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    SubjectRe: 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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