From: =?ISO-8859-15?Q?Luk=E1=A8_Czerner?= Subject: Re: [PATCH] ext4: Prevent race while waling extent tree Date: Fri, 9 Nov 2012 10:19:52 +0100 (CET) Message-ID: References: <1352372929-18513-1-git-send-email-lczerner@redhat.com> <20121108215233.GB1282@lenny.home.zabbo.net> Mime-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Cc: Lukas Czerner , linux-ext4@vger.kernel.org, tytso@mit.edu To: Zach Brown Return-path: Received: from mx1.redhat.com ([209.132.183.28]:58825 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750881Ab2KIJT7 (ORCPT ); Fri, 9 Nov 2012 04:19:59 -0500 In-Reply-To: <20121108215233.GB1282@lenny.home.zabbo.net> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Thu, 8 Nov 2012, Zach Brown wrote: > Date: Thu, 8 Nov 2012 13:52:33 -0800 > From: Zach Brown > To: Lukas Czerner > Cc: linux-ext4@vger.kernel.org, tytso@mit.edu > Subject: Re: [PATCH] ext4: Prevent race while waling extent tree > > On Thu, Nov 08, 2012 at 12:08:49PM +0100, Lukas Czerner wrote: > > + down_read(&EXT4_I(inode)->i_data_sem); > > error = ext4_ext_walk_space(inode, start_blk, len_blks, > > ext4_ext_fiemap_cb, fieinfo); > > + up_read(&EXT4_I(inode)->i_data_sem); > > Can this deadlock? ext4_ext_fiemap_cb() seems to be doing all kinds of > exciting things that might also try and acquire the i_data_sem, like > GFP_KERNEL allocs (reclaim -> writepage) and copying to userspace (mmap > fault -> readpage -> get blocks). > > It seems like the safer fix is to broaden the sampling lock coverage to > include referencing all the extent data but to release it around the > callback. > > No? > > - z Yeah, you're right. Having the lock around the whole ext4_ext_walk_space() might deadlock. I'll fix this. Thanks for noticing this! -Lukas