From: Damien Le Moal damien.lemoal@wdc.com
commit cb8acabbe33b110157955a7425ee876fb81e6bbc upstream.
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 Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- block/mq-deadline.c | 19 +++++++++---------- 1 file changed, 9 insertions(+), 10 deletions(-)
--- 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_reque
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 re * 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 req
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); } }