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

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

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