From 47fecca15c0944924423390fe21b956eea57da30 Mon Sep 17 00:00:00 2001
From: Oleg Nesterov <oleg@redhat.com>
Date: Thu, 9 Oct 2014 15:25:49 -0700
Subject: [PATCH] fs/proc/task_nommu.c: don't use priv->task->mm

I do not know if CONFIG_PREEMPT/SMP is possible without CONFIG_MMU
but the usage of task->mm in m_stop(). The task can exit/exec before
we take mmap_sem, in this case m_stop() can hit NULL or unlock the
wrong rw_semaphore.

Also, this code uses priv->task != NULL to decide whether we need
up_read/mmput. This is correct, but we will probably kill priv->task.
Change m_start/m_stop to rely on IS_ERR_OR_NULL() like task_mmu.c does.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Acked-by: Greg Ungerer <gerg@uclinux.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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