Re: [PATCH v4] PM / Memory-hotplug: Avoid task freezing failures

["Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>]

On 11/21/2011 11:53 PM, Tejun Heo wrote:
> 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. :)
> 

Thank you too :-)

Regards,
Srivatsa S. Bhat

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