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

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Message-ID: <Pine.LNX.4.44L0.0801061734150.20032-100000@netrider.rowland.org>
Date:	Sun, 6 Jan 2008 17:39:28 -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:

> > 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

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