Quick-hack to avoid `lock against myself' panic. It is not the real

fix!

The ufs_link() assumes that vnode is not unlocked and tries to lock it
in certain case.  Because union_link calls VOP_LINK after locking vnode,
vn_lock in ufs_link causes above panic.

Currently, I don't know the real fix for a locking violation in
union_link, but I think it is important to avoid panic.

A vnode is unlocked before calling VOP_LINK and is locked after it if
the vnode is not union fs.  Even though panic went away, the process
that access the union fs in which link was made will hang-up.

Hang-up can be easily reproduced by following operation:

	mount -t union a b
	cd b
	ln foo bar
	ls

1 files changed, 15 insertions, 2 deletions

diff --git a/sys/miscfs/union/union_vnops.c b/sys/miscfs/union/union_vnops.c
index e05372f..4b70f2c 100644
--- a/sys/miscfs/union/union_vnops.c
+++ b/sys/miscfs/union/union_vnops.c
@@ -35,7 +35,7 @@
  * SUCH DAMAGE.
  *
  *	@(#)union_vnops.c	8.32 (Berkeley) 6/23/95
- * $Id: union_vnops.c,v 1.22 1997/04/13 13:12:12 kato Exp $
+ * $Id: union_vnops.c,v 1.23 1997/04/14 05:13:55 kato Exp $
  */
 
 #include <sys/param.h>
@@ -1058,6 +1058,7 @@ union_link(ap)
 	struct union_node *un;
 	struct vnode *vp;
 	struct vnode *tdvp;
+	int isvnunlocked = 0;
 
 	un = VTOUNION(ap->a_tdvp);
 
@@ -1096,7 +1097,19 @@ union_link(ap)
 	un->un_flags |= UN_KLOCK;
 	vput(ap->a_tdvp);
 
-	return (VOP_LINK(vp, tdvp, cnp));
+	/*
+	 * XXX -- This is a quick-hack to avoid panic.  Problem still remains!
+	 * The process which access the union filesystem will be hang-up after
+	 * making link in the union fs.
+	 */
+	if (VOP_ISLOCKED(tdvp) && (tdvp->v_op != union_vnodeop_p)) {
+		isvnunlocked = 1;
+		VOP_UNLOCK(tdvp, 0, p);
+	}
+	error = VOP_LINK(vp, tdvp, cnp);
+	if (isvnunlocked)
+		vn_lock(tdvp, LK_EXCLUSIVE | LK_RETRY, p);
+	return error;
 }
 
 int