Re: [PATCH] PM: Prevent waiting forever on asynchronous resume after abort

[Colin Cross]

On Thu, Sep 2, 2010 at 2:05 PM, Rafael J. Wysocki <rjw@xxxxxxx> wrote:
> On Thursday, September 02, 2010, Colin Cross wrote:
>> On Thu, Sep 2, 2010 at 12:46 PM, Rafael J. Wysocki <rjw@xxxxxxx> wrote:
>> > On Thursday, September 02, 2010, Alan Stern wrote:
>> >> On Wed, 1 Sep 2010, Colin Cross wrote:
>> >>
>> >> > Only wait on a parent device during resume if the parent device is
>> >> > suspended.
>> >> >
>> >> > Consider three drivers, A, B, and C.  The parent of A is C, and C
>> >> > has async_suspend set.  On boot, C->power.completion is initialized
>> >> > to 0.
>> >> >
>> >> > During the first suspend:
>> >> > suspend_devices_and_enter(...)
>> >> >  dpm_resume(...)
>> >> >   device_suspend(A)
>> >> >   device_suspend(B) returns error, aborts suspend
>> >> >  dpm_resume_end(...)
>> >> >    dpm_resume(...)
>> >> >     device_resume(A)
>> >> >      dpm_wait(A->parent == C)
>> >> >       wait_for_completion(C->power.completion)
>> >> >
>> >> > The wait_for_completion will never complete, because
>> >> > complete_all(C->power.completion) will only be called from
>> >> > device_suspend(C) or device_resume(C), neither of which is called
>> >> > if suspend is aborted before C.
>> >>
>> >> This would work okay if C->power.completion had been initialized to the
>> >> completed state during boot, right?
>> >>
>> >> > After a successful suspend->resume cycle, where B doesn't abort
>> >> > suspend, C->power.completion is left in the completed state by the
>> >> > call to device_resume(C), and the same call path will work if B
>> >> > aborts suspend.
>> >> >
>> >> > Signed-off-by: Colin Cross <ccross@xxxxxxxxxxx>
>> >> > ---
>> >> >  drivers/base/power/main.c |    3 ++-
>> >> >  1 files changed, 2 insertions(+), 1 deletions(-)
>> >> >
>> >> > diff --git a/drivers/base/power/main.c b/drivers/base/power/main.c
>> >> > index cb784a0..e159910 100644
>> >> > --- a/drivers/base/power/main.c
>> >> > +++ b/drivers/base/power/main.c
>> >> > @@ -526,7 +526,8 @@ static int device_resume(struct device *dev, pm_message_t state, bool async)
>> >> >     TRACE_DEVICE(dev);
>> >> >     TRACE_RESUME(0);
>> >> >
>> >> > -   dpm_wait(dev->parent, async);
>> >> > +   if (dev->parent && dev->parent->power.status >= DPM_OFF)
>> >> > +           dpm_wait(dev->parent, async);
>> >> >     device_lock(dev);
>> >> >
>> >> >     dev->power.status = DPM_RESUMING;
>> >>
>> >> I think it would be better to change device_pm_init() and add a
>> >> complete_all().
>> >
>> > I agree.
>> That would work, and was my first solution, but it increases the
>> reliance on the completion variable being left completed between state
>> transitions, which is undocumented and unnecessary.
>
> In fact it is necessary, because dpm_wait() may be called by external code
> through device_pm_wait_for_dev() which is exported for a reason.  That may
> lead to problems analogous to the one you described if the completion
> variables are not completed initially.
>
>> It seems more straightforward to me to only wait on the parent if the parent is
>> suspended.
>>
>> > Who's writing the patch?
>> I'll write it if you still don't like this one.
>
> Yes, please.
OK - do you prefer it in dpm_prepare or device_pm_init?
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