linux-kernel - Re: [PATCH v2] tools/memory-model: Weaken ctrl dependency definition in explanation.txt

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Date:   Mon, 12 Sep 2022 07:11:16 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Paul Heidekrüger <Paul.Heidekrueger@...tum.de>
Cc:     Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <parri.andrea@...il.com>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Akira Yokosawa <akiyks@...il.com>,
        Daniel Lustig <dlustig@...dia.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Palmer Dabbelt <palmerdabbelt@...gle.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        Marco Elver <elver@...gle.com>,
        Charalampos Mainas <charalampos.mainas@...il.com>,
        Pramod Bhatotia <pramod.bhatotia@...tum.de>,
        Soham Chakraborty <s.s.chakraborty@...elft.nl>,
        Martin Fink <martin.fink@...tum.de>
Subject: Re: [PATCH v2] tools/memory-model: Weaken ctrl dependency definition
 in explanation.txt

On Mon, Sep 12, 2022 at 02:38:58PM +0100, Paul Heidekrüger wrote:
> On 12. Sep 2022, at 12:38, Paul E. McKenney <paulmck@...nel.org> 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@in.tum.de/
> >> Cc: Marco Elver <elver@...gle.com>
> >> Cc: Charalampos Mainas <charalampos.mainas@...il.com>
> >> Cc: Pramod Bhatotia <pramod.bhatotia@...tum.de>
> >> Cc: Soham Chakraborty <s.s.chakraborty@...elft.nl>
> >> Cc: Martin Fink <martin.fink@...tum.de>
> >> Signed-off-by: Paul Heidekrüger <paul.heidekrueger@...tum.de>
> >> Co-developed-by: Alan Stern <stern@...land.harvard.edu>
> > 
> > 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@joelfernandes.org/
> [2]: https://lore.kernel.org/all/D7E3D42D-2ABE-4D16-9DCA-0605F0C84F7D@in.tum.de/
> [3]: https://lore.kernel.org/all/20220903165718.4186763-1-paul.heidekrueger@in.tum.de/
> 
> 
> >> ---
> >> 
> >> 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
> 
>