linux-kernel - Re: Block: Prevent busy looping

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Message-ID: <20080416164219.GL12774@kernel.dk>
Date:	Wed, 16 Apr 2008 18:42:19 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Elias Oltmanns <eo@...ensachen.de>
Cc:	linux-kernel@...r.kernel.org, stable@...nel.org
Subject: Re: Block: Prevent busy looping

On Wed, Apr 16 2008, Jens Axboe wrote:
> On Wed, Apr 16 2008, Elias Oltmanns wrote:
> > blk_run_queue() as well as blk_start_queue() plug the device on reentry
> > and schedule blk_unplug_work() right afterwards. However,
> > blk_plug_device() takes care of that already and makes sure that there is
> > a short delay before blk_unplug_work() is scheduled. This is important
> > to prevent busy looping and possibly system lockups as observed here:
> > <http://permalink.gmane.org/gmane.linux.ide/28351>.
> 
> If you call blk_start_queue() and blk_run_queue(), you better mean it.
> There should be no delay. The only reason it does blk_plug_device() is
> so that the work queue function will actually do some work. In the newer
> kernels we just do:
> 
>         set_bit(QUEUE_FLAG_PLUGGED, &q->queue_flags);
>         kblockd_schedule_work(q, &q->unplug_work);
> 
> instead, which is much better.

actually that's only in my devel tree, not in mainline yet (which still
does blk_plug_device() instead of just setting the plugged bit).

-- 
Jens Axboe

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