linux-kernel - Re: [PATCH 1/1] do not abuse ->cred_guard_mutex in threadgroup

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Date:	Sat, 9 Mar 2013 12:15:50 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Li Zefan <lizefan@...wei.com>, Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	cgroups@...r.kernel.org
Subject: Re: [PATCH 1/1] do not abuse ->cred_guard_mutex in threadgroup_lock()

On Sat, Mar 9, 2013 at 12:01 PM, Oleg Nesterov <oleg@...hat.com> wrote:
> threadgroup_lock() takes signal->cred_guard_mutex to ensure that
> thread_group_leader() is stable. This doesn't look nice, the scope
> of this lock in do_execve() is huge.
>
> And as Dave pointed out this can lead to deadlock, we have the
> following dependencies:
>
>         do_execve:              cred_guard_mutex -> i_mutex
>         cgroup_mount:           i_mutex -> cgroup_mutex
>         attach_task_by_pid:     cgroup_mutex -> cred_guard_mutex
>
> Change de_thread() to take threadgroup_change_begin() around the
> switch-the-leader code and change threadgroup_lock() to avoid
> ->cred_guard_mutex.
>
> Note that de_thread() can't sleep with ->group_rwsem held, this
> can obviously deadlock with the exiting leader if the writer is
> active, so it does threadgroup_change_end() before schedule().
>
> Reported-by: Dave Jones <davej@...hat.com>
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>

Acked-by: Tejun Heo <tj@...nel.org>

Thanks.

-- 
tejun
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