linux-kernel - Re: [patch v2 for-3.2-rc3] cpusets: stall when updating mems_allowed for mempolicy or disjoint nodemask

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Date:	Thu, 17 Nov 2011 16:00:19 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Miao Xie <miaox@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Paul Menage <paul@...lmenage.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [patch v2 for-3.2-rc3] cpusets: stall when updating
 mems_allowed for mempolicy or disjoint nodemask

On Thu, 17 Nov 2011 15:08:08 -0800 (PST)
David Rientjes <rientjes@...gle.com> wrote:

> cpusets: stall when updating mems_allowed for mempolicy or disjoint nodemask
> 
> c0ff7453bb5c ("cpuset,mm: fix no node to alloc memory when changing
> cpuset's mems") adds get_mems_allowed() to prevent the set of allowed
> nodes from changing for a thread.  This causes any update to a set of
> allowed nodes to stall until put_mems_allowed() is called.
> 
> This stall is unncessary, however, if at least one node remains unchanged
> in the update to the set of allowed nodes.  This was addressed by
> 89e8a244b97e ("cpusets: avoid looping when storing to mems_allowed if one
> node remains set"), but it's still possible that an empty nodemask may be
> read from a mempolicy because the old nodemask may be remapped to the new
> nodemask during rebind.  To prevent this, only avoid the stall if there
> is no mempolicy for the thread being changed.
> 
> This is a temporary solution until all reads from mempolicy nodemasks can
> be guaranteed to not be empty without the get_mems_allowed()
> synchronization.
> 
> Also moves the check for nodemask intersection inside task_lock() so that
> tsk->mems_allowed cannot change.  This ensures that nothing can set this
> tsk's mems_allowed out from under us and also protects tsk->mempolicy.

Nothing in this changelog makes me understand why you think we need this
change in 3.2.  What are the user-visible effects of this change?
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