linux-kernel - Re: [PATCHv4 17/17] writeback: lessen sync

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Date:	Mon, 31 May 2010 12:04:04 +0300
From:	Artem Bityutskiy <dedekind1@...il.com>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Al Viro <viro@...IV.linux.org.uk>,
	LKML <linux-kernel@...r.kernel.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCHv4 17/17] writeback: lessen sync_supers wakeup count

> On Mon, May 31, 2010 at 11:25:52AM +0300, Artem Bityutskiy wrote:
> > Hmm, but it looks like we cannot do that either. If we do
> > 
> > set_current_state(TASK_INTERRUPTIBLE);
> > if (supers_dirty)
> > 	bdi_arm_supers_timer();
> > schedule();
> > 
> > and the kernel is preemptive, is it possible that we get preempted
> > before we run 'bdi_arm_supers_timer()', but after we do
> > 'set_current_state(TASK_INTERRUPTIBLE)'. And we will never wake up if
> > the timer armed in mark_sb_dirty() went off.
> > 
> > So it looks like this is the way to go:
> > 
> > /*
> >  * Disable preemption for a while to make sure we are not
> >  * preempted before the timer is armed.
> >  */
> > preempt_disable();
> > set_current_state(TASK_INTERRUPTIBLE);
> > if (supers_dirty)
> > 	bdi_arm_supers_timer();
> > preempt_enable();
> > schedule();
> 
> This should not be required because preempt is transparent to these
> task sleep/schedule APIs.
> 
> The preempt event will not clear TASK_INTERRUPTIBLE, and so the timer
> wakeup will set it to TASK_RUNNING (whether or not it has called
> schedule() yet and whether or not it is currently preempted).

Nick, I'm sorry, but could you please elaborate:

set_current_state(TASK_INTERRUPTIBLE);
/* 
 * XXX: what if we are preempted here. No timer is armed. Our state is
 * TASK_INTERRUPTIBLE, supers_dirty is 1, so no one will ever wake us
 * up. Thus, we'll sleep forever.
 */
if (supers_dirty)
	bdi_arm_supers_timer();
schedule();

Not sure, but I did quick search and it looks like in preemptive kernel,
an interrupt may happen in the XXX place above, then it will call
'preempt_schedule_irq()', which sill call 'schedule()'.

-- 
Best Regards,
Artem Bityutskiy (Артём Битюцкий)

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