From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751793Ab2ALAbT (ORCPT ); Wed, 11 Jan 2012 19:31:19 -0500 Received: from mail-gx0-f174.google.com ([209.85.161.174]:37290 "EHLO mail-gx0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751663Ab2ALAbR (ORCPT ); Wed, 11 Jan 2012 19:31:17 -0500 Date: Wed, 11 Jan 2012 16:31:02 -0800 From: Mandeep Singh Baines To: Oleg Nesterov Cc: Mandeep Singh Baines , Frederic Weisbecker , Li Zefan , Tejun Heo , LKML , Containers , Cgroups , KAMEZAWA Hiroyuki , Paul Menage , Andrew Morton , "Paul E. McKenney" Subject: Re: Q: cgroup: Questions about possible issues in cgroup locking Message-ID: <20120112003102.GB9511@google.com> References: <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> <20120106182535.GJ9511@google.com> <20120111160730.GA24556@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120111160730.GA24556@redhat.com> X-Operating-System: Linux/2.6.38.8-gg621 (x86_64) User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Oleg, Oleg Nesterov (oleg@redhat.com) wrote: > On 01/06, Mandeep Singh Baines wrote: > > > > Oleg Nesterov (oleg@redhat.com) wrote: > > > > > > > > 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: > > (I assume, you mean the lockless case) > Correct. > > * 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 > > Well yes, but it is not easy to exactly define what after/before > means in this case. > > > 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. > > I don't understand how this helps... and how this can work even > ignoring the barriers. > > OK, we have the main thream M and the sub-thread T, we are doing > > do { > do_something(t); > } while_each_thread(M, t, M); > > why we can't miss T if it does exec? > So for: struct task *M; /* assuming this is passed in to us */ struct task *L = M->group_leader; struct task *I = M; do { do_something(T); } while_each_thread(M, T, L); Here is my thinking. If some thread K does exec, you won't miss it because: 1) Ignoring the group_leader check, you'll visit K just by following next_thread(). That's the case today and is what you except when iterating over an rcu_list. 2) (t->group_leader == o) will fail iff t is the exec thread. Since we test t->group_leader before re-assigning it (t=next_thread()), the test will fail only after visiting the exec thread. So you'll visit the exec thread and then terminate the loop. I realize its a klutzy interface (requires 3 variables) but it seems correct (ignoring barriers) and meets all the requirements. I'm hoping it inspires a solution which is less klutzy and meet its all the requirements. Regards, Mandeep > Oleg. >