linux-kernel - Re: [PATCH] scsi: sd: add runtime pm to open / release

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Date:   Mon, 29 Jun 2020 09:56:49 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Alan Stern <stern@...land.harvard.edu>,
        Martin Kepplinger <martin.kepplinger@...i.sm>
Cc:     jejb@...ux.ibm.com, Can Guo <cang@...eaurora.org>,
        martin.petersen@...cle.com, linux-scsi@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel@...i.sm
Subject: Re: [PATCH] scsi: sd: add runtime pm to open / release

On 2020-06-29 09:15, Alan Stern wrote:
> Aha.  Looking at this more closely, it's apparent that the code in 
> blk-core.c contains a logic bug: It assumes that if the BLK_MQ_REQ_PREEMPT 
> flag is set then the request can be issued regardless of the queue's 
> runtime status.  That is not correct when the queue is suspended.

Please clarify why this is not correct.

> Index: usb-devel/block/blk-core.c
> ===================================================================
> --- usb-devel.orig/block/blk-core.c
> +++ usb-devel/block/blk-core.c
> @@ -423,7 +423,8 @@ int blk_queue_enter(struct request_queue
>  			 * responsible for ensuring that that counter is
>  			 * globally visible before the queue is unfrozen.
>  			 */
> -			if (pm || !blk_queue_pm_only(q)) {
> +			if ((pm && q->rpm_status != RPM_SUSPENDED) ||
> +			    !blk_queue_pm_only(q)) {
>  				success = true;
>  			} else {
>  				percpu_ref_put(&q->q_usage_counter);

Does the above change make it impossible to bring a suspended device
back to the RPM_ACTIVE state if the BLK_MQ_REQ_NOWAIT flag is set?

> @@ -448,8 +449,7 @@ int blk_queue_enter(struct request_queue
>  
>  		wait_event(q->mq_freeze_wq,
>  			   (!q->mq_freeze_depth &&
> -			    (pm || (blk_pm_request_resume(q),
> -				    !blk_queue_pm_only(q)))) ||
> +			    blk_pm_resume_queue(pm, q)) ||
>  			   blk_queue_dying(q));
>  		if (blk_queue_dying(q))
>  			return -ENODEV;
> Index: usb-devel/block/blk-pm.h
> ===================================================================
> --- usb-devel.orig/block/blk-pm.h
> +++ usb-devel/block/blk-pm.h
> @@ -6,11 +6,14 @@
>  #include <linux/pm_runtime.h>
>  
>  #ifdef CONFIG_PM
> -static inline void blk_pm_request_resume(struct request_queue *q)
> +static inline int blk_pm_resume_queue(const bool pm, struct request_queue *q)
>  {
> -	if (q->dev && (q->rpm_status == RPM_SUSPENDED ||
> -		       q->rpm_status == RPM_SUSPENDING))
> -		pm_request_resume(q->dev);
> +	if (!q->dev || !blk_queue_pm_only(q))
> +		return 1;	/* Nothing to do */
> +	if (pm && q->rpm_status != RPM_SUSPENDED)
> +		return 1;	/* Request allowed */
> +	pm_request_resume(q->dev);
> +	return 0;
>  }

Does the above change, especially the " && q->rpm_status !=
RPM_SUSPENDED" part, make it impossible to bring a suspended device back
to the RPM_ACTIVE state?

Thanks,

Bart.