linux-kernel - Re: [PATCH] PM: Acquire device locks on suspend

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Date:	Sun, 6 Jan 2008 23:34:05 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Greg KH <gregkh@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Len Brown <lenb@...nel.org>, Ingo Molnar <mingo@...e.hu>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	pm list <linux-pm@...ts.linux-foundation.org>
Subject: Re: [PATCH] PM: Acquire device locks on suspend

On Sunday, 6 of January 2008, Alan Stern wrote:
> On Sun, 6 Jan 2008, Rafael J. Wysocki wrote:
> 
> > > Still, shouldn't we fail the removal of the device apart from giving the
> > > warning?
> > 
> > Actually, having thought about it a bit more, I don't see the point in
> > preventing the removal of the device from the list in device_pm_remove() if
> > we allow all of the operations in device_del() preceding it to be performed.
> 
> That's not the issue.  We _don't_ allow all of the operations in 
> device_del() preceding the call to device_pm_remove().  In particular, 
> the call to the device's driver's remove method will deadlock because 
> device_release_driver() always has to acquire dev->sem.
> 
> > Shouldn't we just take pm_sleep_rwsem in device_del() upfront and block on that
> > if locked?
> 
> 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.  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.

Rafael
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