On Sat, Sep 03, 2022 at 04:57:17PM +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> > Co-developed-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> > Signed-off-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> > Signed-off-by: Paul Heidekrüger <paul.heidekrueger@xxxxxxxxx> I have pulled this one in, thank you both! It can still be updated if need be. ;-) Thanx, Paul > --- > > 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 >
