linux-kernel - Re: [RFC PATCH 1/2] lib/percpu-list: Per-cpu list with associated per-cpu locks

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Date:	Wed, 17 Feb 2016 17:22:16 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <waiman.long@....com>
Cc:	Dave Chinner <david@...morbit.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Jan Kara <jack@...e.com>,
	Jeff Layton <jlayton@...chiereds.net>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Tejun Heo <tj@...nel.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...hat.com>,
	Andi Kleen <andi@...stfloor.org>,
	Dave Chinner <dchinner@...hat.com>,
	Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>
Subject: Re: [RFC PATCH 1/2] lib/percpu-list: Per-cpu list with associated
 per-cpu locks

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.