Greg,

On 2019/10/03 15:40, gregkh@linuxfoundation.org wrote:

The patch below does not apply to the 4.19-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.

thanks,

greg k-h

I sent the backported (and tested) patch. Please consider it for addition to 4.19.

Thank you.

Best regards.

------------------ original commit in Linus's tree ------------------

From cb8acabbe33b110157955a7425ee876fb81e6bbc Mon Sep 17 00:00:00 2001 From: Damien Le Moal damien.lemoal@wdc.com Date: Wed, 28 Aug 2019 13:40:20 +0900 Subject: [PATCH] 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

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 @@ static struct request *__dd_dispatch_request(struct deadline_data *dd)

• 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); }

}