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

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Message-ID: <alpine.DEB.2.20.1602170910140.13779@east.gentwo.org>
Date:	Wed, 17 Feb 2016 09:13:25 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Waiman Long <Waiman.Long@....com>
cc:	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>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	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 Tue, 16 Feb 2016, Waiman Long wrote:

> List entry insertion is strictly per cpu. List deletion, however, can
> happen in a cpu other than the one that did the insertion. So we still
> need lock to protect the list. Because of that, there may still be
> a small amount of contention when deletion is being done.

Is there a way to avoid locking completely? You could use cmpxchg_double
to swap both list head and tail in an atomic fashion with some work. There is
even a this_cpu_cmpxchg_double available.