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

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Date:	Sun, 6 Jan 2008 17:21:50 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
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 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.

Alan Stern

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