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

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Message-ID: <bfe737e8-5e5b-4fef-9346-06bbac57b2b3@paulmck-laptop>
Date: Thu, 19 Dec 2024 09:48:03 -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 Thu, Dec 19, 2024 at 05:45:15PM +0100, Florian Weimer wrote:
> * Paul E. McKenney:
> 
> > 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.
> 
> I think the reference was originally to machine code.

Very well, then compiling this to a to-memory increment instruction
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?
> 
> I don't know.  It seems that Clang/LLVM is cheating.  It's doing this
> optimization even for
> 
>   i = i + 1;
> 
> with a volatile i.  That doesn't look like “strictly according to the
> abstract machine” anymore.

How so?  It is in fact adding one to that volatile variable, which is
in accord with the abstract machine.

>                             A proper implementation would need explicit
> representation of volatile increment/decrement in the IR.  Given that
> volatile increment/decrement is deprecated, that seems quite a bit of
> effort.

If I remember correctly, there was a discussion in SG21 about
de-deprecating volatile increment/decrement.

							Thanx, Paul