Why does xfs_fs_thaw() call xfs_unmountfs_writesb()?

["Stephenson, Dale" <dale.stephenson@quantum.com>]

The xfs_freeze command uses xfs_fs_freeze() to flush out and freeze new
writes to a file system, and xfs_fs_thaw() to let writes through again.  In
between an xfs_freeze -f and xfs_freeze -u, the file system should be
effectively frozen.

The last thing xfs_fs_freeze() does before returning is call
xfs_unmountfs_writesb().  The first thing xfs_fs_thaw() does call it again.
Since all new writes between the two calls should be waiting for the
filesystem to be thawed, why does xfs_unmountfs_writesb() need to be called
again?  What does xfs_unmountfs_writesb() accomplish in these two functions?

Dale Stephenson
steph@snapserver.com