Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754746Ab3CLA3D (ORCPT ); Mon, 11 Mar 2013 20:29:03 -0400 Received: from mail-pb0-f53.google.com ([209.85.160.53]:55703 "EHLO mail-pb0-f53.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754446Ab3CLA3B convert rfc822-to-8bit (ORCPT ); Mon, 11 Mar 2013 20:29:01 -0400 MIME-Version: 1.0 In-Reply-To: <20130312001722.GB973@dcvr.yhbt.net> References: <20130307112639.GA25130@dcvr.yhbt.net> <20130308013027.GA31830@dcvr.yhbt.net> <20130308204944.GA27379@dcvr.yhbt.net> <20130309071037.GA13360@dcvr.yhbt.net> <20130312001722.GB973@dcvr.yhbt.net> Date: Mon, 11 Mar 2013 17:29:00 -0700 Message-ID: Subject: Re: epoll: possible bug from wakeup_source activation From: =?ISO-8859-1?Q?Arve_Hj=F8nnev=E5g?= To: Eric Wong Cc: NeilBrown , "Rafael J. Wysocki" , linux-kernel@vger.kernel.org, Davide Libenzi , Alexander Viro , linux-fsdevel@vger.kernel.org, Andrew Morton Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2008 Lines: 47 On Mon, Mar 11, 2013 at 5:17 PM, Eric Wong wrote: > Arve Hj?nnev?g wrote: >> On Fri, Mar 8, 2013 at 11:10 PM, Eric Wong wrote: >> > Arve Hj?nnev?g wrote: >> >> On Fri, Mar 8, 2013 at 12:49 PM, Eric Wong wrote: >> >> > What happens if ep_modify calls ep_destroy_wakeup_source >> >> > while __pm_stay_awake is running on the same epi->ws? >> >> >> >> Yes, that looks like a problem. I think calling >> >> ep_destroy_wakeup_source with ep->lock held should fix that. It is not >> >> clear how useful changing EPOLLWAKEUP in ep_modify is, so >> >> alternatively we could remove that feature and instead only allow it >> >> to be set in ep_insert. >> > >> > ep->lock would work, but ep->lock is already a source of heavy >> > contention in my multithreaded+epoll webservers. >> >> This should not have any significant impact on that since you would be >> adding a lock to a code path that is, as far as I know, unused. >> >> > Perhaps RCU can be used? I've no experience with RCU, but I've been >> > meaning to get acquainted with RCU. >> >> That adds code to the common path however. The wakeup_source is not >> touch without holding one of the locks so holding both locks before >> deleting it seems like a simpler solution. > > True. However, I've been looking into eliminating ep->lock in more > places (maybe entirely)[1]. > > I don't think the current overhead of RCU in epoll is significant, > either. > > > [1] I'll be testing Mathieu's wait-free concurrent queue soon: > http://mid.gmane.org/20130311213602.GB9829@Krystal OK, but is there any way you could use the same locking scheme for the wakeup_source and the queue? -- Arve Hj?nnev?g -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/