linux-kernel - Re: [PATCH v4] tools/memory-model: Weaken ctrl dependency definition in explanation.txt

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Date:   Sat, 3 Sep 2022 13:19:07 -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>,
        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 v4] tools/memory-model: Weaken ctrl dependency definition
 in explanation.txt

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@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>
> Co-developed-by: Alan Stern <stern@...land.harvard.edu>
> Signed-off-by: Alan Stern <stern@...land.harvard.edu>
> Signed-off-by: Paul Heidekrüger <paul.heidekrueger@...tum.de>

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
>