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Date:   Fri, 01 May 2020 09:42:17 +0800
From:   Can Guo <cang@...eaurora.org>
To:     Bart Van Assche <bvanassche@....org>
Cc:     asutoshd@...eaurora.org, nguyenb@...eaurora.org,
        hongwus@...eaurora.org, rnayak@...eaurora.org,
        stanley.chu@...iatek.com, alim.akhtar@...sung.com,
        beanhuo@...ron.com, Avri.Altman@....com,
        bjorn.andersson@...aro.org, linux-scsi@...r.kernel.org,
        kernel-team@...roid.com, saravanak@...gle.com, salyzyn@...gle.com,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 1/1] scsi: pm: Balance pm_only counter of request queue
 during system resume

On 2020-05-01 04:32, Bart Van Assche wrote:
> 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.

Hi Bart,

Slightly revised my previous mail.

Please let me address your concern.

First of all, it is allowed to call scsi_device_quiesce() multiple 
times,
but one sdev's request queue's pm_only counter can only be increased 
once
by scsi_device_quiesce(), because if a sdev has already been quiesced,
in scsi_device_quiesce(), scsi_device_set_state(sdev, SDEV_QUIESCE) 
would
return -ENIVAL (illegal state transform), then blk_clear_pm_only() shall
be called to decrease pm_only once, so no matter how many times
scsi_device_quiesce() is called, it can only increase pm_only once.

scsi_device_resume() is same, it calls blk_clear_pm_only only once and
only if the sdev was quiesced().

So, in a word, after scsi_device_resume() returns in 
scsi_dev_type_resume(),
if a sdev has block layer runtime PM enabled (sdev->request_queue->dev 
is not
NULL), its queue's pm_only counter should be 1 (if the sdev's runtime 
power
status is RPM_SUSPENDED) or 0 (if the sdev's runtime power status is 
RPM_ACTIVE).
If a sdev has block layer runtime PM disabled (sdev->request_queue->dev 
is NULL),
its queue's pm_only counter should be 0.

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?

Yes, I used to make the patch like that way, and it also worked well, as
both ways are equal actually. I kinda like the current code because we
should be confident that after scsi_dev_type_resume() returns, pm_only
must be 0. Different reviewers may have different opinions, either way
works well anyways.

Thanks,

Can Guo.

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