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block: mq-deadline: Fix queue restart handling

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Commit 7211aef86f ("block: mq-deadline: Fix write completion
handling") added a call to blk_mq_sched_mark_restart_hctx() in
dd_dispatch_request() to make sure that write request dispatching does
not stall when all target zones are locked. This fix left a subtle race
when a write completion happens during a dispatch execution on another
CPU:

CPU 0: Dispatch			CPU1: write completion

dd_dispatch_request()
    lock(&dd->lock);
    ...
    lock(&dd->zone_lock);	dd_finish_request()
    rq = find request		lock(&dd->zone_lock);
    unlock(&dd->zone_lock);
    				zone write unlock
				unlock(&dd->zone_lock);
				...
				__blk_mq_free_request
                                      check restart flag (not set)
				      -> queue not run
    ...
    if (!rq && have writes)
        blk_mq_sched_mark_restart_hctx()
    unlock(&dd->lock)

Since the dispatch context finishes after the write request completion
handling, marking the queue as needing a restart is not seen from
__blk_mq_free_request() and blk_mq_sched_restart() not executed leading
to the dispatch stall under 100% write workloads.

Fix this by moving the call to blk_mq_sched_mark_restart_hctx() from
dd_dispatch_request() into dd_finish_request() under the zone lock to
ensure full mutual exclusion between write request dispatch selection
and zone unlock on write request completion.

Fixes: 7211aef86f ("block: mq-deadline: Fix write completion handling")
Cc: stable@vger.kernel.org
Reported-by: Hans Holmberg <Hans.Holmberg@wdc.com>
Reviewed-by: Hans Holmberg <hans.holmberg@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Damien Le Moal 2019-08-28 13:40:20 +09:00 committed by Jens Axboe
parent 0feacaa216
commit cb8acabbe3
1 changed files with 9 additions and 10 deletions
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19
block/mq-deadline.c
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@ -377,13 +377,6 @@ done:
* hardware queue, but we may return a request that is for a
* different hardware queue. This is because mq-deadline has shared
* state for all hardware queues, in terms of sorting, FIFOs, etc.
*
* For a zoned block device, __dd_dispatch_request() may return NULL
* if all the queued write requests are directed at zones that are already
* locked due to on-going write requests. In this case, make sure to mark
* the queue as needing a restart to ensure that the queue is run again
* and the pending writes dispatched once the target zones for the ongoing
* write requests are unlocked in dd_finish_request().
*/
static struct request *dd_dispatch_request(struct blk_mq_hw_ctx *hctx)
{
@ -392,9 +385,6 @@ static struct request *dd_dispatch_request(struct blk_mq_hw_ctx *hctx)
spin_lock(&dd->lock);
rq = __dd_dispatch_request(dd);
if (!rq && blk_queue_is_zoned(hctx->queue) &&
!list_empty(&dd->fifo_list[WRITE]))
blk_mq_sched_mark_restart_hctx(hctx);
spin_unlock(&dd->lock);
return rq;
@ -561,6 +551,13 @@ static void dd_prepare_request(struct request *rq, struct bio *bio)
* spinlock so that the zone is never unlocked while deadline_fifo_request()
* or deadline_next_request() are executing. This function is called for
* all requests, whether or not these requests complete successfully.
*
* For a zoned block device, __dd_dispatch_request() may have stopped
* dispatching requests if all the queued requests are write requests directed
* at zones that are already locked due to on-going write requests. To ensure
* write request dispatch progress in this case, mark the queue as needing a
* restart to ensure that the queue is run again after completion of the
* request and zones being unlocked.
*/
static void dd_finish_request(struct request *rq)
{
@ -572,6 +569,8 @@ static void dd_finish_request(struct request *rq)
spin_lock_irqsave(&dd->zone_lock, flags);
blk_req_zone_write_unlock(rq);
if (!list_empty(&dd->fifo_list[WRITE]))
blk_mq_sched_mark_restart_hctx(rq->mq_hctx);
spin_unlock_irqrestore(&dd->zone_lock, flags);
}
}