From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1161275AbcBQLFb (ORCPT <rfc822;w@1wt.eu>);
	Wed, 17 Feb 2016 06:05:31 -0500
Received: from casper.infradead.org ([85.118.1.10]:51019 "EHLO
	casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1161182AbcBQLF3 (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Wed, 17 Feb 2016 06:05:29 -0500
Date: Wed, 17 Feb 2016 12:05:20 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Dave Chinner <david@fromorbit.com>
Cc: Waiman Long <Waiman.Long@hpe.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>, Jan Kara <jack@suse.com>,
        Jeff Layton <jlayton@poochiereds.net>,
        "J. Bruce Fields" <bfields@fieldses.org>, Tejun Heo <tj@kernel.org>,
        Christoph Lameter <cl@linux-foundation.org>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        Ingo Molnar <mingo@redhat.com>, Andi Kleen <andi@firstfloor.org>,
        Dave Chinner <dchinner@redhat.com>,
        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
Message-ID: <20160217110520.GN6375@twins.programming.kicks-ass.net>
References: <1455672680-7153-1-git-send-email-Waiman.Long@hpe.com>
 <1455672680-7153-2-git-send-email-Waiman.Long@hpe.com>
 <20160217095318.GO14668@dastard>
 <20160217110040.GB6357@twins.programming.kicks-ass.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20160217110040.GB6357@twins.programming.kicks-ass.net>
User-Agent: Mutt/1.5.21 (2012-12-30)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Wed, Feb 17, 2016 at 12:00:40PM +0100, Peter Zijlstra wrote:
> On Wed, Feb 17, 2016 at 08:53:18PM +1100, Dave Chinner wrote:
> > > +/**
> > > + * for_all_percpu_list_entries - iterate over all the per-cpu list with locking

But the locking is 'pointless'. You only lock one per-cpu sublist at a
time, therefore the list can be modified concurrently anyhow.

So why not use RCU for the list iteration and avoid potentially large
lock hold times?

> > > + * @pos:	the type * to use as a loop cursor for the current .
> > > + * @next:	an internal type * variable pointing to the next entry
> > > + * @pchead:	an internal struct list * of percpu list head
> > > + * @pclock:	an internal variable for the current per-cpu spinlock
> > > + * @head:	the head of the per-cpu list
> > > + * @member:	the name of the per-cpu list within the struct
> > > + */