Q: cgroup: Questions about possible issues in cgroup locking

[Mandeep Singh Baines]

Frederic Weisbecker (fweisbec at gmail.com) wrote:
> On Wed, Dec 21, 2011 at 02:08:48PM +0100, Oleg Nesterov wrote:
> > On 12/21, Frederic Weisbecker wrote:
> > > Hi,
> > >
> > > Starring at some parts of cgroups, I have a few questions:
> > >
> > > - Is cgroup_enable_task_cg_list()'s while_each_thread() safe
> > > against concurrent exec()? The leader may change in de_thread()
> > > and invalidate the test done in while_each_thread().
> > 
> > Yes. Oh, we need to do something with while_each_thread.
> 
> Would something like this work?
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index c0c5876..e002a00 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -2293,8 +2293,12 @@ extern bool current_is_single_threaded(void);
>  #define do_each_thread(g, t) \
>  	for (g = t = &init_task ; (g = t = next_task(g)) != &init_task ; ) do
>  
> -#define while_each_thread(g, t) \
> -	while ((t = next_thread(t)) != g)
> +#define while_each_thread(g, t)					\
> +	while (({						\
> +		struct task_struct *__prev = t;			\
> +		t = next_thread(t);				\
> +		t != __prev && t != g;				\

Hi,

Don't you still have an (highly unlikely) race if you exec
and then pthread_create()?

Instead of:

t != __prev && t != g;

How about:

t != t->group_leader;

Regards,
Mandeep

> +	}))
>  
>  static inline int get_nr_threads(struct task_struct *tsk)
>  {