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

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Date:	Thu, 10 Jan 2008 17:59:09 +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 Thursday, 10 of January 2008, Alan Stern wrote:
> On Thu, 10 Jan 2008, Rafael J. Wysocki wrote:
> 
> > 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.
> 
> Not unregister_dropped_devices()!

Sure, it will need locking around the check in while().

> > > > > 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.
> 
> So it counts as being indirectly invoked by a resume method.

Rather, by the resume core.  Technically, it's invoked by
enable_nonboot_cpus(), which is not a resume method literally.

Greetings,
Rafael
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