From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752357Ab2DQXo2 (ORCPT ); Tue, 17 Apr 2012 19:44:28 -0400 Received: from zeniv.linux.org.uk ([195.92.253.2]:59233 "EHLO ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751286Ab2DQXo0 (ORCPT ); Tue, 17 Apr 2012 19:44:26 -0400 Date: Wed, 18 Apr 2012 00:44:24 +0100 From: Al Viro To: Linus Torvalds Cc: "J. Bruce Fields" , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org Subject: Re: [git pull] vfs and fs fixes Message-ID: <20120417234423.GY6589@ZenIV.linux.org.uk> References: <20120417052511.GU6589@ZenIV.linux.org.uk> <20120417180129.GW6589@ZenIV.linux.org.uk> <20120417182825.GX6589@ZenIV.linux.org.uk> <20120417211419.GC27426@fieldses.org> 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 Tue, Apr 17, 2012 at 03:08:26PM -0700, Linus Torvalds wrote: > > Or I could increment that counter for all the conflicting operations and > > rely on it instead of the i_mutex. ?I was trying to avoid adding > > something like that (an inc, a dec, another error path) to every > > operation. ?And hoping to avoid adding another field to struct inode. > > Oh well. > > We could just say that we can do a double inode lock, but then > standardize on the order. And the only sane order is comparing inode > pointers, not inode numbers like ext4 apparently does. > > With a standard order, I don't think it would be at all wrong to just > take the inode lock on rename. In principle, yes, but have you tried to grep for i_mutex? Note that we have *another* place where multiple ->i_mutex might be held on non-directories (and unless I'm missing something, ext4 move_extent.c stuff doesn't play well with it): quota writes. Which can, AFAICS, happen while write(2) is holding ->i_mutex on a regular file. So it's not _that_ easy - we want something like "and quota file is goes last", since there we don't get to change the locking order - the first ->i_mutex is taken too far outside. I really don't like how messy i_mutex had become these days. Right now I'm staring at 700-odd lines all over the place where it's taken/released and it's a wastebucket lock - used to protect random bits and scraps, with a lot of filesystems, etc. using it for purposes of their own ;-/