- Disallow XFS mounting in write mode. The write support never worked really

and there is no need to maintain it.
- Fix vn_get() in order to let it call vget(9) with a valid locking
  request.  vget(9) returns the vnode locked in order to prevent recycling,
  but in this case internal XFS locks alredy prevent it from happening, so
  it is safe to drop the vnode lock before to return by vn_get().
- Add a VNASSERT() in vget(9) in order to catch malformed locking requests.

Discussed with:	kan, kib
Tested by:	Lothar Braun <lothar at lobraun dot de>

sys/gnu/fs/xfs/FreeBSD/xfs_mountops.c

@@ -199,6 +199,8 @@ _xfs_mount(struct mount		*mp,
 
 	if (mp->mnt_flag & MNT_UPDATE)
 		return (0);
+	if ((mp->mnt_flag & MNT_RDONLY) == 0)
+		return (EPERM);
 
         xmp = xfsmount_allocate(mp);
         if (xmp == NULL)

sys/gnu/fs/xfs/FreeBSD/xfs_vnode.c

@@ -104,13 +104,18 @@ vn_get(
 
 	vp = vmap->v_vp;
 
-	error = vget(vp, 0, curthread);
-	if (error) {
-		vdrop(vp);
-		return (NULL);
-	}
-
+	error = vget(vp, LK_EXCLUSIVE, curthread);
 	vdrop(vp);
+	if (error)
+		return (NULL);
+
+	/*
+	 * Drop the vnode returned by vget here.
+	 * VOP_RECLAIM(9) should block on internal XFS locks so that
+	 * the reclaiming scheme still remains consistent even if the
+	 * vp is not locked.
+	 */
+	VOP_UNLOCK(vp, 0);
 	if (vp->v_data != xfs_vp) {
 		vput(vp);
 		return (NULL);

sys/kern/vfs_subr.c

@@ -2038,6 +2038,8 @@ vget(struct vnode *vp, int flags, struct thread *td)
 
 	error = 0;
 	VFS_ASSERT_GIANT(vp->v_mount);
+	VNASSERT((flags & LK_TYPE_MASK) != 0, vp,
+	    ("vget: invalid lock operation"));
 	if ((flags & LK_INTERLOCK) == 0)
 		VI_LOCK(vp);
 	vholdl(vp);