Re: [C/R v20][PATCH 15/96] cgroup freezer: Fix buggy resume test for tasks frozen with cgroup freezer

[Oren Laadan <orenl@cs.columbia.edu>]

Rafael J. Wysocki wrote:
> On Wednesday 17 March 2010, Oren Laadan wrote:
>> From: Matt Helsley <matthltc@us.ibm.com>
>>
>> When the cgroup freezer is used to freeze tasks we do not want to thaw
>> those tasks during resume. Currently we test the cgroup freezer
>> state of the resuming tasks to see if the cgroup is FROZEN.  If so
>> then we don't thaw the task. However, the FREEZING state also indicates
>> that the task should remain frozen.
>>
>> This also avoids a problem pointed out by Oren Ladaan: the freezer state
>> transition from FREEZING to FROZEN is updated lazily when userspace reads
>> or writes the freezer.state file in the cgroup filesystem. This means that
>> resume will thaw tasks in cgroups which should be in the FROZEN state if
>> there is no read/write of the freezer.state file to trigger this
>> transition before suspend.
>>
>> NOTE: Another "simple" solution would be to always update the cgroup
>> freezer state during resume. However it's a bad choice for several reasons:
>> Updating the cgroup freezer state is somewhat expensive because it requires
>> walking all the tasks in the cgroup and checking if they are each frozen.
>> Worse, this could easily make resume run in N^2 time where N is the number
>> of tasks in the cgroup. Finally, updating the freezer state from this code
>> path requires trickier locking because of the way locks must be ordered.
>>
>> Instead of updating the freezer state we rely on the fact that lazy
>> updates only manage the transition from FREEZING to FROZEN. We know that
>> a cgroup with the FREEZING state may actually be FROZEN so test for that
>> state too. This makes sense in the resume path even for partially-frozen
>> cgroups -- those that really are FREEZING but not FROZEN.
>>
>> Reported-by: Oren Ladaan <orenl@cs.columbia.edu>
>> Signed-off-by: Matt Helsley <matthltc@us.ibm.com>
>> Cc: Cedric Le Goater <legoater@free.fr>
>> Cc: Paul Menage <menage@google.com>
>> Cc: Li Zefan <lizf@cn.fujitsu.com>
>> Cc: Rafael J. Wysocki <rjw@sisk.pl>
>> Cc: Pavel Machek <pavel@ucw.cz>
>> Cc: linux-pm@lists.linux-foundation.org
> 
> Looks reasonable.
> 
> Is anyone handling that already or do you want me to take it to my tree?

Yes, please do.

Thanks !

Oren.

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