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Subject: Re: [PATCH v3 1/1] scsi: pm: Balance pm_only counter of request queue
 during system resume
To:     Can Guo <cang@codeaurora.org>
Cc:     asutoshd@codeaurora.org, nguyenb@codeaurora.org,
        hongwus@codeaurora.org, rnayak@codeaurora.org,
        stanley.chu@mediatek.com, alim.akhtar@samsung.com,
        beanhuo@micron.com, Avri.Altman@wdc.com,
        bjorn.andersson@linaro.org, linux-scsi@vger.kernel.org,
        kernel-team@android.com, saravanak@google.com, salyzyn@google.com,
        "James E.J. Bottomley" <jejb@linux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@oracle.com>,
        open list <linux-kernel@vger.kernel.org>
References: <1588219805-25794-1-git-send-email-cang@codeaurora.org>
 <9e15123e-4315-15cd-3d23-2df6144bd376@acm.org>
 <1ef85ee212bee679f7b2927cbbc79cba@codeaurora.org>
From:   Bart Van Assche <bvanassche@acm.org>
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Date:   Thu, 30 Apr 2020 13:32:55 -0700
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On 2020-04-29 22:40, Can Guo wrote:
> On 2020-04-30 13:08, Bart Van Assche wrote:
>> On 2020-04-29 21:10, Can Guo wrote:
>>> During system resume, scsi_resume_device() decreases a request queue's
>>> pm_only counter if the scsi device was quiesced before. But after that,
>>> if the scsi device's RPM status is RPM_SUSPENDED, the pm_only counter is
>>> still held (non-zero). Current scsi resume hook only sets the RPM status
>>> of the scsi device and its request queue to RPM_ACTIVE, but leaves the
>>> pm_only counter unchanged. This may make the request queue's pm_only
>>> counter remain non-zero after resume hook returns, hence those who are
>>> waiting on the mq_freeze_wq would never be woken up. Fix this by calling
>>> blk_post_runtime_resume() if pm_only is non-zero to balance the pm_only
>>> counter which is held by the scsi device's RPM ops.
>>
>> How was this issue discovered? How has this patch been tested?
> 
> As the issue was found after system resumes, so the issue was discovered
> during system suspend/resume test, and it is very easy to be replicated.
> After system resumes, if this issue hits some scsi devices, all bios sent
> to their request queues are blocked, which may cause a system hang if the
> scsi devices are vital to system functionality.
> 
> To make sure the patch work well, we have tested system suspend/resume
> and made sure no system hang happen due to request queues got blocked
> by imbalanced pm_only counter.

Thanks, that's very interesting information. My concern with this patch
is that the power management code is not the only caller of
blk_set_pm_only() / blk_clear_pm_only(). E.g. the SCSI SPI code also
calls scsi_device_quiesce() and scsi_device_resume(). These last
functions call blk_set_pm_only() and blk_clear_pm_only(). More calls of
scsi_device_quiesce() and scsi_device_resume() might be added in the future.

Has it been considered to test directly whether a SCSI device has been
runtime suspended instead of relying on blk_queue_pm_only()? How about
using pm_runtime_status_suspended() or adding a function in
block/blk-pm.h that checks whether q->rpm_status == RPM_SUSPENDED?

Thanks,

Bart.