[RFC, PATCH] filesystem freeze: fix sys_umount induced perpetual freeze

["Fernando Luis Vázquez Cao" <fernando@oss.ntt.co.jp>]

The current locking scheme for filesystem freeze avoids races between
freeze_bdev() and do_umount() by taking the s_umount semaphore.

If freeze_bdev() wins the race the process that invoked sys_umount
will sleep until thaw_bdev releases the semaphore. Unfortunately, this
will never happen because thaw_bdev bails out early the
bd_fsfreeze_count check having failed (the count is 0).

The problem is that the block_device that ioctl_fsthaw() passes to
thaw_bdev() is not the one that we freezed because before sleeping in
deactivate_super() do_umount() released the dentry (dput()) and freed
the vfs mount (free_vfsmnt()).

This patch works around this issue by checking the freeze level in
do_umount().

Signed-off-by: Fernando Luis Vazquez Cao <fernando@oss.ntt.co.jp>
---

diff -urNp linux-2.6.31-rc7-orig/fs/namespace.c linux-2.6.31-rc7/fs/namespace.c
--- linux-2.6.31-rc7-orig/fs/namespace.c	2009-08-25 16:39:46.000000000 +0900
+++ linux-2.6.31-rc7/fs/namespace.c	2009-08-26 11:30:10.000000000 +0900
@@ -1033,6 +1033,8 @@ static int do_umount(struct vfsmount *mn
  	if (retval)
  		return retval;

+	vfs_check_frozen(sb, SB_FREEZE_WRITE);
+
  	/*
  	 * Allow userspace to request a mountpoint be expired rather than
  	 * unmounting unconditionally. Unmount only happens if: