linux-kernel - Re: [PATCH v2 2/3] locking/percpu-rwsem: Rework writer block/wake to not use wait-queues

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Date:   Mon, 5 Dec 2016 18:19:17 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Davidlohr Bueso <dave@...olabs.net>, mingo@...nel.org,
        john.stultz@...aro.org, dimitrysh@...gle.com,
        linux-kernel@...r.kernel.org, Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH v2 2/3] locking/percpu-rwsem: Rework writer block/wake
        to not use wait-queues

On 12/05, Peter Zijlstra wrote:
>
> > +	for (;;) {
> > +		set_current_state(TASK_UNINTERRUPTIBLE);
> > +
> > +		if (readers_active_check(sem))
> > +			break;
> > +
> > +		schedule();
> > +	}
> > +
> > +	rcu_assign_pointer(sem->writer, NULL);
>
> And this one does not, and the value being NULL this actually reverts to
> WRITE_ONCE().

Do we really care? We do not even need WRITE_ONCE() afaics, this is like
__set_current_state(TASK_RUNNING) after the main loop. We can't avoid the
spurious wakeups anyway after return from percpu_down_write().

Oleg.