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@xxxxxxxxx/ Cc: Marco Elver <elver@xxxxxxxxxx> Cc: Charalampos Mainas <charalampos.mainas@xxxxxxxxx> Cc: Pramod Bhatotia <pramod.bhatotia@xxxxxxxxx> Cc: Soham Chakraborty <s.s.chakraborty@xxxxxxxxxx> Cc: Martin Fink <martin.fink@xxxxxxxxx> Co-developed-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> Signed-off-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> Signed-off-by: Paul Heidekrüger <paul.heidekrueger@xxxxxxxxx> --- v4: - Replace "a memory access event" with "a write event" v3: - Address Alan and Joel's feedback re: the wording around switch statements and the use of "guarding" v2: - Fix typos - Fix indentation of code snippet v1: @Alan, since I got it wrong the last time, I'm adding you as a co-developer after my SOB. I'm sorry if this creates extra work on your side due to you having to resubmit the patch now with your SOB if I understand correctly, but since it's based on your wording from the other thread, I definitely wanted to give you credit. 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 ee819a402b69..11a1d2d4f681 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; -- 2.35.1
