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: Fri, 23 Mar 2012 18:51:57 +0100 [thread overview]
Message-ID: <20120323175157.GA20675@redhat.com> (raw)
In-Reply-To: <20120321185955.GK27051@google.com>
On 03/21, Mandeep Singh Baines wrote:
>
> Oleg Nesterov (oleg@redhat.com) wrote:
> > +/*
> > + * rcu-safe, but should start at ->group_leader.
> > + * thread_group_leader(g) protects against the race with exec which
> > + * removes the leader from list.
> > + * smp_rmb() pairs with implicit mb() implied by unlock + lock in
> > + * de_thread()->release_task() path.
> > + */
> > +#define while_each_thread_rcu(g, t) \
> > + while ((t = next_thread(t)) != g && \
> > + ({ smp_rmb(); thread_group_leader(g); }))
> > +
>
> Couldn't you miss the exec_thread if:
>
> t = exec_thread && !thread_group_leader(g)
Yes, we already discussed this, iirc.
I was going to write that this is fine, but then I changed my mind.
Indeed, it is not good while_each_thread_rcu() can miss the new leader.
> Could we change do_prlimit()? Especially since its slow path.
But do_prlimit() is correct. It sees the unhashed task under tasklist,
task->group_leader should be correct.
> I really like you're earlier solution (ignoring barrier):
>
> #define while_each_thread(g, t) \
> while (t->group_leader == g->group_leader && (t = next_thread(t)) != g)
The problem is "ignoring barrier and races".
OK, I'll try to think again.
Oleg.
next prev parent reply other threads:[~2012-03-23 17:59 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
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 [this message]
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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