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Date: Wed, 17 Feb 2016 17:22:16 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <waiman.long@hpe.com>
Cc: Dave Chinner <david@fromorbit.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>, Jan Kara <jack@suse.com>,
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        Scott J Norton <scott.norton@hp.com>,
        Douglas Hatch <doug.hatch@hp.com>
Subject: Re: [RFC PATCH 1/2] lib/percpu-list: Per-cpu list with associated
 per-cpu locks
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On Wed, Feb 17, 2016 at 11:16:10AM -0500, Waiman Long wrote:
> >So why not use RCU for the list iteration and avoid potentially large
> >lock hold times?
> >
> 
> I know we can use RCU for singly linked list, but I don't think we can use
> that for doubly linked list as there is no easy way to make atomic changes
> to both prev and next pointers simultaneously unless you are taking about
> 16b cmpxchg which is only supported in some architecture.

See include/linux/rculist.h, the only constraint is that reverse
iteration is not allowed, because of what you say.