From mboxrd@z Thu Jan  1 00:00:00 1970
From: Artem Bityutskiy <dedekind1@gmail.com>
Subject: Re: [PATCHv4 17/17] writeback: lessen sync_supers wakeup count
Date: Mon, 31 May 2010 11:25:52 +0300
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Cc: Al Viro <viro@ZenIV.linux.org.uk>,
	LKML <linux-kernel@vger.kernel.org>,
	Jens Axboe <jens.axboe@oracle.com>,
	linux-fsdevel@vger.kernel.org
To: Nick Piggin <npiggin@suse.de>
Return-path: <linux-kernel-owner@vger.kernel.org>
In-Reply-To: <20100527154435.GS22536@laptop>
Sender: linux-kernel-owner@vger.kernel.org
List-Id: linux-fsdevel.vger.kernel.org

On Fri, 2010-05-28 at 01:44 +1000, Nick Piggin wrote:
> > if (supers_dirty)
> > 	bdi_arm_supers_timer();
> > set_current_state(TASK_INTERRUPTIBLE);
> > schedule();

> But we cannot do the above, because again the timer might go off
> before we set current state. We'd lose the wakeup and never wake
> up again.
>=20
> Putting it inside set_current_state() should be OK. I suppose.

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();

--=20
Best Regards,
Artem Bityutskiy (=D0=90=D1=80=D1=82=D1=91=D0=BC =D0=91=D0=B8=D1=82=D1=8E=
=D1=86=D0=BA=D0=B8=D0=B9)