linux-kernel - [PATCH memory-model 1/5] tools/memory-model: Weaken ctrl dependency definition in explanation.txt

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Date:   Wed, 19 Oct 2022 16:06:47 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        kernel-team@...com, mingo@...nel.org
Cc:     stern@...land.harvard.edu, parri.andrea@...il.com, will@...nel.org,
        peterz@...radead.org, boqun.feng@...il.com, npiggin@...il.com,
        dhowells@...hat.com, j.alglave@....ac.uk, luc.maranget@...ia.fr,
        akiyks@...il.com,
        Paul Heidekrüger <paul.heidekrueger@...tum.de>,
        Marco Elver <elver@...gle.com>,
        Charalampos Mainas <charalampos.mainas@...il.com>,
        Pramod Bhatotia <pramod.bhatotia@...tum.de>,
        Soham Chakraborty <s.s.chakraborty@...elft.nl>,
        Martin Fink <martin.fink@...tum.de>,
        "Paul E . McKenney" <paulmck@...nel.org>
Subject: [PATCH memory-model 1/5] tools/memory-model: Weaken ctrl dependency definition in explanation.txt

From: Paul Heidekrüger <paul.heidekrueger@...tum.de>

The current informal control dependency definition in explanation.txt is
too broad and, as discussed, needs to be updated.

Consider the following example:

> if(READ_ONCE(x))
>   return 42;
>
> WRITE_ONCE(y, 42);
>
> return 21;

The read event determines whether the write event will be executed "at all"
- as per the current definition - but the formal LKMM does not recognize
this as a control dependency.

Introduce a new definition which includes the requirement for the second
memory access event to syntactically lie within the arm of a non-loop
conditional.

Link: https://lore.kernel.org/all/20220615114330.2573952-1-paul.heidekrueger@in.tum.de/
Cc: Marco Elver <elver@...gle.com>
Cc: Charalampos Mainas <charalampos.mainas@...il.com>
Cc: Pramod Bhatotia <pramod.bhatotia@...tum.de>
Cc: Soham Chakraborty <s.s.chakraborty@...elft.nl>
Cc: Martin Fink <martin.fink@...tum.de>
Co-developed-by: Alan Stern <stern@...land.harvard.edu>
Signed-off-by: Alan Stern <stern@...land.harvard.edu>
Signed-off-by: Paul Heidekrüger <paul.heidekrueger@...tum.de>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 tools/memory-model/Documentation/explanation.txt | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/tools/memory-model/Documentation/explanation.txt b/tools/memory-model/Documentation/explanation.txt
index ee819a402b698..11a1d2d4f681c 100644
--- a/tools/memory-model/Documentation/explanation.txt
+++ b/tools/memory-model/Documentation/explanation.txt
@@ -464,9 +464,10 @@ to address dependencies, since the address of a location accessed
 through a pointer will depend on the value read earlier from that
 pointer.
 
-Finally, a read event and another memory access event are linked by a
-control dependency if the value obtained by the read affects whether
-the second event is executed at all.  Simple example:
+Finally, a read event X and a write event Y are linked by a control
+dependency if Y syntactically lies within an arm of an if statement and
+X affects the evaluation of the if condition via a data or address
+dependency (or similarly for a switch statement).  Simple example:
 
 	int x, y;
 
-- 
2.31.1.189.g2e36527f23