linux-kernel - Re: [PATCH v1 02/25] mm/swap: Don't abuse the seqcount latching API

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Message-ID: <20200525154505.GZ325280@hirez.programming.kicks-ass.net>
Date:   Mon, 25 May 2020 17:45:05 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Ahmed S. Darwish" <a.darwish@...utronix.de>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        "Sebastian A. Siewior" <bigeasy@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        linux-mm@...ck.org
Subject: Re: [PATCH v1 02/25] mm/swap: Don't abuse the seqcount latching API

On Mon, May 25, 2020 at 05:24:01PM +0200, Ahmed S. Darwish wrote:
> Peter Zijlstra <peterz@...radead.org> wrote:
> > On Tue, May 19, 2020 at 11:45:24PM +0200, Ahmed S. Darwish wrote:

> > > +	WRITE_ONCE(lru_drain_gen, lru_drain_gen + 1);
> > > +	smp_wmb();
> >
> > You can leave this smp_wmb() out and rely on the smp_mb() implied by
> > queue_work_on()'s test_and_set_bit().
> >
> 
> Won't this be too implicit?
> 
> Isn't it possible that, over the years, queue_work_on() impementation
> changes and the test_and_set_bit()/smp_mb() gets removed?
> 
> If that happens, this commit will get *silently* broken and the local
> CPU pages won't be drained.

Add a comment to queue_work_on() that points here? That way people are
aware.