Messages in this thread Patch in this message | | | From | "Paul E. McKenney" <> | Subject | [PATCH memory-model 1/5] tools/memory-model: Weaken ctrl dependency definition in explanation.txt | Date | Wed, 19 Oct 2022 16:06:47 -0700 |
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From: Paul Heidekrüger <paul.heidekrueger@in.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@google.com> Cc: Charalampos Mainas <charalampos.mainas@gmail.com> Cc: Pramod Bhatotia <pramod.bhatotia@in.tum.de> Cc: Soham Chakraborty <s.s.chakraborty@tudelft.nl> Cc: Martin Fink <martin.fink@in.tum.de> Co-developed-by: Alan Stern <stern@rowland.harvard.edu> Signed-off-by: Alan Stern <stern@rowland.harvard.edu> Signed-off-by: Paul Heidekrüger <paul.heidekrueger@in.tum.de> Signed-off-by: Paul E. McKenney <paulmck@kernel.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
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