linux-kernel - Re: [Bug #14388] keyboard under X with 2.6.31

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Date:	Thu, 15 Oct 2009 21:00:24 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Paul Fulghum <paulkf@...rogate.com>,
	Boyan <btanastasov@...oo.co.uk>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Kernel Testers List <kernel-testers@...r.kernel.org>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Ed Tomlinson <edt@....ca>
Subject: Re: [Bug #14388] keyboard under X with 2.6.31

On 10/16, OGAWA Hirofumi wrote:
>
> > +void flush_delayed_work(struct delayed_work *dwork)
> > +{
> > +	if (del_timer(&dwork->timer)) {
> > +		struct cpu_workqueue_struct *cwq;
> > +		cwq = wq_per_cpu(keventd_wq, get_cpu());
> > +		__queue_work(cwq, &dwork->work);
> > +		put_cpu();
> > +	}
> > +	flush_work(&dwork->work);
> > +}
> > +EXPORT_SYMBOL(flush_delayed_work);
> > +
> > +/**
>
> Sorry if I'm missing the point. Doesn't this have (possible) race with
> schedule_delayed_work() (i.e. by tty writer)?
>
>              cpu0                                      cpu1
>
>     if (del_timer(&dwork->timer)) {

If dwork->timer is pending - _PENDING must be set.
If del_timer() succeeds, nobody else can clear this bit.

>                                             // cpu0 doesn't set _PENDING
>                                             schedule_delayed_work()

and in this case schedule_delayed_work()->queue_delayed_work_on()
can't succeed because it does test_and_set_bit(_PENDING).


But. Since this helper was merged, I think it should use del_timer_sync()
to be correct. Yes, it is slower, but otherwise flush is racy.

And I think it should return a bolean to match flush_work(). IOW,

	int flush_delayed_work(struct delayed_work *dwork)
	{
		int requeued = false;

		if (del_timer(&dwork->timer)) {
			struct cpu_workqueue_struct *cwq;
			cwq = wq_per_cpu(keventd_wq, get_cpu());
			__queue_work(cwq, &dwork->work);
			put_cpu();

			requeued = true;
		}

		return flush_work(&dwork->work) || requeued;
	}

Not that I think this is terribly important, but still.

I'll send the patch.

Oleg.

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