[Devel] Re: [PATCH] cgroup_freezer: Freezing and task move race fix

[Matt Helsley]

On Thu, Aug 12, 2010 at 02:53:25AM +0200, Tomasz Buchert wrote:
> Matt Helsley a écrit :
> > On Wed, Aug 11, 2010 at 09:35:43AM +0200, Tomasz Buchert wrote:
> >> Matt Helsley a écrit :
> >>> On Wed, Aug 11, 2010 at 12:18:44AM +0200, Tomasz Buchert wrote:
> >>>> Matt Helsley a écrit :
> >>>>> On Tue, Aug 10, 2010 at 09:53:21PM +0200, Tomasz Buchert wrote:
> >>>>>> Writing 'FROZEN' to freezer.state file does not
> >>>>>> forbid the task to be moved away from its cgroup
> >>>>>> (for a very short time). Nevertheless the moved task
> >>>>>> can become frozen OUTSIDE its cgroup which puts
> >>>>>> discussed task in a permanent 'D' state.
> >>>>>>
> >>>>>> This patch forbids migration of either FROZEN
> >>>>>> or FREEZING tasks.
> >>>>>>
> >>>>>> This behavior was observed and easily reproduced on
> >>>>>> a single core laptop. Program and instructions how
> >>>>>> to reproduce the bug can be fetched from:
> >>>>>> http://pentium.hopto.org/~thinred/repos/linux-misc/freezer_bug.c
> >>>>> Thanks for the report and the test code.
> >>>>>
> >>>>> I'm will try to reproduce this race in the next few hours and analyze
> >>>>> it since I'm not sure the patch really fixes the race -- it may only
> >>>>> make the race trigger less frequently.
> >>>>>
> >>>>> At the very least the patch won't break the current code since it's
> >>>>> essentially a more-strict version of is_task_frozen_enough() -- it lets
> >>>>> fewer tasks attach/detach to/from frozen cgroups.
> >>>>>
> >>>>> Cheers,
> >>>>> 	-Matt Helsley
> >>>> Hi Matt!
> >>>> I am a novice if it comes to the kernel and I find the cgroup_freezer
> >>>> code especially complicated, so definetely this may be not enough to fix that.
> >>>> Notice also that if you uncomment the line 55 in my testcase this will also
> >>>> trigger the race! This, however, makes sense since process may not be in the cgroup anymore
> >>>> and consequently won't be thawed.
> >>> OK, I triggered it with that. Interesting.
> >>>
> >> Good!
> >>
> >>>> I think that this patch fixes these problems because it does the flag checking in a right order:
> >>>> first freezing() is used and then frozen() which assures (see frozen_process()) that
> >>>> the race will not happen. Right? :)
> >>> I see what you mean. It still seems like it wouldn't actually fix the race -- just make it
> >>> harder to trigger. I think you're saying this is what happens without the patch:
> >>>
> >>> Time	"bug" goes through these states		cgroup code checks for these states
> >>> -----------------------------------------------------------------------------------
> >>> |	freezing
> >>> |						is_frozen? Nope.
> >>> |	frozen
> >>> |						is_freezing? Nope.
> >>> |						<move>
> >>> V
> >>>
> >> My first scenario was a bit different:
> >> Time	"bug" goes through these states		cgroup code checks for these states
> >> -----------------------------------------------------------------------------------
> >> |	freezing
> >> |						is_task_frozen_enough? Nope.
> >> |						<move>
> >> |	frozen
> >> V
> >> but the problem is the same.
> > 
> > I think I found a bug in the cgroup freezer which your patch fixes.
> > However I don't think it's a race.
> > 
> > /* Task is frozen or will freeze immediately when next it gets woken */
> > static bool is_task_frozen_enough(struct task_struct *task)
> > {
> >         return frozen(task) ||
> >                 (task_is_stopped_or_traced(task) && freezing(task));
> > }
> > 
> > So it will only refuse to attach freezing tasks which have been stopped
> > or traced! Yet for attach we need to refuse to move _any_ freezing tasks.
> > 
> > Though stopped/traced _is_ relevant to the cgroup freezer state itself.
> > If we uses frozen(task) || freezing(task) to determine whether a cgroup
> > is frozen then it would be possible for the task to still be active
> > when the cgroup is finally reported FROZEN. However that's not possible
> > when the task is stopped/traced *and* freezing since even if woken it
> > won't exit the kernel before entering the refrigerator.
> > 
> >>> But, without having carefully investigated the details, this could just as easily happen
> >>> with your patch:
> >>>
> >>> Time	"bug" goes through these states		cgroup code checks for these states
> >>> -----------------------------------------------------------------------------------
> >>> |						is_freezing? Nope.
> >>> |						is_frozen? Nope.
> >>> |	freezing
> >>> |						<move>
> >>> |	frozen
> >>> V
> >>>
> >> This can't happen as far as I know because there is cgroup_lock around the code in freezer_write()
> >> and freezer_can_attach().
> >> The task can't enter 'freezing' state when can_attach is executed.
> > 
> > You're right, cgroup_mutex should protect against that. However presumably
> > that same logic applies to the first case. So again I don't think the bug
> > is a race.
> > 
> > <snipped the remaining cases which should be the same>
> > 
> > At this point I think the bug description in your patch needs to change
> > but the patch itself is perfect. Assuming you agree with my assessment
> > of the bug I think the process is: you'll rewrite the description, add -stable
> > maintaners to your current Cc's (since this bug is simple, exists in previous
> > versions, and is somewhat nasty), add:
> > 
> > Reviewed-by: Matt Helsley <matthltc at us.ibm.com>
> > Tested-by: Matt Helsley <matthltc at us.ibm.com>
> > 
> > and send it to Andrew Morton. Hopefully then (if not before) Paul and Li
> > will ack it.
> > 
> > Thanks!
> > 
> > Cheers,
> > 	-Matt Helsley
> 
> I agree with your assessment, Matt. The wrong the definition of being 'frozen enough'
> allows a task to sneak out of its freezing cgroup.
> The problem is that I found another, closely related bug. Right now, I have
> a new queue of 3 patches to fix both bugs in a more general setting. They are not well tested
> yet but I'm fairly confident that they do the right thing. I'm going to test them tomorrow.
> Do you still want me to push the first patch? I propose to let me work a bit on
> the new patches and prepare the code for the incoming fix to the related bug.
> What is your opinion?

OK, I'll have a look at the 3 new patches and your test(s) then we can discuss
what to do.

Cheers,
	-Matt
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