From: Oleg Nesterov <oleg@redhat.com>
To: Mandeep Singh Baines <msb@chromium.org>
Cc: 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: Thu, 12 Jan 2012 18:07:28 +0100 [thread overview]
Message-ID: <20120112170728.GA25717@redhat.com> (raw)
In-Reply-To: <20120112003102.GB9511@google.com>
Hi Mandeep,
On 01/11, Mandeep Singh Baines wrote:
> > >
> > > #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;
L == 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.
Still can't understand... Lets look at this trivial example again.
We start from the main thread M, it is ->group_leader. There is
another thread T in this thread group. We are doing
OLD = M;
t = M;
do {
do_smth(t);
}
while (t->group_leader == OLD && ((t = next_thread(t)) != M);
The first iteration does do_smth(M).
T calls de_thread() and, in particular, it does M->group_leader = T
(see "leader->group_leader = tsk" in de_thread).
after that t->group_leader == OLD fails. t == M, its group_leader == T.
do_smth(T) won't be called.
No?
Oleg.
next prev parent reply other threads:[~2012-01-12 17:14 UTC|newest]
Thread overview: 29+ 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 13:08 ` Oleg Nesterov
2011-12-21 17:56 ` Frederic Weisbecker
2011-12-21 19:01 ` Mandeep Singh Baines
2011-12-21 19:08 ` Frederic Weisbecker
2011-12-21 19:24 ` Mandeep Singh Baines
2011-12-21 20:04 ` Frederic Weisbecker
2011-12-22 15:30 ` Oleg Nesterov
2012-01-04 19:36 ` Mandeep Singh Baines
2012-01-06 15:23 ` Oleg Nesterov
2012-01-06 18:25 ` Mandeep Singh Baines
2012-01-11 16:07 ` Oleg Nesterov
2012-01-12 0:31 ` Mandeep Singh Baines
2012-01-12 17:07 ` Oleg Nesterov [this message]
2012-01-12 17:57 ` Mandeep Singh Baines
2012-01-13 15:20 ` Oleg Nesterov
2012-01-13 18:27 ` Mandeep Singh Baines
2012-01-14 17:36 ` Oleg Nesterov
2012-01-18 23:17 ` Mandeep Singh Baines
2012-01-19 15:45 ` Oleg Nesterov
2012-01-19 18:18 ` Mandeep Singh Baines
2012-01-20 15:06 ` Oleg Nesterov
2012-03-20 19:34 ` Oleg Nesterov
2012-03-21 18:59 ` Mandeep Singh Baines
2012-03-23 17:51 ` Oleg Nesterov
2011-12-21 17:59 ` Frederic Weisbecker
2011-12-21 18:11 ` Oleg Nesterov
2011-12-21 18:23 ` Frederic Weisbecker
2012-02-01 16:28 ` Frederic Weisbecker
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