FAILED: patch "[PATCH] block: mq-deadline: Fix queue restart handling" failed to apply to 4.19-stable tree

FAILED: patch "[PATCH] block: mq-deadline: Fix queue restart handling" failed to apply to 4.19-stable tree

[gregkh]

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

------------------ 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);
 	}
 }

Re: FAILED: patch "[PATCH] block: mq-deadline: Fix queue restart handling" failed to apply to 4.19-stable tree

[Damien Le Moal]

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);
>  	}
>  }
> 
> 


-- 
Damien Le Moal
Western Digital Research