From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753670Ab0AXVhO (ORCPT ); Sun, 24 Jan 2010 16:37:14 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753444Ab0AXVhL (ORCPT ); Sun, 24 Jan 2010 16:37:11 -0500 Received: from zeniv.linux.org.uk ([195.92.253.2]:48711 "EHLO ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752366Ab0AXVhJ (ORCPT ); Sun, 24 Jan 2010 16:37:09 -0500 Date: Sun, 24 Jan 2010 21:37:07 +0000 From: Al Viro To: Dmitry Monakhov Cc: linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] fs: fix filesystem_sync vs write race on rw=>ro remount Message-ID: <20100124213707.GY19799@ZenIV.linux.org.uk> References: <87sk9vd92c.fsf@openvz.org> <20100124195309.GX19799@ZenIV.linux.org.uk> <87r5pfw6ew.fsf@openvz.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87r5pfw6ew.fsf@openvz.org> 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 On Mon, Jan 25, 2010 at 12:15:51AM +0300, Dmitry Monakhov wrote: > > It's not a solution. You get an _attempted_ remount ro making writes > > fail, even if it's going to be unsuccessful. No go... > We have two options for new writers: > 1) Fail it via -EROFS > Yes, remount may fail, but it is really unlikely. > 2) Defer(block) new writers on until we complete or fail remount > for example like follows. Do you like second solution ? Umm... I wonder what the locking implications would be... Frankly, I suspect that what we really want is this: * per-superblock write count of some kind, bumped when we decide that writeback is inevitable and dropped when we are done with it (the same thing goes for async part of unlink(), etc.) * fs_may_remount_ro() checking that write count So basically we try to push those short-term writers to completion and if new ones had come while we'd been doing that (or some are really stuck) we fail remount with -EBUSY. As a short-term solution the second patch would do probably (-stable and .33), but in the next cycle I'd rather see something addressing the real problem. fs_may_remount_ro() in its current form is really broken by design - it should not scan any lists (which is where your race comes from, BTW)