On Thursday, November 17, 2011, Srivatsa S. Bhat wrote:
> The lock_system_sleep() function is used in the memory hotplug code at
> several places in order to implement mutual exclusion with hibernation.
> However, this function tries to acquire the 'pm_mutex' lock using
> mutex_lock() and hence blocks in TASK_UNINTERRUPTIBLE state if it doesn't
> get the lock. This would lead to task freezing failures and hence
> hibernation failure as a consequence, even though the hibernation call path
> successfully acquired the lock.
>
> This patch fixes this issue by modifying lock_system_sleep() to use
> mutex_trylock() in a loop until the lock is acquired, instead of using
> mutex_lock(), in order to avoid going to uninterruptible sleep.
> Also, we use msleep() to avoid busy looping and breaking expectations
> that we go to sleep when we fail to acquire the lock.
> And we also call try_to_freeze() in order to cooperate with the freezer,
> without which we would end up in almost the same situation as mutex_lock(),
> due to uninterruptible sleep caused by msleep().
>
> v3: Tejun suggested avoiding busy-looping by adding an msleep() since
> it is not guaranteed that we will get frozen immediately.
>
> v2: Tejun pointed problems with using mutex_lock_interruptible() in a
> while loop, when signals not related to freezing are involved.
> So, replaced it with mutex_trylock().
>
> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
> ---
>
> include/linux/suspend.h | 37 ++++++++++++++++++++++++++++++++++++-
> 1 files changed, 36 insertions(+), 1 deletions(-)
>
> diff --git a/include/linux/suspend.h b/include/linux/suspend.h
> index 57a6924..0af3048 100644
> --- a/include/linux/suspend.h
> +++ b/include/linux/suspend.h
> @@ -5,6 +5,8 @@
> #include <linux/notifier.h>
> #include <linux/init.h>
> #include <linux/pm.h>
> +#include <linux/freezer.h>
> +#include <linux/delay.h>
> #include <linux/mm.h>
> #include <asm/errno.h>
>
> @@ -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);
The number here seems to be somewhat arbitrary. Is there any reason not to
use 100 or any other number?
> + }
> }
>
> static inline void unlock_system_sleep(void)
Rafael
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