linux-kernel - Re: [RFC][PATCH 0/7] PM: Implement autosleep and "wake locks", take2

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Date:	Fri, 24 Feb 2012 20:43:28 -0800
From:	Arve Hjønnevåg <arve@...roid.com>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	John Stultz <john.stultz@...aro.org>,
	Linux PM list <linux-pm@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Magnus Damm <magnus.damm@...il.com>, markgross@...gnar.org,
	Matthew Garrett <mjg@...hat.com>,
	Greg KH <gregkh@...uxfoundation.org>,
	Brian Swetland <swetland@...gle.com>,
	Neil Brown <neilb@...e.de>,
	Alan Stern <stern@...land.harvard.edu>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>
Subject: Re: [RFC][PATCH 0/7] PM: Implement autosleep and "wake locks", take2

On Fri, Feb 24, 2012 at 3:21 PM, Rafael J. Wysocki <rjw@...k.pl> wrote:
> On Friday, February 24, 2012, Srivatsa S. Bhat wrote:
>> On 02/24/2012 03:02 AM, Rafael J. Wysocki wrote:
>>
>> > On Thursday, February 23, 2012, Rafael J. Wysocki wrote:
>> >> On Thursday, February 23, 2012, Srivatsa S. Bhat wrote:
>> >>> On 02/23/2012 03:40 AM, Rafael J. Wysocki wrote:
> [...]
>> >>>
>> >>> By the way, I am just curious.. how difficult will this make it for userspace
>> >>> to disable autosleep? I mean, would a trylock mean that the user has to keep
>> >>> fighting until he finally gets a chance to disable autosleep?
>> >>
>> >> That's a good point, so I think it may be a good idea to do
>> >> mutex_lock_interruptible() in pm_autosleep_set_state() instead.
>> >
>> > Now that I think of it, perhaps it's a good idea to just make
>> > pm_autosleep_lock() do mutex_lock_interruptible() _and_ make
>> > pm_autosleep_set_state() use pm_autosleep_lock().
>> >
>> > What do you think?
>> >
>>
>>
>> Well, I don't think mutex_lock_interruptible() would help us much..
>> Consider what would happen, if we use it:
>>
>> * pm-suspend got initiated as part of autosleep. Acquired autosleep lock.
>> * Userspace is about to get frozen.
>> * Now, the user tries to write "off" to autosleep. And hence, he is waiting
>>   for autosleep lock, interruptibly.
>> * The freezer sent a fake signal to all userspace processes and hence
>>   this process also got interrupted.. it is no longer waiting on autosleep
>>   lock - it got the signal and returned, and got frozen.
>>   (And when the userspace gets thawed later, this process won't have the
>>    autosleep lock - which is a different (but yet another) problem).
>>
>> So ultimately the only thing we achieved is to ensure that freezing of
>> userspace goes smoothly. But the user process could not succeed in
>> disabling autosleep. Of course we can work around that by having the
>> mutex_lock_interruptible() in a loop and so on, but that gets very
>> ugly pretty soon.
>>
>> So, I would suggest the following solution:
>>
>> We want to achieve 2 things here:
>>  a. A user process trying to write to /sys/power/state or
>>     /sys/power/autosleep should not cause freezing failures.
>>  b. When a user process writes "off" to autosleep, the suspend/hibernate
>>     attempt that is on-going, if any, must be immediately aborted, to give
>>     the user the feeling that his preference has been noticed and respected.
>>
>> And to achieve this, we note that a user process can write "off" to autosleep
>> only until the userspace gets frozen. No chance after that.
>>
>> So, let's do this:
>> 1. Drop the autosleep lock before entering pm-suspend/hibernate.
>> 2. This means, a user process can get hold of this lock and successfully
>>    disable autosleep a moment after we initiated suspend, but before userspace
>>    got frozen fully.
>> 3. So, to respect the user's wish, we add a check immediately after the
>>    freezing of userspace is complete - we check if the user disabled autosleep
>>    and bail out, if he did. Otherwise, we continue and suspend the machine.
>>
>> IOW, this is like hitting 2 birds with one stone ;-)
>> We don't hold autosleep lock throughout suspend/hibernate, but still react
>> instantly when the user disables autosleep. And of course, freezing of tasks
>> won't fail, ever! :-)
>
> Well, you essentially are postulating to restore the "interface" wakeup source
> that was present in the previous version of this patch and that I dropped in
> order to simplify the code.
>
> I guess I can do that ...
>

If this wakeup source is reported as active whenever user-space has
not requested suspend that would be useful in the stats. It does not
look like your original patch did this however, but you could have a
main wakeup-source that you release when any form of suspend is
requested and activate when turning off auto suspend or returning from
a one-shot suspend operation.

-- 
Arve Hjønnevåg
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