RE: Re: [PATCH v19 2/3] scsi: ufs: L2P map management for HPB read

[Daejun Park <daejun7.park@samsung.com>]

>>> @@ -342,13 +1208,14 @@ void ufshpb_suspend(struct ufs_hba *hba)
>>> >          struct scsi_device *sdev;
>>> >
>>> >          shost_for_each_device(sdev, hba->host) {
>>> > -                hpb = sdev->hostdata;
>>> > +                hpb = ufshpb_get_hpb_data(sdev);
>>> >                  if (!hpb)
>>> >                          continue;
>>> >
>>> >                  if (ufshpb_get_state(hpb) != HPB_PRESENT)
>>> >                          continue;
>>> >                  ufshpb_set_state(hpb, HPB_SUSPEND);
>>> > +                ufshpb_cancel_jobs(hpb);
>>> 
>>> Here may have a dead lock problem - in the case of runtime suspend,
>>> when ufshpb_suspend() is invoked, all of hba's children scsi devices
>>> are in RPM_SUSPENDED state. When this line tries to cancel a running
>>> map work, i.e. when ufshpb_get_map_req() calls below lines, it will
>>> be stuck at blk_queue_enter().
>>> 
>>> req = blk_get_request(hpb->sdev_ufs_lu->request_queue,
>>>                       REQ_OP_SCSI_IN, 0);
>>> 
>>> Please check block layer power management, and see also commit 
>>> d55d15a33
>>> ("scsi: block: Do not accept any requests while suspended").
>> 
>> I am agree with your comment.
>> How about add BLK_MQ_REQ_NOWAIT flag on blk_get_request() to avoid 
>> hang?
>> 
> 
>That won't work - BLK_MQ_REQ_NOWAIT allows one to fast fail from 
>blk_mq_get_tag(),
>but blk_queue_enter() comes before __blk_mq_alloc_request();
> 
In blk_queue_enter(), BLK_MQ_REQ_NOWAIT flag can make error than wait rpm
resume. Please refer following code.

int blk_queue_enter(struct request_queue *q, blk_mq_req_flags_t flags)
{
	const bool pm = flags & BLK_MQ_REQ_PM;

	while (true) {
		bool success = false;

		rcu_read_lock();
		if (percpu_ref_tryget_live(&q->q_usage_counter)) {
			/*
			 * The code that increments the pm_only counter is
			 * responsible for ensuring that that counter is
			 * globally visible before the queue is unfrozen.
			 */
			if ((pm && queue_rpm_status(q) != RPM_SUSPENDED) ||
			    !blk_queue_pm_only(q)) {
				success = true;
			} else {
				percpu_ref_put(&q->q_usage_counter);
			}
		}
		rcu_read_unlock();

		if (success)
			return 0;

		if (flags & BLK_MQ_REQ_NOWAIT)
			return -EBUSY;	<-- out from the function.
			
Thanks,
Daejun