linux-kernel - Re: Lockdep problem involving sysfs

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Date:	Fri, 20 Mar 2009 18:44:58 -0400
From:	Theodore Tso <tytso@....edu>
To:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, Greg Kroah-Hartman <gregkh@...e.de>,
	Kay Sievers <kay.sievers@...y.org>
Subject: Re: Lockdep problem involving sysfs_mutex in ext4 in linux-next

On Fri, Mar 20, 2009 at 06:25:22PM -0400, Theodore Tso wrote:
> So if I'm reading this right, the problem is happening because
> someplace in the kernel, fs/sysfs code is grabbing mmap_sem while it
> is holding sysfs_mutex, right?  But I can't see where that might be
> happening...

Never mind, I see it.  sysfs_readdir() calls filldir() while holding
sysfs_mutex, and filldir() in turn calls copy_to_user(), which can
grab mmap_sem.

The problem is that in other code paths, mmap_sem() is grabbed while
we grab inode->i_alloc_sem() (in ext4_page_mkwrite), and that in turns
has a dependency on jbd2_handle, which has a dependency on s_lock, and
hence lock_super().  Ugh.  What a mess.

							- Ted
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