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SubjectRe: [PATCH v3] PM/Memory-hotplug: Avoid task freezing failures
Hello, Srivatsa.

On Thu, Nov 17, 2011 at 02:00:50PM +0530, Srivatsa S. Bhat wrote:
> @@ -380,7 +382,40 @@ static inline void unlock_system_sleep(void) {}
>
> static inline void lock_system_sleep(void)
> {
> - mutex_lock(&pm_mutex);
> + /*
> + * "To sleep, or not to sleep, that is the question!"
> + *
> + * We should not use mutex_lock() here because, in case we fail to
> + * acquire the lock, it would put us to sleep in TASK_UNINTERRUPTIBLE
> + * state, which would lead to task freezing failures. As a
> + * consequence, hibernation would fail (even though it had acquired
> + * the 'pm_mutex' lock).
> + * Using mutex_lock_interruptible() in a loop is not a good idea,
> + * because we could end up treating non-freezing signals badly.
> + * So we use mutex_trylock() in a loop instead.
> + *
> + * Also, we add try_to_freeze() to the loop, to co-operate with the
> + * freezer, to avoid task freezing failures due to busy-looping.
> + *
> + * But then, since it is not guaranteed that we will get frozen
> + * rightaway, we could keep spinning for some time, breaking the
> + * expectation that we go to sleep when we fail to acquire the lock.
> + * So we add an msleep() to the loop, to dampen the spin (but we are
> + * careful enough not to sleep for too long at a stretch, lest the
> + * freezer whine and give up again!).
> + *
> + * Now that we no longer busy-loop, try_to_freeze() becomes all the
> + * more important, due to a subtle reason: if we don't cooperate with
> + * the freezer at this point, we could end up in a situation very
> + * similar to mutex_lock() due to the usage of msleep() (which sleeps
> + * uninterruptibly).
> + *
> + * Phew! What a delicate balance!
> + */
> + while (!mutex_trylock(&pm_mutex)) {
> + try_to_freeze();
> + msleep(10);
> + }

I tried to think about a better way to do it but couldn't, so I
suppose this is what we should go with for now. That said, I think
the comment is a bit too umm.... verbose. What we want here is
freezable but !interruptible mutex_lock() and while I do appreciate
the detailed comment, I think it makes it look a lot more complex than
it actually is. Other than that,
Acked-by: Tejun Heo <tj@kernel.org>

Thank you very much.

--
tejun


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