Date:   Mon, 24 Aug 2020 15:36:36 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Linux PM <linux-pm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux ACPI <linux-acpi@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>
Subject: Re: [PATCH] PM: sleep: core: Fix the handling of pending runtime resume requests

On Friday, August 21, 2020 9:34:42 PM CEST Alan Stern wrote:
> On Fri, Aug 21, 2020 at 07:41:02PM +0200, Rafael J. Wysocki wrote:
> > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > 
> > It has been reported that system-wide suspend may be aborted in the
> > absence of any wakeup events due to unforseen interactions of it with
> > the runtume PM framework.
> > 
> > One failing scenario is when there are multiple devices sharing an
> > ACPI power resource and runtime-resume needs to be carried out for
> > one of them during system-wide suspend (for example, because it needs
> > to be reconfigured before the whole system goes to sleep).  In that
> > case, the runtime-resume of that device involves turning the ACPI
> > power resource "on" which in turn causes runtime resume requests
> > to be queued up for all of the other devices sharing it.  Those
> > requests go to the runtime PM workqueue which is frozen during
> > system-wide suspend, so they are not actually taken care of until
> > the resume of the whole system, but the pm_runtime_barrier()
> > call in __device_suspend() sees them and triggers system wakeup
> > events for them which then cause the system-wide suspend to be
> > aborted if wakeup source objects are in active use.
> > 
> > Of course, the logic that leads to triggering those wakeup events is
> > questionable in the first place, because clearly there are cases in
> > which a pending runtime resume request for a device is not connected
> > to any real wakeup events in any way (like the one above).  Moreover,
> > if there is a pending runtime resume request for a device while
> > __device_suspend() is running for it, the physical state of the
> > device may not be in agreement with the "suspended" runtime PM status
> > of it (which may be the very reason for queuing up the runtime resume
> > request for it).
> > 
> > For these reasons, rework __device_suspend() to carry out synchronous
> > runtime-resume for devices with pending runtime resume requests before
> > attempting to invoke system-wide suspend callbacks for them with the
> > expectation that their drivers will trigger system-wide wakeup events
> > in the process of handling the runtime resume, if necessary.
> > 
> > Fixes: 1e2ef05bb8cf8 ("PM: Limit race conditions between runtime PM and system sleep (v2)")
> > Reported-by: Mika Westerberg <mika.westerberg@...ux.intel.com>
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > ---
> >  drivers/base/power/main.c |   12 ++++++------
> >  1 file changed, 6 insertions(+), 6 deletions(-)
> > 
> > Index: linux-pm/drivers/base/power/main.c
> > ===================================================================
> > --- linux-pm.orig/drivers/base/power/main.c
> > +++ linux-pm/drivers/base/power/main.c
> > @@ -1606,13 +1606,13 @@ static int __device_suspend(struct devic
> >  	}
> >  
> >  	/*
> > -	 * If a device configured to wake up the system from sleep states
> > -	 * has been suspended at run time and there's a resume request pending
> > -	 * for it, this is equivalent to the device signaling wakeup, so the
> > -	 * system suspend operation should be aborted.
> > +	 * If there's a runtime resume request pending for the device, resume
> > +	 * it before proceeding with invoking the system-wide suspend callbacks
> > +	 * for it, because the physical state of the device may not reflect the
> > +	 * "suspended" runtime PM status already in that case.
> >  	 */
> > -	if (pm_runtime_barrier(dev) && device_may_wakeup(dev))
> > -		pm_wakeup_event(dev, 0);
> > +	if (pm_runtime_barrier(dev))
> > +		pm_runtime_resume(dev);
> 
> Is this really right?  Note that whenever pm_runtime_barrier() returns a 
> nonzero value, it already calls rpm_resume(dev, 0).  So the 
> pm_runtime_resume() call added here is redundant.

Yes, it is (it seems that I've looked at __pm_runtime_barrier() alone, not sure
why).

> Furthermore, by the logic used in this patch, the call to 
> pm_wakeup_event() in the original code is also redundant: Any required 
> wakeup event should have been generated when the runtime resume inside 
> pm_runtime_barrer() was carried out.

It should be redundant in the real wakeup event cases, but it may cause
spurious suspend aborts to occur when there are no real system wakeup
events.

Actually, the original code is racy with respect to system wakeup events,
because it depends on the exact time when the runtime-resume starts.  Namely,
if it manages to start before the freezing of pm_wq, the wakeup will be lost
unless the driver takes care of reporting it, which means that drivers really
need to do that anyway.  And if they do that (which hopefully is the case), the
pm_wakeup_event() call in the core may be dropped.

> Removing a redundant function call can't fix a bug!

That's true, but this particular call is redundant only if there is a real
wakeup event.  Otherwise it may produce a spurious wakeup.

> This means that the code could be simplified to just:
> 
> 	pm_runtime_barrier(dev);

Yes, it could, so I'm going to re-spin the patch with this code simplification
and updated changelog.

> Will this fix the reported bug?

I think so.

> It seems likely to me that the actual 
> problem with the failure scenario in the patch description was that 
> turning on an ACPI power resource causes runtime-resume requests to be 
> queued for all devices sharing that resource.  Wouldn't it make more 
> sense to resume only the device that requested it and leave the others 
> in runtime suspend?

Please see the Mika's reply for a good explanation of this.

Cheers!

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