diff options
authorDamien Le Moal <damien.lemoal@wdc.com>2019-08-28 13:40:20 +0900
committerJens Axboe <axboe@kernel.dk>2019-09-03 07:59:51 -0600
commitcb8acabbe33b110157955a7425ee876fb81e6bbc (patch)
parent0feacaa21634014148068035b02eade71f853496 (diff)
block: mq-deadline: Fix queue restart handling
Commit 7211aef86f79 ("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: 7211aef86f79 ("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>
1 files changed, 9 insertions, 10 deletions
diff --git a/block/mq-deadline.c b/block/mq-deadline.c
index 2a2a2e82832e..35e84bc0ec8c 100644
--- a/block/mq-deadline.c
+++ b/block/mq-deadline.c
@@ -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)
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);
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);
+ if (!list_empty(&dd->fifo_list[WRITE]))
+ blk_mq_sched_mark_restart_hctx(rq->mq_hctx);
spin_unlock_irqrestore(&dd->zone_lock, flags);

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