linux-kernel - Re: [PATCH] PM: Prevent waiting forever on asynchronous resume after abort

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Date:	Thu, 2 Sep 2010 15:45:19 -0700
From:	Colin Cross <ccross@...roid.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>, linux-kernel@...r.kernel.org,
	linux-pm@...ts.linux-foundation.org, Pavel Machek <pavel@....cz>,
	Len Brown <len.brown@...el.com>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Randy Dunlap <randy.dunlap@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] PM: Prevent waiting forever on asynchronous resume after abort

On Thu, Sep 2, 2010 at 2:34 PM, Alan Stern <stern@...land.harvard.edu> wrote:
> On Thu, 2 Sep 2010, Colin Cross wrote:
>
>> That would work, but I still don't see why it's better.  With either
>> of your changes, the power.completion variable is storing state, and
>> not just used for notification.  However, the exact meaning of that
>> state is unclear, especially during the transition from an aborted
>> suspend to resume, and the state is duplicating power.status.  Setting
>> it to complete in dpm_prepare is especially confusing, because at that
>> point nothing is completed, it hasn't even been started.
>
> The state being waited for varies from time to time and is only
> partially related to power.status.  Instead of using a completion I
> suppose we could have used a new "transition_complete" variable
> together with a waitqueue.  Would you prefer that?  It's effectively
> the same thing as a completion, but without the nice packaging already
> provided by the kernel.
No, that doesn't change anything.  What I'd prefer to see is a
wait_for_condition on the desired state of the parent.  As is,
power.completion means one thing during suspend (the device has
started, but not finished, suspending), and a different thing during
resume (the device has not finished resuming, and may not have started
resuming).  That difference is exactly what caused the bug - the
completion has to be set on init so that it is set before the device
starts suspend.

I'll send the complete_all on init patch, as it's the only way to fix
the problem given the current implementation of dpm_wait.
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