Re: [PATCH] ext4_freeze: don't return to userspace with a mutex held

[tytso@mit.edu]

On Sun, Apr 04, 2010 at 04:52:37PM -0400, tytso@MIT.EDU wrote:
> On Mon, Mar 29, 2010 at 05:34:43PM -0500, Eric Sandeen wrote:
> > 
> > I don't -think- we need to do this; by now we should have s_frozen
> > set, and nobody else should be coming down the pipe to get to
> > the journal.  However, just to be on the safe side, I added
> > a couple of vfs_check_frozen() calls in ext4 functions which will
> > arrive at start_this_handle(), which should ensure that we never
> > get any journal traffic generated while frozen.
> 
> Um, I think the addition of vfs_check_frozen(), esp. to
> ext4_journal_start_sb() is absolutely necessary.  What else do we have
> to prevent filesystem modifications from going to the file systme
> layer?  I didn't see anything in the VFS layer that checks s_frozen;
> am I missing something?

Added to the ext4 patch queue, with the following patch comment:

ext4: don't return to userspace after freezing the fs with a mutex held

ext4_freeze() used jbd2_journal_lock_updates() which takes
the j_barrier mutex, and then returns to userspace.  The
kernel does not like this:

================================================
[ BUG: lock held when returning to user space! ]
------------------------------------------------
lvcreate/1075 is leaving the kernel with locks still held!
1 lock held by lvcreate/1075:
 #0:  (&journal->j_barrier){+.+...}, at: [<ffffffff811c6214>]
jbd2_journal_lock_updates+0xe1/0xf0

Use vfs_check_frozen() added to ext4_journal_start_sb() and
ext4_force_commit() instead.

Addresses-Red-Hat-Bugzilla: #568503

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>


					- Ted