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Message-ID: <20120323175157.GA20675@redhat.com>
Date: Fri, 23 Mar 2012 18:51:57 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: Mandeep Singh Baines <msb@...omium.org>
Cc: Frederic Weisbecker <fweisbec@...il.com>,
Li Zefan <lizf@...fujitsu.com>, Tejun Heo <tj@...nel.org>,
LKML <linux-kernel@...r.kernel.org>,
Containers <containers@...ts.linux-foundation.org>,
Cgroups <cgroups@...r.kernel.org>,
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
Paul Menage <paul@...lmenage.org>,
Andrew Morton <akpm@...ux-foundation.org>,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: Q: cgroup: Questions about possible issues in cgroup locking
On 03/21, Mandeep Singh Baines wrote:
>
> Oleg Nesterov (oleg@...hat.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.
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