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

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Message-ID: <f91651fd-dda3-4fcd-9c12-dd12d46d39c9@paulmck-laptop>
Date: Thu, 19 Dec 2024 08:10:55 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Florian Weimer <fweimer@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.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

On Wed, Dec 18, 2024 at 08:56:07PM +0100, Florian Weimer wrote:
> * 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?

Peter's "(*(volatile unsigned int *)&s->sequence)++;" qualifies as sane.

> 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.)

Very good!  Should I do something like file a bug somewhere to help
this along?

							Thanx, Paul