[PATCH] blk-mq: fix hang issue in blk_queue_enter()

[PATCH] blk-mq: fix hang issue in blk_queue_enter()

[Yang Yang]

There is a race between blk_queue_enter() and block layer's runtime
suspend.

 CPU0                                CPU1
 ---------------------------------   -------------------------------
 blk_pre_runtime_suspend(q) {        blk_queue_enter() {
   /* q->rpm_status=RPM_ACTIVE */
   blk_set_pm_only(q)
   /* q->pm_only=1 */
   blk_freeze_queue_start(q)
   blk_mq_unfreeze_queue(q)
                                       if (percpu_ref_tryget_live()) {
                                         /* pm=0 && q->pm_only=1 */
                                         if (pm || !blk_queue_pm_only(q)) {
                                         } else {
                                           percpu_ref_put()
                                         }
                                       }
                                       wait_event(q->mq_freeze_wq,
                                         (!q->mq_freeze_depth &&
                                         /* q->rpm_status=RPM_ACTIVE
                                            q->pm_only=1 */
                                         (pm || (blk_pm_request_resume(q),
                                          !blk_queue_pm_only(q)))) ||
                                          blk_queue_dying(q))
                                     }
   spin_lock_irq(&q->queue_lock)
   q->rpm_status = RPM_SUSPENDING
   spin_unlock_irq(&q->queue_lock)
 }

At this point blk_pm_request_resume() missed the chance to resume the
queue, so blk_queue_enter() may wait here forever.
The solution is to wake up the mq_freeze_wq after runtime suspend
completed, make blk_pm_request_resume() reexamine the q->rpm_status flag.

Signed-off-by: Yang Yang <yang.yang@vivo.com>
---
 block/blk-pm.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/block/blk-pm.c b/block/blk-pm.c
index b85234d758f7..dec7d0aef606 100644
--- a/block/blk-pm.c
+++ b/block/blk-pm.c
@@ -132,6 +132,8 @@ void blk_post_runtime_suspend(struct request_queue *q, int err)
 
 	if (err)
 		blk_clear_pm_only(q);
+	else
+		wake_up_all(&q->mq_freeze_wq);
 }
 EXPORT_SYMBOL(blk_post_runtime_suspend);
 
-- 
2.17.1

Re: [PATCH] blk-mq: fix hang issue in blk_queue_enter()

[Bart Van Assche]

On 2020-09-14 00:19, Yang Yang wrote:
> There is a race between blk_queue_enter() and block layer's runtime
> suspend.
> 
>  CPU0                                CPU1
>  ---------------------------------   -------------------------------
>  blk_pre_runtime_suspend(q) {        blk_queue_enter() {
>    /* q->rpm_status=RPM_ACTIVE */
>    blk_set_pm_only(q)
>    /* q->pm_only=1 */
>    blk_freeze_queue_start(q)
>    blk_mq_unfreeze_queue(q)
>                                        if (percpu_ref_tryget_live()) {
>                                          /* pm=0 && q->pm_only=1 */
>                                          if (pm || !blk_queue_pm_only(q)) {
>                                          } else {
>                                            percpu_ref_put()
>                                          }
>                                        }
>                                        wait_event(q->mq_freeze_wq,
>                                          (!q->mq_freeze_depth &&
>                                          /* q->rpm_status=RPM_ACTIVE
>                                             q->pm_only=1 */
>                                          (pm || (blk_pm_request_resume(q),
>                                           !blk_queue_pm_only(q)))) ||
>                                           blk_queue_dying(q))
>                                      }
>    spin_lock_irq(&q->queue_lock)
>    q->rpm_status = RPM_SUSPENDING
>    spin_unlock_irq(&q->queue_lock)
>  }
> 
> At this point blk_pm_request_resume() missed the chance to resume the
> queue, so blk_queue_enter() may wait here forever.
> The solution is to wake up the mq_freeze_wq after runtime suspend
> completed, make blk_pm_request_resume() reexamine the q->rpm_status flag.
> 
> Signed-off-by: Yang Yang <yang.yang@vivo.com>
> ---
>  block/blk-pm.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/block/blk-pm.c b/block/blk-pm.c
> index b85234d758f7..dec7d0aef606 100644
> --- a/block/blk-pm.c
> +++ b/block/blk-pm.c
> @@ -132,6 +132,8 @@ void blk_post_runtime_suspend(struct request_queue *q, int err)
>  
>  	if (err)
>  		blk_clear_pm_only(q);
> +	else
> +		wake_up_all(&q->mq_freeze_wq);
>  }
>  EXPORT_SYMBOL(blk_post_runtime_suspend);

Please verify whether the following patch series also fixes the reported
hang: https://lore.kernel.org/linux-block/20200906012219.17893-1-bvanassche@acm.org/T/#t

Thanks,

Bart.

Re: [PATCH] blk-mq: fix hang issue in blk_queue_enter()

[Yang Yang]

On 2020/9/14 21:35, Markus Elfring wrote:
> …
>> The solution is to wake up the mq_freeze_wq after runtime suspend
>> completed, make blk_pm_request_resume() reexamine the q->rpm_status flag.
> 
> * Would an imperative wording become helpful for the change description?
> 
> * How do you think about to add the tag “Fixes” to the commit message?
> 

Thank you for your suggestions.

--
Yang Yang

Re: [PATCH] blk-mq: fix hang issue in blk_queue_enter()

[Yang Yang]

On 2020/9/14 21:35, Markus Elfring wrote:
> …
>> The solution is to wake up the mq_freeze_wq after runtime suspend
>> completed, make blk_pm_request_resume() reexamine the q->rpm_status flag.
> 
> * Would an imperative wording become helpful for the change description?
> 
> * How do you think about to add the tag “Fixes” to the commit message?
> 

Thank you for your suggestions.

--
Yang Yang