linux-kernel - Re: Block: Prevent busy looping

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Message-ID: <20080416163152.GK12774@kernel.dk>
Date:	Wed, 16 Apr 2008 18:31:54 +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, 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.

-- 
Jens Axboe

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