Is not locking task_lock in cgroup_fork() safe?

Tejun Heo tj at
Thu Oct 18 20:07:05 UTC 2012

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.

> 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.

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




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