Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754060AbaLMXrn (ORCPT ); Sat, 13 Dec 2014 18:47:43 -0500 Received: from zeniv.linux.org.uk ([195.92.253.2]:58307 "EHLO ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753151AbaLMXrm (ORCPT ); Sat, 13 Dec 2014 18:47:42 -0500 Date: Sat, 13 Dec 2014 23:47:30 +0000 From: Al Viro To: Linus Torvalds Cc: Dave Jones , Chris Mason , Mike Galbraith , Ingo Molnar , Peter Zijlstra , =?iso-8859-1?Q?D=E2niel?= Fraga , Sasha Levin , "Paul E. McKenney" , Linux Kernel Mailing List , Thomas Gleixner Subject: Re: frequent lockups in 3.18rc4 Message-ID: <20141213234730.GP22149@ZenIV.linux.org.uk> References: <20141212185454.GB4716@redhat.com> <20141213165915.GA12756@redhat.com> <20141213223616.GA22559@redhat.com> <20141213233508.GN22149@ZenIV.linux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Dec 13, 2014 at 03:38:57PM -0800, Linus Torvalds wrote: > On Sat, Dec 13, 2014 at 3:35 PM, Al Viro wrote: > > > > Er... There's much more direct reason - suppose we get a timer interrupt > > right in the middle of mnt_drop_write(). And lost the timeslice. > > So? > > You didn't have preemption disabled in *between* the mnt_want_write() > and mnt_drop_write(), there's absolutely no reason to have it inside > of them. > > Nobody cares if you get preempted and go away for a while. It's > exactly equivalent to sleeping while doing the write that the pair was > protecting. > > Seriously, the preemption disable looks like just voodoo code. It > doesn't protect anything, it doesn't fix anything, it doesn't change > anything. All it does is disable preemption over a random sequence of > code. Huh? Sure, we can enable it after mnt_inc_writers() and disable just prior to mnt_dec_writers(), but we absolutely *do* need it disabled during either. Is that what you are talking about? If so, yes, we can do that. But that applies only to __mnt_want_write() - __mnt_drop_write() is pure mnt_dec_writers() and we can't call that one with preemption enabled. Seriously, look at the mnt_dec_writers(): static inline void mnt_dec_writers(struct mount *mnt) { #ifdef CONFIG_SMP this_cpu_dec(mnt->mnt_pcp->mnt_writers); #else mnt->mnt_writers--; #endif } It's load/modify/store, without any kind of atomicity; get preempted in the middle of that sequence by another caller of mnt_dec_writers() and obvious bad things will happen... Al, really confused by now... -- 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/