Re: [PATCH 1/7] cgroup: cgroup_subsys->fork() should be called after the task is added to css_set

[Oleg Nesterov <oleg@redhat.com>]

Hi Tejun,

On 10/21, Tejun Heo wrote:
>
> On Sun, Oct 21, 2012 at 09:11:41PM +0200, Oleg Nesterov wrote:
>
> > And it seems that fork() can race with cgroup iterator. post_fork
> > will notice use_task_css_set_links, but until then the child belongs
> > to the parent's css and it is not "visible" to iterator (and right
> > after cgroup_fork() it is not visible to do_each_thread() if
> > use_task_css_set_links is not set).
> >
> > For example. Suppose that the child migrates to another cgroup after
> > copy_process() makes it visible to the user-space. Then update_if_frozen
> > sets CGROUP_FROZEN (again, cgroup_iter_next do not see this child).
> >
> > Now, post_fork calls freezer_fork() and hits BUG_ON(CGROUP_FROZEN).
> >
> > But again, I do not blame this patch.
>
> I'm planning to update it to,
>
> * Clear ->cgroup to %NULL during copy_process().

I completely agree. new_child->cgroups copied from parent looks simply
strange until post_fork. If nothing else, the new task is still under
construction by the time cgroup_fork() is called.

> > I am starting to think again about a big-rw-lock around copy_process.
> > Recently I tried to add one around dup_mmap for uprobes, but perhaps
> > cgroups can use it too...
>
> If some other subsystems need it, maybe just make threadgroup locking
> coarser?

What do you mean?

> I *think* I can make cgroup work correctly without a agiant
> rwlock

Yes, probably cgroup doesn't really need it. Although we could probably
kill signal->group_rwsem, but this is minor and "write-lock" will be much
slower.

Oleg.