linux-kernel - Re: [PATCH 4/4] cpuset,mm: use rwlock to protect task->mempolicy and mems

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Message-ID: <20100304033017.GN8653@laptop>
Date:	Thu, 4 Mar 2010 14:30:17 +1100
From:	Nick Piggin <npiggin@...e.de>
To:	Miao Xie <miaox@...fujitsu.com>
Cc:	David Rientjes <rientjes@...gle.com>,
	Lee Schermerhorn <lee.schermerhorn@...com>,
	Paul Menage <menage@...gle.com>,
	Linux-Kernel <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH 4/4] cpuset,mm: use rwlock to protect task->mempolicy
 and mems_allowed

On Wed, Mar 03, 2010 at 06:52:39PM +0800, Miao Xie wrote:
> if MAX_NUMNODES > BITS_PER_LONG, loading/storing task->mems_allowed or mems_allowed in
> task->mempolicy are not atomic operations, and the kernel page allocator gets an empty
> mems_allowed when updating task->mems_allowed or mems_allowed in task->mempolicy. So we
> use a rwlock to protect them to fix this probelm.

Thanks for working on this. However, rwlocks are pretty nasty to use
when you have short critical sections and hot read-side (they're twice
as heavy as even spinlocks in that case).

It's being used in the page allocator path, so I would say rwlocks are
almost a showstopper. Wouldn't it be possible to use a seqlock for this?
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