linux-kernel - Re: [PATCH] mm: add missing mutex lock arround notify

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Message-ID: <20111219020637.GA1653@ZenIV.linux.org.uk>
Date:	Mon, 19 Dec 2011 02:06:37 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Dave Chinner <david@...morbit.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Djalal Harouni <tixxdz@...ndz.org>,
	Hugh Dickins <hughd@...gle.com>,
	Minchan Kim <minchan.kim@...il.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Wu Fengguang <fengguang.wu@...el.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Neil Brown <neilb@...e.de>,
	Mikulas Patocka <mikulas@...ax.karlin.mff.cuni.cz>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH] mm: add missing mutex lock arround notify_change

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...
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