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    SubjectRe: [PATCH v4] PM / Memory-hotplug: Avoid task freezing failures
    On Mon, Nov 21, 2011 at 11:42:54PM +0530, 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.
    >
    > But it is to be noted that, since this task tries to acquire pm_mutex, if it
    > blocks due to this, we are *100% sure* that this task is not going to run
    > as long as hibernation sequence is in progress, since hibernation releases
    > 'pm_mutex' only at the very end, when everything is done.
    > And this means, this task is going to be anyway blocked for much more longer
    > than what the freezer intends to achieve; which means, freezing and thawing
    > doesn't really make any difference to this task!
    >
    > So, to fix freezing failures, we just ask the freezer to skip freezing this
    > task, since it is already "frozen enough".
    >
    > But instead of calling freezer_do_not_count() and freezer_count() as it is,
    > we use only the relevant parts of those functions, because restrictions
    > such as 'the task should be a userspace one' etc., might not be relevant in
    > this scenario.
    >
    > v4: Redesigned the whole fix, to ask the freezer to skip freezing the task
    > which is blocked trying to acquire 'pm_mutex' lock.
    >
    > 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@linux.vnet.ibm.com>

    Acked-by: Tejun Heo <tj@kernel.org>

    Thanks a lot. :)

    --
    tejun


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