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

[Alan Stern]

On Mon, 7 Jan 2008, Rafael J. Wysocki wrote:

> On Monday, 7 of January 2008, Alan Stern wrote:
> > On Mon, 7 Jan 2008, Rafael J. Wysocki wrote:
> > 
> > > Please see the patch at: http://lkml.org/lkml/2008/1/6/298 .  It represents my
> > > current idea about how to do that.
> > 
> > It has some problems.
> > 
> > First, note that the list manipulations in dpm_suspend(), 
> > device_power_down(), and so on aren't protected by dpm_list_mtx.  So 
> > your patch could corrupt the list pointers.
> 
> Yes, they need the locking.  I have overlooked that, mostly because the locking
> was removed by gregkh-driver-pm-acquire-device-locks-prior-to-suspending.patch
> too (because you assumed there woundn't be any need to remove a device during
> a suspend, right?).

Right.

> > Are you assuming that no other threads can be running at this time?
> 
> No, I'm not.
> 
> > Note also that device_pm_destroy_suspended() does up(&dev->sem), but it 
> > doesn't know whether or not dev->sem was locked to begin with.
> 
> Do you mean it might have been released already by another thread
> calling device_pm_destroy_suspended() on the same device?

I was thinking that it might be called before lock_all_devices().

However let's ignore that possibility and simplify the discussion by 
assuming that destroy_suspended_device() is never called except by a 
suspend or resume method for that device or one of its ancestors.  
(This still leaves the possibility that it might get called by mistake 
during a runtime suspend or resume...)

> > Do you want to rule out the possibility of a driver's suspend or remove 
> > methods calling destroy_suspended_device() on its own device?  With 
> > your synchronous approach, this would mean that the suspend/resume 
> > method would indirectly end up calling the remove method.  This is 
> > dangerous at best; with USB it would be a lockdep violation.  With an 
> > asynchronous approach, on the other hand, this wouldn't be a problem.
> 
> Well, the asynchronous apprach has the problem that the device may end up
> on a wrong list when removed by one of the .suspend() callbacks (and I don't
> see how to avoid that without extra complexity).  Perhaps that's something we
> can live with, though.

The same problem affects the synchronous approach.  We can fix it by
having dpm_suspend() do the list_move() before calling
suspend_device().  Then if the suspend fails move the device back.

> One more question: is there any particular reason not to call
> device_pm_remove() at the beginning of device_del()?

I think it's done this way to avoid having a window where the device 
isn't on a PM list and is still owned by the bus and the driver.  But 
if a suspend occurs during that window, it shouldn't matter that the 
device will be left unsuspended.  After all, the same thing would have 
happened if the suspend occurred after bus_remove_device().

So no, there shouldn't be a problem with moving the call.

Alan Stern

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