Message-ID: <82bcdf73-54c5-4220-86c0-540a5cb59bb7@vivo.com>
Date: Mon, 1 Dec 2025 17:46:45 +0800
From: YangYang <yang.yang@...o.com>
To: "Rafael J. Wysocki" <rafael@...nel.org>,
 Bart Van Assche <bvanassche@....org>
Cc: Jens Axboe <axboe@...nel.dk>, Pavel Machek <pavel@...nel.org>,
 Len Brown <lenb@...nel.org>, Greg Kroah-Hartman
 <gregkh@...uxfoundation.org>, Danilo Krummrich <dakr@...nel.org>,
 linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-pm@...r.kernel.org
Subject: Re: [PATCH 1/2] PM: runtime: Fix I/O hang due to race between resume
 and runtime disable

On 2025/11/27 20:34, Rafael J. Wysocki wrote:
> On Wed, Nov 26, 2025 at 11:47 PM Bart Van Assche <bvanassche@....org> wrote:
>>
>> On 11/26/25 1:30 PM, Rafael J. Wysocki wrote:
>>> On Wed, Nov 26, 2025 at 10:11 PM Bart Van Assche <bvanassche@....org> wrote:
>>>>
>>>> On 11/26/25 12:17 PM, Rafael J. Wysocki wrote:
>>>>> --- a/block/blk-core.c
>>>>> +++ b/block/blk-core.c
>>>>> @@ -309,6 +309,8 @@ int blk_queue_enter(struct request_queue
>>>>>                 if (flags & BLK_MQ_REQ_NOWAIT)
>>>>>                         return -EAGAIN;
>>>>>
>>>>> +             /* if necessary, resume .dev (assume success). */
>>>>> +             blk_pm_resume_queue(pm, q);
>>>>>                 /*
>>>>>                  * read pair of barrier in blk_freeze_queue_start(), we need to
>>>>>                  * order reading __PERCPU_REF_DEAD flag of .q_usage_counter and
>>>>
>>>> blk_queue_enter() may be called from the suspend path so I don't think
>>>> that the above change will work.
>>>
>>> Why would the existing code work then?
>>
>> The existing code works reliably on a very large number of devices.
> 
> Well, except that it doesn't work during system suspend and
> hibernation when the PM workqueue is frozen.  I think that we agree
> here.
> 
> This needs to be addressed because it may very well cause system
> suspend to deadlock.
> 
> There are two possible ways to address it I can think of:
> 
> 1. Changing blk_pm_resume_queue() and its users to carry out a
> synchronous resume of q->dev instead of calling pm_request_resume()
> and (effectively) waiting for the queued-up runtime resume of q->dev
> to take effect.
> 
> This would be my preferred option, but at this point I'm not sure if
> it's viable.
> 

After __pm_runtime_disable() is called from device_suspend_late(), 
dev->power.disable_depth is set, preventing rpm_resume() from making 
progress until the system resume completes, regardless of whether 
rpm_resume() is invoked synchronously or asynchronously.
Performing a synchronous resume of q->dev seems to have a similar 
effect to removing the following code block from 
__pm_runtime_barrier(), which is invoked by __pm_runtime_disable():

1428     if (dev->power.request_pending) {
1429         dev->power.request = RPM_REQ_NONE;
1430         spin_unlock_irq(&dev->power.lock);
1431
1432         cancel_work_sync(&dev->power.work);
1433
1434         spin_lock_irq(&dev->power.lock);
1435         dev->power.request_pending = false;
1436     }

> 2. Stop freezing the PM workqueue before system suspend/hibernation
> and adapt device_suspend_late() to that.
> 
> This should be doable, even though it is a bit risky because it may
> uncover some latent bugs (the freezing of the PM workqueue has been
> there forever), but it wouldn't address the problem entirely because
> device_suspend_late() would still need to disable runtime PM for the
> device (and for some devices it is disabled earlier), so
> pm_request_resume() would just start to fail at that point and if
> blk_queue_enter() were called after that point for a device supporting
> runtime PM, it might deadlock.
> 
>> Maybe there is a misunderstanding? RQF_PM / BLK_MQ_REQ_PM are set for
>> requests that should be processed even if the power status is changing
>> (RPM_SUSPENDING or RPM_RESUMING). The meaning of the 'pm' variable is
>> as follows: process this request even if a power state change is
>> ongoing.
> 
> I see.
> 
> The behavior depends on whether or not q->pm_only is set.  If it is
> not set, both blk_queue_enter() and __bio_queue_enter() will allow the
> request to be processed.
> 
> If q->pm_only is set, __bio_queue_enter() will wait until it gets
> cleared and in that case pm_request_resume(q->dev) is called to make
> that happen (did I get it right?).  This is a bit fragile because what
> if the async resume of q->dev fails for some reason?  You deadlock
> instead of failing the request.
> 
> Unlike __bio_queue_enter(), blk_queue_enter() additionally checks the
> runtime PM status of the queue if q->pm_only is set and it will allow
> the request to be processed in that case so long as q->rpm_status is
> not RPM_SUSPENDED.  However, if the queue status is RPM_SUSPENDED,
> pm_request_resume(q->dev) will be called like in the
> __bio_queue_enter() case.
> 
> I'm not sure why pm_request_resume(q->dev) needs to be called from
> within blk_pm_resume_queue().  Arguably, it should be sufficient to
> call it once before using the wait_event() macro, if the conditions
> checked by blk_pm_resume_queue() are not met.
> 
>>> Are you suggesting that q->rpm_status should still be checked before
>>> calling pm_runtime_resume() or do you mean something else?
>> The purpose of the code changes from a previous email is not entirely
>> clear to me so I'm not sure what the code should look like. But to
>> answer your question, calling blk_pm_resume_queue() if the runtime
>> status is RPM_SUSPENDED should be safe.
>>>> As an example, the UFS driver submits a
>>>> SCSI START STOP UNIT command from its runtime suspend callback. The call
>>>> chain is as follows:
>>>>
>>>>      ufshcd_wl_runtime_suspend()
>>>>        __ufshcd_wl_suspend()
>>>>          ufshcd_set_dev_pwr_mode()
>>>>            ufshcd_execute_start_stop()
>>>>              scsi_execute_cmd()
>>>>                scsi_alloc_request()
>>>>                  blk_queue_enter()
>>>>                blk_execute_rq()
>>>>                blk_mq_free_request()
>>>>                  blk_queue_exit()
>>>
>>> In any case, calling pm_request_resume() from blk_pm_resume_queue() in
>>> the !pm case is a mistake.
>>    Hmm ... we may disagree about this. Does what I wrote above make clear
>> why blk_pm_resume_queue() is called if pm == false?
> 
> Yes, it does, thanks!

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