From: Mandeep Singh Baines <msb-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org> To: Oleg Nesterov <oleg-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> Cc: Frederic Weisbecker <fweisbec-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>, Mandeep Singh Baines <msb-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org>, LKML <linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>, Paul Menage <paul-inf54ven1CmVyaH7bEyXVA@public.gmane.org>, Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>, Cgroups <cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>, Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>, "Paul E. McKenney" <paulmck-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>, Containers <containers-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org> Subject: Re: Q: cgroup: Questions about possible issues in cgroup locking Date: Wed, 11 Jan 2012 16:31:02 -0800 [thread overview] Message-ID: <20120112003102.GB9511@google.com> (raw) In-Reply-To: <20120111160730.GA24556-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> Hi Oleg, Oleg Nesterov (oleg-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org) wrote: > On 01/06, Mandeep Singh Baines wrote: > > > > Oleg Nesterov (oleg-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org) 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. >
WARNING: multiple messages have this Message-ID (diff)
From: Mandeep Singh Baines <msb@chromium.org> To: Oleg Nesterov <oleg@redhat.com> Cc: Mandeep Singh Baines <msb@chromium.org>, Frederic Weisbecker <fweisbec@gmail.com>, Li Zefan <lizf@cn.fujitsu.com>, Tejun Heo <tj@kernel.org>, LKML <linux-kernel@vger.kernel.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 Date: Wed, 11 Jan 2012 16:31:02 -0800 [thread overview] Message-ID: <20120112003102.GB9511@google.com> (raw) In-Reply-To: <20120111160730.GA24556@redhat.com> 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. >
next prev parent reply other threads:[~2012-01-12 0:31 UTC|newest] Thread overview: 73+ messages / expand[flat|nested] mbox.gz Atom feed top 2011-12-21 3:43 Q: cgroup: Questions about possible issues in cgroup locking Frederic Weisbecker 2011-12-21 3:43 ` Frederic Weisbecker 2011-12-21 13:08 ` Oleg Nesterov 2011-12-21 13:08 ` Oleg Nesterov 2011-12-21 17:56 ` Frederic Weisbecker 2011-12-21 17:56 ` Frederic Weisbecker 2011-12-21 19:01 ` Mandeep Singh Baines 2011-12-21 19:01 ` Mandeep Singh Baines [not found] ` <20111221190102.GE13529-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> 2011-12-21 19:08 ` Frederic Weisbecker 2011-12-21 19:08 ` Frederic Weisbecker 2011-12-21 19:24 ` Mandeep Singh Baines 2011-12-21 19:24 ` Mandeep Singh Baines [not found] ` <20111221192413.GF13529-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> 2011-12-21 20:04 ` Frederic Weisbecker 2011-12-21 20:04 ` Frederic Weisbecker 2011-12-21 20:04 ` Frederic Weisbecker 2011-12-22 15:30 ` Oleg Nesterov 2011-12-22 15:30 ` Oleg Nesterov [not found] ` <20111222153004.GA30522-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> 2012-01-04 19:36 ` Mandeep Singh Baines 2012-01-04 19:36 ` Mandeep Singh Baines 2012-01-04 19:36 ` Mandeep Singh Baines [not found] ` <20120104193614.GF9511-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> 2012-01-06 15:23 ` Oleg Nesterov 2012-01-06 15:23 ` Oleg Nesterov [not found] ` <20120106152356.GA23995-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> 2012-01-06 18:25 ` Mandeep Singh Baines 2012-01-06 18:25 ` Mandeep Singh Baines 2012-01-06 18:25 ` Mandeep Singh Baines [not found] ` <20120106182535.GJ9511-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> 2012-01-11 16:07 ` Oleg Nesterov 2012-01-11 16:07 ` Oleg Nesterov [not found] ` <20120111160730.GA24556-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> 2012-01-12 0:31 ` Mandeep Singh Baines [this message] 2012-01-12 0:31 ` Mandeep Singh Baines [not found] ` <20120112003102.GB9511-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> 2012-01-12 17:07 ` Oleg Nesterov 2012-01-12 17:07 ` Oleg Nesterov 2012-01-12 17:57 ` Mandeep Singh Baines 2012-01-12 17:57 ` Mandeep Singh Baines [not found] ` <20120112175725.GD9511-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> 2012-01-13 15:20 ` Oleg Nesterov 2012-01-13 15:20 ` Oleg Nesterov [not found] ` <20120113152010.GA19215-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> 2012-01-13 18:27 ` Mandeep Singh Baines 2012-01-13 18:27 ` Mandeep Singh Baines [not found] ` <20120113182750.GD18166-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> 2012-01-14 17:36 ` Oleg Nesterov 2012-01-14 17:36 ` Oleg Nesterov 2012-01-18 23:17 ` Mandeep Singh Baines 2012-01-18 23:17 ` Mandeep Singh Baines [not found] ` <20120118231742.GS18166-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> 2012-01-19 15:45 ` Oleg Nesterov 2012-01-19 15:45 ` Oleg Nesterov 2012-01-19 18:18 ` Mandeep Singh Baines 2012-01-19 18:18 ` Mandeep Singh Baines [not found] ` <20120119181803.GU18166-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> 2012-01-20 15:06 ` Oleg Nesterov 2012-01-20 15:06 ` Oleg Nesterov 2012-03-20 19:34 ` Oleg Nesterov 2012-03-20 19:34 ` Oleg Nesterov [not found] ` <20120320193414.GA21277-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> 2012-03-21 18:59 ` Mandeep Singh Baines 2012-03-21 18:59 ` Mandeep Singh Baines 2012-03-23 17:51 ` Oleg Nesterov 2012-03-23 17:51 ` Oleg Nesterov [not found] ` <20120321185955.GK27051-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> 2012-03-23 17:51 ` Oleg Nesterov [not found] ` <20120119154522.GA14058-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> 2012-01-19 18:18 ` Mandeep Singh Baines 2012-03-20 19:34 ` Oleg Nesterov [not found] ` <20120114173648.GA32543-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> 2012-01-18 23:17 ` Mandeep Singh Baines [not found] ` <20120112170728.GA25717-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> 2012-01-12 17:57 ` Mandeep Singh Baines 2011-12-22 15:30 ` Oleg Nesterov 2011-12-21 19:01 ` Mandeep Singh Baines 2012-02-01 16:28 ` Frederic Weisbecker 2012-02-01 16:28 ` Frederic Weisbecker [not found] ` <20111221130848.GA19679-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> 2011-12-21 17:56 ` Frederic Weisbecker 2011-12-21 17:59 ` Frederic Weisbecker 2011-12-21 17:59 ` Frederic Weisbecker 2011-12-21 18:11 ` Oleg Nesterov 2011-12-21 18:11 ` Oleg Nesterov [not found] ` <20111221181101.GA3092-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> 2011-12-21 18:23 ` Frederic Weisbecker 2011-12-21 18:23 ` Frederic Weisbecker 2011-12-21 18:23 ` Frederic Weisbecker 2012-02-01 16:28 ` Frederic Weisbecker 2011-12-21 13:08 ` Oleg Nesterov -- strict thread matches above, loose matches on Subject: below -- 2011-12-21 3:43 Frederic Weisbecker
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