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

Alan Stern
2008-01-06T22:39:28Z

On Sun, 6 Jan 2008, Rafael J. Wysocki wrote:

> > No -- the whole idea here is to print an error message in the system
> > log if a driver's resume method tries to call device_del(). Deadlock
> > is unavoidable in this case, but at least we'll know which driver is
> > guilty.
>
> Still, if we do that, we won't need to acquire dev->sem in device_pm_remove()
> any more.

There's a window in lock_all_devices() when dpm_list_mtx isn't held.
We don't want device_pm_remove() taking an already-locked device off
the dpm_locked list at that time. So we do need to acquire dev->sem in
device_pm_remove().

> Apart from this, by acqiring pm_sleep_rwsem for reading in
> device_del() we can prevent a suspend from starting while the device is being
> removed.
>
> Consider, for example, the scenario possible with the $subject patch:
> - device_del() starts and notices pm_sleep_rwsem unlocked, so the warning is
> not printed
> - it proceeds and everything before device_pm_remove() succeeds
> - now, device_suspend() is called and locks dev->sem
> - device_del() calls device_pm_remove() and blocks on that with the device
> partialy removed
> I think we should prevent this from happening.

I don't see anything wrong with it. All that will happen is that the
removal will start before the suspend and finish after the resume.

Alan Stern