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

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Date:   Tue, 30 Aug 2022 17:08:15 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Paul Heidekrüger <paul.heidekrueger@...tum.de>,
        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>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Akira Yokosawa <akiyks@...il.com>,
        Daniel Lustig <dlustig@...dia.com>,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Cc:     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] tools/memory-model: Weaken ctrl dependency definition in
 explanation.txt



On 8/30/2022 4:44 PM, Paul Heidekrüger wrote:
> The current informal control dependency definition in explanation.txt is
> too broad and, as dicsussed, 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 defintion 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>
> ---
> 
> @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:

'conditioning guarding Y' sounds confusing to me as it implies to me that the
condition's evaluation depends on Y. I much prefer Alan's wording from the
linked post saying something like 'the branch condition is data or address
dependent on X, and Y lies in one of the arms'.

I have to ask though, why doesn't this imply that the second instruction never
executes at all? I believe that would break the MP-pattern if it were not true.

cheers,

 - Joel