> 
> -If r2 = 0 at the end then P0's store at X overwrites the value
> -that P1's load at Z reads from, so we have Z ->fre X and thus
> -Z ->link X.  In addition, there is a synchronize_rcu() between Y and
> -Z, so therefore we have Y ->gp-link X.
> +If r2 = 0 at the end then P0's store at X overwrites the value that
> +P1's load at Z reads from, so we have Z ->fre X and thus Z ->rcu-link X.
> +In addition, there is a synchronize_rcu() between Y and Z, so therefore
> +we have Y ->gp-link X.
> 
>  If r1 = 1 at the end then P1's load at Y reads from P0's store at W,
> -so we have W ->link Y.  In addition, W and X are in the same critical
> +so we have W ->rcu-link Y.  In addition, W and X are in the same critical
>  section, so therefore we have X ->rscs-link Y.
> 
>  This gives us a cycle, Y ->gp-link X ->rscs-link Y, with one gp-link
> 
>