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    Subject[PATCH] block: always requeue !fs requests at the front
    SCSI marks internal commands with REQ_PREEMPT and push it at the front
    of the request queue using blk_execute_rq(). When entering suspended
    or frozen state, SCSI devices are quiesced using
    scsi_device_quiesce(). In quiesced state, only REQ_PREEMPT requests
    are processed. This is how SCSI blocks other requests out while
    suspending and resuming. As all internal commands are pushed at the
    front of the queue, this usually works.

    Unfortunately, this interacts badly with ordered requeueing. To
    preserve request order on requeueing (due to busy device, active EH or
    other failures), requests are sorted according to ordered sequence on
    requeue if IO barrier is in progress.

    The following sequence deadlocks.

    1. IO barrier sequence issues.

    2. Suspend requested. Queue is quiesced with part or all of IO
    barrier sequence at the front.

    3. During suspending or resuming, SCSI issues internal command which
    gets deferred and requeued for some reason. As the command is
    issued after the IO barrier in #1, ordered requeueing code puts the
    request after IO barrier sequence.

    4. The device is ready to process requests again but still is in
    quiesced state and the first request of the queue isn't
    REQ_PREEMPT, so command processing is deadlocked -
    suspending/resuming waits for the issued request to complete while
    the request can't be processed till device is put back into
    running state by resuming.

    This can be fixed by always putting !fs requests at the front when

    The following thread reports this deadlock.

    Signed-off-by: Tejun Heo <>
    Cc: Jenn Axboe <>
    Cc: David Greaves <>
    > Yep looks good, except for the bad multi-line comment style, but that's
    > minor stuff ;-)

    That's how Jeff likes it in libata and my fingers are hardcoded to it,
    but I do appreciate the paramount importance of each maintainer's
    right to his/her own comment style, so here's the respinned patch. :-)

    block/ll_rw_blk.c | 9 +++++++++
    1 file changed, 9 insertions(+)

    diff --git a/block/ll_rw_blk.c b/block/ll_rw_blk.c
    index 6b5173a..c99b463 100644
    --- a/block/ll_rw_blk.c
    +++ b/block/ll_rw_blk.c
    @@ -340,6 +340,15 @@ unsigned blk_ordered_req_seq(struct request *rq)
    if (rq == &q->post_flush_rq)

    + /*
    + * !fs requests don't need to follow barrier ordering. Always
    + * put them at the front. This fixes the following deadlock.
    + *
    + *
    + */
    + if (!blk_fs_request(rq))
    + return QUEUE_ORDSEQ_DRAIN;
    if ((rq->cmd_flags & REQ_ORDERED_COLOR) ==
    (q->orig_bar_rq->cmd_flags & REQ_ORDERED_COLOR))
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      Last update: 2007-06-15 13:19    [W:0.022 / U:54.684 seconds]
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