From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mandeep Singh Baines Subject: Re: Q: cgroup: Questions about possible issues in cgroup locking Date: Fri, 6 Jan 2012 10:25:35 -0800 Message-ID: <20120106182535.GJ9511__45628.8680121817$1325874368$gmane$org@google.com> References: <20111221034334.GD17668@somewhere> <20111221130848.GA19679@redhat.com> <20111221175632.GF17668@somewhere> <20111221190102.GE13529@google.com> <20111221190817.GI17668@somewhere> <20111221192413.GF13529@google.com> <20111221200422.GJ17668@somewhere> <20111222153004.GA30522@redhat.com> <20120104193614.GF9511@google.com> <20120106152356.GA23995@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <20120106152356.GA23995-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: containers-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org Errors-To: containers-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org To: Oleg Nesterov Cc: Frederic Weisbecker , Mandeep Singh Baines , LKML , Paul Menage , Tejun Heo , Cgroups , Andrew Morton , "Paul E. McKenney" , Containers List-Id: containers.vger.kernel.org Oleg Nesterov (oleg-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org) wrote: > On 01/04, Mandeep Singh Baines wrote: > > > > Oleg Nesterov (oleg-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org) wrote: > > > On 12/21, Frederic Weisbecker wrote: > > > > > > > > On Wed, Dec 21, 2011 at 11:24:13AM -0800, Mandeep Singh Baines wrote: > > > > > > > > > > If you call exec from a thread other than g, g is now unlinked. So > > > > > "t != g" will always be true. If you then pthread_create, you now > > > > > have two threads so "t != __prev" will also always be true. So > > > > > you now have an infinite loop. > > > > > > > > Oh you're right. > > > > > > > > But then we can't use t != t->group_leader because that assumes while_each_thread() > > > > started on the leader. > > > > > > Yes, this can't work. > > > > > > Besides, we need more burriers to rely on the ->group_leader check. > > > > > > See http://marc.info/?t=127688987300002 > > > > > > > I went through the thread. Were there any other concerns other than > > requiring that you start with the group_leader and the barrier? > > > > You could modify zap_other_threads to start with the group leader by > > skipping p: > > > > if (p == t) > > continue; > > Yes, we can but there are other while_each_thread(nonleader) users. > Yes we can fix them too but this looks a bit ugly and we need to > change while_each_thread() anyway. And I do not see why this change > will be simpler if we restrict it to group_leader. > > And note that zap_other_threads() is fine in any case, it is called > under ->siglock. > > > > in particular, http://marc.info/?l=linux-kernel&m=127714242731448 > > > I think this should work, but then we should do something with the > > > users like zap_threads(). > > > > > > > With that patch, won't you potentially miss the exec thread if an exec > > occurs while you're iterating over the list? Is that OK? > > Of course it is not OK ;) Note the "we should do something with" above. > So requirements should be something like this: * Any task alive for the duration of the iteration MUST be visited * No task should be visited more than once * Any task born or exiting after starting the iteration MAY be skipped * You can start at any task in the thread group Would something like this work: #define while_each_thread(g, t, o) \ while (t->group_leader == o && (t = next_thread(t)) != g) Where o should have the value of g->group_leader. Regards, Mandeep > Oleg. >