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    Subject[PATCH 4.4 63/86] block: defer timeouts to a workqueue
    Date
    From: Christoph Hellwig <hch@lst.de>

    commit 287922eb0b186e2a5bf54fdd04b734c25c90035c upstream.

    Timer context is not very useful for drivers to perform any meaningful abort
    action from. So instead of calling the driver from this useless context
    defer it to a workqueue as soon as possible.

    Note that while a delayed_work item would seem the right thing here I didn't
    dare to use it due to the magic in blk_add_timer that pokes deep into timer
    internals. But maybe this encourages Tejun to add a sensible API for that to
    the workqueue API and we'll all be fine in the end :)

    Contains a major update from Keith Bush:

    "This patch removes synchronizing the timeout work so that the timer can
    start a freeze on its own queue. The timer enters the queue, so timer
    context can only start a freeze, but not wait for frozen."

    -------------
    NOTE: Back-ported to 4.4.y.

    The only parts of the upstream commit that have been kept are various
    locking changes, none of which were mentioned in the original commit
    message which therefore describes this change not at all.

    Timeout callbacks continue to be run via a timer. Both blk_mq_rq_timer
    and blk_rq_timed_out_timer will return without without doing any work
    if they cannot acquire the queue (without waiting).
    -------------

    Signed-off-by: Christoph Hellwig <hch@lst.de>
    Acked-by: Keith Busch <keith.busch@intel.com>
    Signed-off-by: Jens Axboe <axboe@fb.com>
    Signed-off-by: Giuliano Procida <gprocida@google.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    block/blk-mq.c | 4 ++++
    block/blk-timeout.c | 3 +++
    2 files changed, 7 insertions(+)

    --- a/block/blk-mq.c
    +++ b/block/blk-mq.c
    @@ -628,6 +628,9 @@ static void blk_mq_rq_timer(unsigned lon
    };
    int i;

    + if (blk_queue_enter(q, GFP_NOWAIT))
    + return;
    +
    blk_mq_queue_tag_busy_iter(q, blk_mq_check_expired, &data);

    if (data.next_set) {
    @@ -642,6 +645,7 @@ static void blk_mq_rq_timer(unsigned lon
    blk_mq_tag_idle(hctx);
    }
    }
    + blk_queue_exit(q);
    }

    /*
    --- a/block/blk-timeout.c
    +++ b/block/blk-timeout.c
    @@ -134,6 +134,8 @@ void blk_rq_timed_out_timer(unsigned lon
    struct request *rq, *tmp;
    int next_set = 0;

    + if (blk_queue_enter(q, GFP_NOWAIT))
    + return;
    spin_lock_irqsave(q->queue_lock, flags);

    list_for_each_entry_safe(rq, tmp, &q->timeout_list, timeout_list)
    @@ -143,6 +145,7 @@ void blk_rq_timed_out_timer(unsigned lon
    mod_timer(&q->timeout, round_jiffies_up(next));

    spin_unlock_irqrestore(q->queue_lock, flags);
    + blk_queue_exit(q);
    }

    /**

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