On Mon, Sep 12, 2022 at 02:38:58PM +0100, Paul Heidekrüger wrote: > 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! My bad, and thank you for checking and letting me know! I have reverted to the proper state. Thanx, Paul > 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 > >
