From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx118.postini.com [74.125.245.118]) by kanga.kvack.org (Postfix) with SMTP id 35E2A6B004D for ; Mon, 19 Dec 2011 00:07:18 -0500 (EST) Date: Mon, 19 Dec 2011 16:07:10 +1100 From: Dave Chinner Subject: Re: [PATCH] mm: add missing mutex lock arround notify_change Message-ID: <20111219050710.GQ23662@dastard> References: <20111216112534.GA13147@dztty> <20111216125556.db2bf308.akpm@linux-foundation.org> <20111217214137.GY2203@ZenIV.linux.org.uk> <20111219014343.GK23662@dastard> <20111219020340.GG2203@ZenIV.linux.org.uk> <20111219020637.GA1653@ZenIV.linux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20111219020637.GA1653@ZenIV.linux.org.uk> Sender: owner-linux-mm@kvack.org List-ID: To: Al Viro Cc: Andrew Morton , Djalal Harouni , Hugh Dickins , Minchan Kim , KAMEZAWA Hiroyuki , Wu Fengguang , linux-mm@kvack.org, linux-kernel@vger.kernel.org, "J. Bruce Fields" , Neil Brown , Mikulas Patocka , Christoph Hellwig On Mon, Dec 19, 2011 at 02:06:37AM +0000, Al Viro wrote: > On Mon, Dec 19, 2011 at 02:03:40AM +0000, Al Viro wrote: > > > OK, I'm definitely missing something. The very first thing > > xfs_file_aio_write_checks() does is > > xfs_rw_ilock(ip, XFS_ILOCK_EXCL); > > which really makes me wonder how the hell does that manage to avoid an > > instant deadlock in case of call via xfs_file_buffered_aio_write() > > where we have: > > struct address_space *mapping = file->f_mapping; > > struct inode *inode = mapping->host; > > struct xfs_inode *ip = XFS_I(inode); > > *iolock = XFS_IOLOCK_EXCL; > > xfs_rw_ilock(ip, *iolock); > > ret = xfs_file_aio_write_checks(file, &pos, &count, new_size, iolock); > > which leads to > > struct inode *inode = file->f_mapping->host; > > struct xfs_inode *ip = XFS_I(inode); > > (IOW, inode and ip are the same as in the caller) followed by > > xfs_rw_ilock(ip, XFS_ILOCK_EXCL); > > and with both xfs_rw_ilock() calls turning into > > mutex_lock(&VFS_I(ip)->i_mutex); > > xfs_ilock(ip, XFS_ILOCK_EXCL); > > we ought to deadlock on that i_mutex. What am I missing and how do we manage > > to survive that? > > Arrrgh... OK, I see... What I missed is that XFS_IOLOCK_EXCL is not > XFS_ILOCK_EXCL. Nice naming, that... Been that way for 15 years. :/ However, the naming makes sense to me - the IO lock is for serialising IO operations on the inode, while the I lock is for serialising metadata operations on the inode. I guess I'm used to it, though, so I'll conceed that it might look strange/confusing to someone who only occassionally looks at the internal XFS locking code.... Cheers, Dave. -- Dave Chinner david@fromorbit.com -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org