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