[PATCH V4] scsi: core: use blk_mq_requeue_request in __scsi_queue_insert

[PATCH V4] scsi: core: use blk_mq_requeue_request in __scsi_queue_insert

[Jianchao Wang]

In scsi core, __scsi_queue_insert should just put request back on
the queue and retry using the same command as before. However, for
blk-mq, scsi_mq_requeue_cmd is employed here which will unprepare
the request. To align with the semantics of __scsi_queue_insert,
use blk_mq_requeue_request with kick_requeue_list == true and put
the reference of scsi_device.

Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jianchao Wang <jianchao.w.wang@oracle.com>
Reviewed-by: Bart Van Assche <bart.vanassche@wdc.com>
---
Changelog:
V3 -> V4:
 - modify the comment and make it more clearly

V2 -> V3:
 - add comment to explain why we need a put_device in
   __scsi_queue_insert
 - add reviewed-by

V1 -> V2:
 - add put_device on scsi_device->sdev_gendev
 drivers/scsi/scsi_lib.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index a86df9c..6ce33f6 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -191,7 +191,19 @@ static void __scsi_queue_insert(struct scsi_cmnd *cmd, int reason, bool unbusy)
 	 */
 	cmd->result = 0;
 	if (q->mq_ops) {
-		scsi_mq_requeue_cmd(cmd);
+		/*
+		 * Before a SCSI command is dispatched,
+		 * get_device(&sdev->sdev_gendev) is called and the host,
+		 * target and device busy counters are increased. Since
+		 * requeuing a request causes these actions to be repeated and
+		 * since scsi_device_unbusy() has already been called,
+		 * put_device(&device->sdev_gendev) must still be called. Call
+		 * put_device() after blk_mq_requeue_request() to avoid that
+		 * removal of the SCSI device can start before requeueing has
+		 * happened.
+		 */
+		blk_mq_requeue_request(cmd->request, true);
+		put_device(&device->sdev_gendev);
 		return;
 	}
 	spin_lock_irqsave(q->queue_lock, flags);
-- 
2.7.4

Re: [PATCH V4] scsi: core: use blk_mq_requeue_request in __scsi_queue_insert

[jianchao.wang]

Hi Martin

Can you take your precious time to review this ?

Thanks in advice.
Jianchao

On 03/03/2018 09:54 AM, Jianchao Wang wrote:
> In scsi core, __scsi_queue_insert should just put request back on
> the queue and retry using the same command as before. However, for
> blk-mq, scsi_mq_requeue_cmd is employed here which will unprepare
> the request. To align with the semantics of __scsi_queue_insert,
> use blk_mq_requeue_request with kick_requeue_list == true and put
> the reference of scsi_device.
> 
> Cc: Christoph Hellwig <hch@lst.de>
> Signed-off-by: Jianchao Wang <jianchao.w.wang@oracle.com>
> Reviewed-by: Bart Van Assche <bart.vanassche@wdc.com>
> ---
> Changelog:
> V3 -> V4:
>  - modify the comment and make it more clearly
> 
> V2 -> V3:
>  - add comment to explain why we need a put_device in
>    __scsi_queue_insert
>  - add reviewed-by
> 
> V1 -> V2:
>  - add put_device on scsi_device->sdev_gendev
>  drivers/scsi/scsi_lib.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
> index a86df9c..6ce33f6 100644
> --- a/drivers/scsi/scsi_lib.c
> +++ b/drivers/scsi/scsi_lib.c
> @@ -191,7 +191,19 @@ static void __scsi_queue_insert(struct scsi_cmnd *cmd, int reason, bool unbusy)
>  	 */
>  	cmd->result = 0;
>  	if (q->mq_ops) {
> -		scsi_mq_requeue_cmd(cmd);
> +		/*
> +		 * Before a SCSI command is dispatched,
> +		 * get_device(&sdev->sdev_gendev) is called and the host,
> +		 * target and device busy counters are increased. Since
> +		 * requeuing a request causes these actions to be repeated and
> +		 * since scsi_device_unbusy() has already been called,
> +		 * put_device(&device->sdev_gendev) must still be called. Call
> +		 * put_device() after blk_mq_requeue_request() to avoid that
> +		 * removal of the SCSI device can start before requeueing has
> +		 * happened.
> +		 */
> +		blk_mq_requeue_request(cmd->request, true);
> +		put_device(&device->sdev_gendev);
>  		return;
>  	}
>  	spin_lock_irqsave(q->queue_lock, flags);
> 

Re: [PATCH V4] scsi: core: use blk_mq_requeue_request in __scsi_queue_insert

[Martin K. Petersen]

Jianchao,

> In scsi core, __scsi_queue_insert should just put request back on
> the queue and retry using the same command as before. However, for
> blk-mq, scsi_mq_requeue_cmd is employed here which will unprepare
> the request. To align with the semantics of __scsi_queue_insert,
> use blk_mq_requeue_request with kick_requeue_list == true and put
> the reference of scsi_device.

Applied to 4.17/scsi-queue, thank you!

-- 
Martin K. Petersen	Oracle Linux Engineering