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    SubjectRe: [RFC][PATCH 0/7] PM: Implement autosleep and "wake locks", take2
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    On Fri, Feb 24, 2012 at 3:21 PM, Rafael J. Wysocki <rjw@sisk.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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