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

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Date:   Fri, 22 May 2020 18:23:18 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Sebastian A. Siewior" <bigeasy@...utronix.de>
Cc:     "Ahmed S. Darwish" <a.darwish@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        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 Fri, May 22, 2020 at 05:17:05PM +0200, Sebastian A. Siewior wrote:
> On 2020-05-22 16:57:07 [+0200], Peter Zijlstra wrote:
> > > @@ -725,21 +735,48 @@ void lru_add_drain_all(void)
> > >  	if (WARN_ON(!mm_percpu_wq))
> > >  		return;
> > >  
> > 
> > > +	this_gen = READ_ONCE(lru_drain_gen);
> > > +	smp_rmb();
> > 
> > 	this_gen = smp_load_acquire(&lru_drain_gen);
> > >  
> > >  	mutex_lock(&lock);
> > >  
> > >  	/*
> > > +	 * (C) Exit the draining operation if a newer generation, from another
> > > +	 * lru_add_drain_all(), was already scheduled for draining. Check (A).
> > >  	 */
> > > +	if (unlikely(this_gen != lru_drain_gen))
> > >  		goto done;
> > >  
> > 
> > > +	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().
> 
> This is to avoid smp_store_release() ?

store_release would have the barrier on the other end. If you read the
comments (I so helpfully cut out) you'll see it wants to order against
later stores, not ealier.