Is not locking task_lock in cgroup_fork() safe?

Frederic Weisbecker fweisbec at gmail.com
Thu Oct 18 20:53:47 UTC 2012


2012/10/18 Tejun Heo <tj at kernel.org>:
> Hello, Frederic.
>
> On Thu, Oct 18, 2012 at 04:50:59PM +0200, Frederic Weisbecker wrote:
>> Ah right I was confused. Hmm, indeed we have a race here on
>> cgroup_fork(). How about using css_try_get() in cgroup_fork() and
>> refetch the parent's css until we succeed? This requires rcu_read_lock
>> though, and freeing the css_set under RCU.
>>
>> Don't know which is better.
>
> For now, I'll revert the patches and cc stable.  Let's think about
> improving it later.

Ok for reverting in cgroup_fork(). Is it necessary for the
cgroup_post_fork() thing? I don't immediately see any race involved
there.

>> Different problem but I really would like we sanitize the cgroup hooks
>> in fork. There is cgroup_fork(), cgroup_post_fork() which takes that
>> big css_set_lock, plus the big threadgroup lock... I hope we can
>> simplify the mess there.
>
> Oh yeah, I've been looking at that one too.  There are a few problems
> in that area.  I think all we need is clearing ->cgroups to NULL on
> copy_process() and all the rest can be moved to cgroup_post_fork().
> I'd also like to make it very explicit that migration can't happen
> before post_fork is complete.

Sounds right.

>
>> > I really don't know.  Why isn't it locking the threadgroup to begin
>> > with?
>>
>> No idea, sounds like something to fix.
>
> Alrighty.

Ok thanks.

> Thanks.
>
> --
> tejun


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