Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756569Ab3JQOAp (ORCPT ); Thu, 17 Oct 2013 10:00:45 -0400 Received: from mx1.redhat.com ([209.132.183.28]:18614 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755478Ab3JQOAn (ORCPT ); Thu, 17 Oct 2013 10:00:43 -0400 Date: Thu, 17 Oct 2013 15:53:39 +0200 From: Oleg Nesterov To: Eric Wong Cc: Linus Torvalds , Dave Jones , Linux Kernel , Al Viro , Davide Libenzi , Pekka Enberg , Peter Hurley , Eric Dumazet Subject: Re: epoll oops. Message-ID: <20131017135339.GA29182@redhat.com> References: <20131014154627.GA9525@redhat.com> <20131015154838.GA32271@redhat.com> <20131016223922.GA18383@dcvr.yhbt.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20131016223922.GA18383@dcvr.yhbt.net> 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 Content-Length: 1448 Lines: 39 On 10/16, Eric Wong wrote: > > Oleg Nesterov wrote: > > Yes. Before that 971316f0503a hack epoll can't even know if the task > > which did signalfd_poll() exits and frees the active signalfd_wqh. > > If for example that task forked a child before exit. > > > > And the whole RCU logic is only needed if exit/ep_remove_wait_queue > > actually race with each other. > > Is there any chance this oops is caused by (or at least more easily > exposed by) commit 91cf5ab60ff82ecf4550a596867787c1e360dd3f ? > (epoll: add a reschedule point in ep_free()) > > I thought 91cf5ab would be benign, except... > > > Yes, ugly, agreed. d80e731ecab4 even tries to docunent that this all > > is the hack. > > .. the following sentence from d80e731ecab4 caught my eye: > > It also assumes that nobody can take tasklist_lock under epoll > locks, this seems to be true. This just reminds that with this patch __wake_up/ep_poll_callback can be called under write_lock(tasklist). > I haven't been able to trace if cond_resched() can take tasklist_lock. No, it can't hold the non-sleepable rwlock_t. And the sentence above doesn't mean the locks like epmutex, it is mostlt about ep->lock. Oleg. -- 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/