Re: Q: cgroup: Questions about possible issues in cgroup locking

[Frederic Weisbecker <fweisbec-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

On Wed, Dec 21, 2011 at 11:01:02AM -0800, Mandeep Singh Baines wrote:
> Frederic Weisbecker (fweisbec-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org) 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()?

I'm not sure what you mean.

> 
> Instead of:
> 
> t != __prev && t != g;
> 
> How about:
> 
> t != t->group_leader;

That might work too but we need a pair of memory barriers.