[PATCH] cgroup_freezer: Freezing and task move race fix

[Matt Helsley]

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.

> 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

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

or:

Time	"bug" goes through these states		cgroup code checks for these states
-----------------------------------------------------------------------------------
|						is_freezing? Nope.
|						is_frozen? Nope.
|	freezing
|	frozen
|						<move>
V

Time	"bug" goes through these states		cgroup code checks for these states
-----------------------------------------------------------------------------------
|						is_freezing? Nope.
|	freezing
|						is_frozen? Nope.
|						<move>
|	frozen
V

or:

Time	"bug" goes through these states		cgroup code checks for these states
-----------------------------------------------------------------------------------
|						is_freezing? Nope.
|	freezing
|						is_frozen? Nope.
|	frozen
|						<move>
V

(even with 1 cpu/core)

Your patch only improves things in the sense that it works for the first
example. We need to prevent the latter cases as well.

Cheers,
	-Matt