From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932832Ab3DFVNM (ORCPT ); Sat, 6 Apr 2013 17:13:12 -0400 Received: from mail.openrapids.net ([64.15.138.104]:41126 "EHLO blackscsi.openrapids.net" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S932713Ab3DFVNL (ORCPT ); Sat, 6 Apr 2013 17:13:11 -0400 Date: Sat, 6 Apr 2013 17:13:07 -0400 From: Mathieu Desnoyers To: Eric Wong Cc: linux-kernel@vger.kernel.org, "Paul E. McKenney" , Lai Jiangshan Message-ID: <20130406211307.GA32683@Krystal> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Bcc: Subject: Re: [PATCH] wfcqueue: add function for unsynchronized prepend Reply-To: In-Reply-To: <20130329081016.GA15979@dcvr.yhbt.net> X-Editor: vi X-Info: http://www.efficios.com * Eric Wong (normalperson@yhbt.net) wrote: > In some situations, it is necessary to prepend a node to a queue. > For epoll, this is necessary for two rare conditions: > > * when the user triggers -EFAULT > * when reinjecting elements from the ovflist (implemented as a stack) > > Signed-off-by: Eric Wong > Cc: Mathieu Desnoyers > --- > This is on top of my other patch to implement __wfcq_enqueue > > include/linux/wfcqueue.h | 35 +++++++++++++++++++++++++++-------- > 1 file changed, 27 insertions(+), 8 deletions(-) > > diff --git a/include/linux/wfcqueue.h b/include/linux/wfcqueue.h > index a452ab9..4cb8f22 100644 > --- a/include/linux/wfcqueue.h > +++ b/include/linux/wfcqueue.h > @@ -56,15 +56,17 @@ > * [5] __wfcq_first > * [6] __wfcq_next > * [7] __wfcq_enqueue > + * [8] __wfcq_prepend > * > - * [1] [2] [3] [4] [5] [6] [7] > - * [1] - - - - - - X > - * [2] - - - - - - X > - * [3] - - X X X X X > - * [4] - - X - X X X > - * [5] - - X X - - X > - * [6] - - X X - - X > - * [7] X X X X X X X > + * [1] [2] [3] [4] [5] [6] [7] [8] > + * [1] - - - - - - X X > + * [2] - - - - - - X X > + * [3] - - X X X X X X > + * [4] - - X - X X X X > + * [5] - - X X - - X X > + * [6] - - X X - - X X > + * [7] X X X X X X X X > + * [8] X X X X X X X X I don't think this table is accurate, and there is likely an issue with the implementation below. AFAIU, you plan to use wait-free enqueue (to tail) concurrently with enqueue to head (prepend). Therefore, the table above should reflect this. I'll look into this and try to come up with a correct implementation. Thanks, Mathieu > * > * Besides locking, mutual exclusion of dequeue, splice and iteration > * can be ensured by performing all of those operations from a single > @@ -441,6 +443,23 @@ static inline enum wfcq_ret __wfcq_splice( > } > > /* > + * __wfcq_prepend: prepend a node into a queue, requiring mutual exclusion. > + * > + * No memory barriers are issued. Mutual exclusion is the responsibility > + * of the caller. > + */ > +static inline void __wfcq_prepend(struct wfcq_head *head, > + struct wfcq_tail *tail, struct wfcq_node *node) > +{ > + node->next = head->node.next; > + head->node.next = node; > + > + /* if the queue was empty before, it is no longer empty now */ > + if (tail->p == &head->node) > + tail->p = node; > +} > + > +/* > * __wfcq_for_each: Iterate over all nodes in a queue, > * without dequeuing them. > * @head: head of the queue (struct wfcq_head pointer). > -- > Eric Wong > -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com