linux-kernel - Re: [RFC][PATCH v3]: documentation,atomic: Add new documents

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Message-ID: <20170731090535.rjgnoewqg7mhzr55@hirez.programming.kicks-ass.net>
Date:   Mon, 31 Jul 2017 11:05:35 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     Will Deacon <will.deacon@....com>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [RFC][PATCH v3]: documentation,atomic: Add new documents

On Wed, Jul 26, 2017 at 08:47:50PM +0800, Boqun Feng wrote:

> > +
> > +Further, while something like:
> > +
> > +  smp_mb__before_atomic();
> > +  atomic_dec(&X);
> > +
> > +is a 'typical' RELEASE pattern, the barrier is strictly stronger than
> > +a RELEASE. Similarly for something like:
> > +
> 
> .. at here. Maybe you planned to put stronger ACQUIRE pattern?

Yes, although I struggled to find a sensible one. The problem is that
ACQUIRE is on loads and value returning atomics have an ACQUIRE variant,
so why would you ever want to use smp_mb__after_atomic() for this.


That is, the best I could come up with is something like:

	val = atomic_fetch_or_relaxed(1, &var);
	smp_mb__after_atomic();

But in that case we should've just written:

	val = atomic_fetch_or_acquire(1, &var);


Suggestions?