linux-kernel - Re: [PATCH v2 -mm 2/2] current_is_single_threaded: don't use ->mmap

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Date:	Fri, 10 Jul 2009 15:07:42 +0100
From:	David Howells <dhowells@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	dhowells@...hat.com, Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	James Morris <jmorris@...ei.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Roland McGrath <roland@...hat.com>,
	Stephen Smalley <sds@...ho.nsa.gov>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 -mm 2/2] current_is_single_threaded: don't use ->mmap_sem

Oleg Nesterov <oleg@...hat.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@...hat.com>

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