On 12. Sep 2022, at 12:38, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote: > On Tue, Aug 30, 2022 at 09:08:20PM +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> > > Hearing no objections, I reverted the old version and replaced it > with this version. Thank you both! > > Thanx, Paul Oh, wait, there was further discussion [1, 2], and we finally agreed on [3]. So [3] is the final version. I think me sending a v2 immediately after the v1 led to this out-of-order discussion - sorry! Many thanks, Paul [1]: https://lore.kernel.org/all/663d568d-a343-d44b-d33d-29998bff8f70@xxxxxxxxxxxxxxxxx/ [2]: https://lore.kernel.org/all/D7E3D42D-2ABE-4D16-9DCA-0605F0C84F7D@xxxxxxxxx/ [3]: https://lore.kernel.org/all/20220903165718.4186763-1-paul.heidekrueger@xxxxxxxxx/ >> --- >> >> 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..0bca50cac5f4 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 another memory access event Y are linked by >> +a control dependency if Y syntactically lies within an arm of an if, >> +else or switch statement and the condition guarding Y is either data or >> +address-dependent on X. Simple example: >> >> int x, y; >> >> -- >> 2.35.1
