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

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Date:	Wed, 9 Jan 2008 23:14:48 +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 Tue, 8 Jan 2008, Rafael J. Wysocki wrote:
> 
> > Appended is what I managed to put together today.
> > 
> > It probably still has some problems, but I'm not seeing them right now (too
> > tired).  At least, it doesn't break my system. ;-)
> > 
> > Please review.
> 
> Okay, this seems to be better.  I like the way the complicated tests 
> are all localized in power/main.c.
> 
> 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.

> The same is true for dpm_suspend().  Once all the device have been 
> locked, there shouldn't be any other tasks accessing the dpm lists.  
> Hence there should be no need to protect the list.

Except for against theoretical races with device_pm_schedule_removal().

> Which reminds me, the kerneldoc for device_pm_schedule_removal() is 
> inaccurate.  The routine always just moves the device to dpm_destroy 
> list for later processing.

Correct.

> 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).

> It looks good.

Thanks for the review.

I'll fix the comments and repost the patch from scratch for merging in a
separate thread.

Greetings,
Rafael
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