linux-kernel - Re: [PATCH] seqlock: Use WRITE

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Message-ID: <87v7vgzrxk.fsf@oldenburg.str.redhat.com>
Date: Wed, 18 Dec 2024 20:56:07 +0100
From: Florian Weimer <fweimer@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: "Paul E. McKenney" <paulmck@...nel.org>,  Daniel Xu <dxu@...uu.xyz>,
  mingo@...hat.com,  will@...nel.org,  longman@...hat.com,
  boqun.feng@...il.com,  linux-kernel@...r.kernel.org,
  linux-toolchains@...r.kernel.org
Subject: Re: [PATCH] seqlock: Use WRITE_ONCE() when updating sequence

* Peter Zijlstra:

> +linux-toolchains
>
> On Wed, Dec 18, 2024 at 08:59:47AM -0800, Paul E. McKenney wrote:
>
>> > Perhaps something like: (*(volatile unsigned int *)&s->sequence)++; ?
>> > I'd have to check what the compiler makes of that.
>> > 
>> > /me mucks about with godbolt for a bit...
>> > 
>> > GCC doesn't optimize that, but Clang does.
>> > 
>> > I would still very much refrain from making this change until both
>> > compilers can generate sane code for it.
>> 
>> Is GCC on track to do this, or do we need to encourage them?
>
> I have no clue; probably wise to offer encouragement.

What do you consider sane code?

Clang's choice to generate an incl instruction (on x86-64 at least) is a
bit surprising.  Curiously, the C11 abstract machine has a value-less
increment-in-place operation, so it's probably not in violation of the
volatile rules.  (C doesn't specify x++ in terms of ++x and x += 1.)

Thanks,
Florian