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Date:   Fri, 22 May 2020 17:17:05 +0200
From:   "Sebastian A. Siewior" <bigeasy@linutronix.de>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     "Ahmed S. Darwish" <a.darwish@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Konstantin Khlebnikov <khlebnikov@yandex-team.ru>,
        linux-mm@kvack.org
Subject: Re: [PATCH v1 02/25] mm/swap: Don't abuse the seqcount latching API
Message-ID: <20200522151705.vwfua5a4lhzcagea@linutronix.de>
References: <20200519214547.352050-1-a.darwish@linutronix.de>
 <20200519214547.352050-3-a.darwish@linutronix.de>
 <20200522145707.GO325280@hirez.programming.kicks-ass.net>
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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() ?

Sebastian