linux-kernel - Re: [PATCH 1/2] PM: runtime: Fix I/O hang due to race between resume and runtime disable

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Message-ID: <CAJZ5v0h=GCLggsj_BGQbZzjDj3+JdKn7UfSMcjyZipteqiAG6A@mail.gmail.com>
Date: Tue, 2 Dec 2025 13:18:11 +0100
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: YangYang <yang.yang@...o.com>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>, Bart Van Assche <bvanassche@....org>, 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 Tue, Dec 2, 2025 at 11:33 AM YangYang <yang.yang@...o.com> wrote:
>
> On 2025/12/2 2:55, Rafael J. Wysocki wrote:
> > On Mon, Dec 1, 2025 at 1:56 PM YangYang <yang.yang@...o.com> wrote:
> >>
> >> On 2025/12/1 17:46, YangYang wrote:
> >>> 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     }
> >>>
> >>
> >> Since both synchronous and asynchronous resumes face similar issues,
> >
> > No, they don't.
> >
> >> it may be sufficient to keep using the asynchronous resume path as long as
> >> pending work items are not canceled while the PM workqueue is frozen.
> >
> > Except for two things:
> >
> > 1. If blk_queue_enter() or __bio_queue_enter() is allowed to race with
> > disabling runtime PM, queuing up the resume work item may fail in the
> > first place.
> >
>
> Perhaps my understanding is incorrect, but during the execution of
> device_suspend_late(), the PM workqueue should already be frozen.
> In that case, queuing a resume work item would not fail; it would
> simply not be executed until the workqueue is unfrozen, as long as
> it is not canceled.

rpm_resume() returns an error if runtime PM is disabled for the given
device and the device status is RPM_SUSPENDED even if it is called
with RPM_ASYNC or RPM_NOWAIT in the flags.