Re: [PATCH] PM: Acquire device locks on suspend

Rafael J. Wysocki
2008-01-09T23:27:26Z

On Wednesday, 9 of January 2008, Alan Stern wrote:
> On Wed, 9 Jan 2008, Rafael J. Wysocki wrote:
>
> > > In dpm_resume() you shouldn't need to use dpm_list_mtx at all, because
> > > the list_move_tail() comes before the resume_device(). It's the same
> > > as in dpm_power_up().
> >
> > Still, device_pm_schedule_removal() can (in theory) be called concurrently
> > with dpm_resume() by another thread and this might corrupt the list without
> > the locking.
>
> Any thread doing that would be in violation of the restrictions you're
> going to add to the kerneldoc for destroy_suspended_device().
>
> However the overhead for the locking isn't critical. There won't be
> any contention (if everything is working right) and it isn't a hot path
> anyway. So you can leave the extra locking in if you want. But then
> you should put it in all the routines where the lists get manipulated,
> not just some of them. That is: device_power_down(), dpm_power_up(),
> and even unregister_dropped_devices().

Except for those run on one CPU with interrupts disabled, I think.

> > > Also, the kerneldoc for destroy_suspended_device() should contain an
> > > extra paragraph warning that the routine should never be called except
> > > within the scope of a system sleep transition. In practice this means
> > > it has to be directly or indirectly invoked by a suspend or resume
> > > method.
> >
> > Or by a CPU hotplug notifier (that will be the majority of cases, IMO).
>
> In your patch the call is made in response to a CPU_UP_CANCELED_FROZEN
> notification. Isn't it true that this notification is issued only as
> part of a system sleep transition?

Yes, it is.

> We wouldn't want to allow destroy_suspended_device() to be called when an
> arbitrary CPU hotplug notification occurs.

Of course.

Greetings,
Rafael