On 3. Sep 2022, at 03:27, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote: > On Fri, Sep 02, 2022 at 09:13:40PM +0000, Paul Heidekrüger wrote: >> 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> >> Signed-off-by: Paul Heidekrüger <paul.heidekrueger@xxxxxxxxx> >> Co-developed-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> > > Signed-off-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> > >> --- >> >> 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 | 8 +++++--- >> 1 file changed, 5 insertions(+), 3 deletions(-) >> >> diff --git a/tools/memory-model/Documentation/explanation.txt b/tools/memory-model/Documentation/explanation.txt >> index ee819a402b69..0b7e1925a673 100644 >> --- a/tools/memory-model/Documentation/explanation.txt >> +++ b/tools/memory-model/Documentation/explanation.txt >> @@ -464,9 +464,11 @@ 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 another memory access 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; Hang on, shouldn't this read "a write event" instead of "another memory access event"? Control dependencies only provide ordering from READ_ONCE to WRITE_ONCE, not from READ_ONCE to (READ | WRITE)_ONCE? Or am I missing something? Many thanks, Paul
