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SubjectRe: [PATCH] cgroup_freezer: Freezing and task move race fix
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.
> 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.

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

Same thing here.

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

Again.

> 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)

Well, once more.

>
> 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

What do you think?

Tomasz
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