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From: David Howells <dhowells@redhat.com>
In-Reply-To: <20090710014857.GC4723@redhat.com>
References: <20090710014857.GC4723@redhat.com> <20090709212847.GA10238@redhat.com> <20090710013322.GA27719@infradead.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: dhowells@redhat.com, Christoph Hellwig <hch@infradead.org>,
       Andrew Morton <akpm@linux-foundation.org>,
       James Morris <jmorris@namei.org>,
       "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
       Roland McGrath <roland@redhat.com>, Stephen Smalley <sds@tycho.nsa.gov>,
       linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 -mm 2/2] current_is_single_threaded: don't use ->mmap_sem
Date: Fri, 10 Jul 2009 15:07:42 +0100
Message-ID: <25973.1247234862@redhat.com>
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1007
Lines: 24

Oleg Nesterov <oleg@redhat.com> wrote:

> current_is_single_threaded() can safely miss a freshly forked CLONE_VM
> task, but in this case it must not miss its parent. That is why we take
> mm->mmap_sem for writing to make sure a thread/task with the same ->mm
> can't pass exit_mm() and disappear.
> 
> However we can avoid ->mmap_sem and rely on rcu/barriers:
> 
> 	- if we do not see the exiting parent on thread/process list
> 	  we see the result of list_del_rcu(), in this case we must
> 	  also see the result of list_add_rcu() which does wmb().
> 
> 	- if we do see the parent but its ->mm == NULL, we need rmb()
> 	  to make sure we can't miss the child.
> 
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>

Acked-by: David Howells <dhowells@redhat.com>
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