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Date: Wed, 21 Dec 2011 18:56:35 +0100
From: Frederic Weisbecker <fweisbec@gmail.com>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>, Tejun Heo <tj@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Mandeep Singh Baines <msb@chromium.org>,
        Containers <containers@lists.linux-foundation.org>,
        Cgroups <cgroups@vger.kernel.org>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
        Paul Menage <paul@paulmenage.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Subject: Re: Q: cgroup: Questions about possible issues in cgroup locking
Message-ID: <20111221175632.GF17668@somewhere>
References: <20111221034334.GD17668@somewhere>
 <20111221130848.GA19679@redhat.com>
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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)
 {