Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752540AbaJYTRb (ORCPT ); Sat, 25 Oct 2014 15:17:31 -0400 Received: from mail-qg0-f47.google.com ([209.85.192.47]:62674 "EHLO mail-qg0-f47.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750893AbaJYTR2 (ORCPT ); Sat, 25 Oct 2014 15:17:28 -0400 MIME-Version: 1.0 X-Originating-IP: [46.139.80.5] In-Reply-To: <20141025081845.GJ7996@ZenIV.linux.org.uk> References: <20141023232539.GA4662@tucsk.piliscsaba.szeredi.hu> <20141024022055.GH7996@ZenIV.linux.org.uk> <20141024032422.GI7996@ZenIV.linux.org.uk> <20141025081845.GJ7996@ZenIV.linux.org.uk> Date: Sat, 25 Oct 2014 11:53:52 +0200 Message-ID: Subject: Re: [GIT PULL] overlay filesystem v25 From: Miklos Szeredi To: Al Viro Cc: Linus Torvalds , Linux-Fsdevel , Kernel Mailing List , linux-unionfs@vger.kernel.org Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Oct 25, 2014 at 10:18 AM, Al Viro wrote: > On Fri, Oct 24, 2014 at 09:24:45AM +0200, Miklos Szeredi wrote: > >> The reason I didn't do your "fix" is that it >> >> - adds more lines than it takes, >> >> - I wasn't sure at all if the lockless access is actually correct >> without the ACCESS_ONCE and all the memory barrier magic that might be >> necessary on weird architectures. > > _What_ lockless accesses? There is an extremely embarrassing bug in that > commit, all right, but it has nothing to do with barriers... All > barrier-related issues are taken care of by ovl_path_upper() (and without > that you'd have tons of worse problems). Fetching ->upperfile outside of > ->i_mutex is fine - in the worst case we'll fetch NULL, open the sucker > grab ->i_mutex and find out that it has already been taken care of. > In which case we fput() what we'd opened and move on (fput() under > ->i_mutex is fine - it's going to be delayed until return from syscall > anyway). Yes, but it's not about race with copy-up (which the ovl_path_upper() protects against), but race of two fsync calls with each other. If there's no synchronization between them, then that od->upperfile does indeed count as lockless access, no matter that the assignment was done under lock. Thanks, Miklos -- 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/