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

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Date:	Thu, 10 Jan 2008 00:29:25 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Johannes Berg <johannes@...solutions.net>,
	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 Wednesday, 9 of January 2008, Alan Stern wrote:
> On Wed, 9 Jan 2008, Rafael J. Wysocki wrote:
> 
> > > In dpm_resume() you shouldn't need to use dpm_list_mtx at all, because
> > > the list_move_tail() comes before the resume_device().  It's the same
> > > as in dpm_power_up().
> > 
> > Still, device_pm_schedule_removal() can (in theory) be called concurrently
> > with dpm_resume() by another thread and this might corrupt the list without
> > the locking.
> 
> Any thread doing that would be in violation of the restrictions you're 
> going to add to the kerneldoc for destroy_suspended_device().
> 
> However the overhead for the locking isn't critical.  There won't be
> any contention (if everything is working right) and it isn't a hot path
> anyway.  So you can leave the extra locking in if you want.  But then
> you should put it in all the routines where the lists get manipulated,
> not just some of them.  That is: device_power_down(), dpm_power_up(),
> and even unregister_dropped_devices().

Except for those run on one CPU with interrupts disabled, I think.

> > > Also, the kerneldoc for destroy_suspended_device() should contain an 
> > > extra paragraph warning that the routine should never be called except 
> > > within the scope of a system sleep transition.  In practice this means 
> > > it has to be directly or indirectly invoked by a suspend or resume 
> > > method.
> > 
> > Or by a CPU hotplug notifier (that will be the majority of cases, IMO).
> 
> In your patch the call is made in response to a CPU_UP_CANCELED_FROZEN
> notification.  Isn't it true that this notification is issued only as
> part of a system sleep transition?

Yes, it is.

> We wouldn't want to allow destroy_suspended_device() to be called when an
> arbitrary CPU hotplug notification occurs.

Of course.

Greetings,
Rafael
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