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Subject: Re: [PATCH v2] tools/memory-model: Weaken ctrl dependency definition
 in explanation.txt
From:   =?utf-8?Q?Paul_Heidekr=C3=BCger?= <Paul.Heidekrueger@in.tum.de>
In-Reply-To: <20220912113838.GG246308@paulmck-ThinkPad-P17-Gen-1>
Date:   Mon, 12 Sep 2022 14:38:58 +0100
Cc:     Alan Stern <stern@rowland.harvard.edu>,
        Andrea Parri <parri.andrea@gmail.com>,
        Will Deacon <will@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Nicholas Piggin <npiggin@gmail.com>,
        David Howells <dhowells@redhat.com>,
        Jade Alglave <j.alglave@ucl.ac.uk>,
        Luc Maranget <luc.maranget@inria.fr>,
        Akira Yokosawa <akiyks@gmail.com>,
        Daniel Lustig <dlustig@nvidia.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Palmer Dabbelt <palmerdabbelt@google.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
        Marco Elver <elver@google.com>,
        Charalampos Mainas <charalampos.mainas@gmail.com>,
        Pramod Bhatotia <pramod.bhatotia@in.tum.de>,
        Soham Chakraborty <s.s.chakraborty@tudelft.nl>,
        Martin Fink <martin.fink@in.tum.de>
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Message-Id: <26A8E8E5-FA0B-4D5B-BDD1-3DA8E654965E@in.tum.de>
References: <20220830210821.3763660-1-paul.heidekrueger@in.tum.de>
 <20220912113838.GG246308@paulmck-ThinkPad-P17-Gen-1>
To:     "Paul E. McKenney" <paulmck@kernel.org>
Precedence: bulk

On 12. Sep 2022, at 12:38, Paul E. McKenney <paulmck@kernel.org> wrote:

> On Tue, Aug 30, 2022 at 09:08:20PM +0000, Paul Heidekr=C3=BCger wrote:
>> The current informal control dependency definition in explanation.txt =
is
>> too broad and, as discussed, needs to be updated.
>>=20
>> Consider the following example:
>>=20
>>> if(READ_ONCE(x))
>>>  return 42;
>>>=20
>>> WRITE_ONCE(y, 42);
>>>=20
>>> return 21;
>>=20
>> 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.
>>=20
>> 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.
>>=20
>> Link: =
https://lore.kernel.org/all/20220615114330.2573952-1-paul.heidekrueger@in.=
tum.de/
>> Cc: Marco Elver <elver@google.com>
>> Cc: Charalampos Mainas <charalampos.mainas@gmail.com>
>> Cc: Pramod Bhatotia <pramod.bhatotia@in.tum.de>
>> Cc: Soham Chakraborty <s.s.chakraborty@tudelft.nl>
>> Cc: Martin Fink <martin.fink@in.tum.de>
>> Signed-off-by: Paul Heidekr=C3=BCger <paul.heidekrueger@in.tum.de>
>> Co-developed-by: Alan Stern <stern@rowland.harvard.edu>
>=20
> Hearing no objections, I reverted the old version and replaced it
> with this version.  Thank you both!
>=20
> 							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!

Many thanks,
Paul

[1]: =
https://lore.kernel.org/all/663d568d-a343-d44b-d33d-29998bff8f70@joelferna=
ndes.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/


>> ---
>>=20
>> v2:
>> - Fix typos
>> - Fix indentation of code snippet
>>=20
>> 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.
>>=20
>> tools/memory-model/Documentation/explanation.txt | 7 ++++---
>> 1 file changed, 4 insertions(+), 3 deletions(-)
>>=20
>> 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.
>>=20
>> -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:
>>=20
>> 	int x, y;
>>=20
>> --
>> 2.35.1