linux-kernel - Re: Q: cgroup: Questions about possible issues in cgroup locking

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Message-ID: <20120120150620.GB9081@redhat.com>
Date:	Fri, 20 Jan 2012 16:06:20 +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 01/19, Mandeep Singh Baines wrote:
>
> Oleg Nesterov (oleg@...hat.com) wrote:
> > But we did
> > have a reason (reasons?). Hopefully, the only really important reason
> > was already removed by 087806b1.
>
> Ah. So the leader->group_leader may have been necessary earlier in order
> to prevent two tasks, old leader and new leader from both returning true
> for thread_group_leader(tsk).

Not really. de_thread() does release_task(old_leader). Please look, for
example, at __exit_signal(). thread_group_leader() must not be true in
this case.

Oleg.

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