linux-kernel - Re: [PATCH v3] tools/memory-model: Weaken ctrl dependency definition in explanation.txt

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Date:   Sat, 3 Sep 2022 13:41:34 +0200
From:   Paul Heidekrüger <Paul.Heidekrueger@...tum.de>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     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>,
        Joel Fernandes <joel@...lfernandes.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-arch <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 v3] tools/memory-model: Weaken ctrl dependency definition
 in explanation.txt

On 3. Sep 2022, at 03:27, Alan Stern <stern@...land.harvard.edu> wrote:

> On Fri, Sep 02, 2022 at 09:13:40PM +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>
> 
> Signed-off-by: Alan Stern <stern@...land.harvard.edu>
> 
>> ---
>> 
>> 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 | 8 +++++---
>> 1 file changed, 5 insertions(+), 3 deletions(-)
>> 
>> diff --git a/tools/memory-model/Documentation/explanation.txt b/tools/memory-model/Documentation/explanation.txt
>> index ee819a402b69..0b7e1925a673 100644
>> --- a/tools/memory-model/Documentation/explanation.txt
>> +++ b/tools/memory-model/Documentation/explanation.txt
>> @@ -464,9 +464,11 @@ 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
>> +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;

Hang on, shouldn't this read "a write event" instead of "another memory
access event"? Control dependencies only provide ordering from READ_ONCE to
WRITE_ONCE, not from READ_ONCE to (READ | WRITE)_ONCE?

Or am I missing something?

Many thanks,
Paul