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

[Rafael J. Wysocki]

On Tuesday 23 March 2010, Oren Laadan wrote:
> 
> Rafael J. Wysocki wrote:
> > On Wednesday 17 March 2010, Oren Laadan wrote:
> >> From: Matt Helsley <matthltc at 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 at cs.columbia.edu>
> >> Signed-off-by: Matt Helsley <matthltc at us.ibm.com>
> >> Cc: Cedric Le Goater <legoater at free.fr>
> >> Cc: Paul Menage <menage at google.com>
> >> Cc: Li Zefan <lizf at cn.fujitsu.com>
> >> Cc: Rafael J. Wysocki <rjw at sisk.pl>
> >> Cc: Pavel Machek <pavel at ucw.cz>
> >> Cc: linux-pm at 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.

Applied.

Rafael
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