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

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Date:	Mon, 19 Dec 2011 16:07:10 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Al Viro <viro@...IV.linux.org.uk>
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 02:06:37AM +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@...morbit.com
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