gfs_lookup_dot() does not have to acquire any locks

In fact, that was dangerous.  For example, zfsctl_snapshot_reclaim()
calls gfs_dir_lookup() on ".." path and that ends up calling
gfs_lookup_dot() which violated locking order by acquiring the parent's
directory vnode lock after the child's vnode lock.

Also, the previous behavior was inconsistent as gfs_dir_lookup()
returned a locked vnode for . and .. lookups, but not for any other.

Now gfs_lookup_dot() just references a resulting vnode and the locking
is done in its consumers, where necessary.
Note that we do not enable shared locking support for any gfs / zfsctl
vnodes.

This commit partially reverts r273641.

MFC after:	5 weeks

2 files changed, 13 insertions, 14 deletions

diff --git a/sys/cddl/contrib/opensolaris/uts/common/fs/gfs.c b/sys/cddl/contrib/opensolaris/uts/common/fs/gfs.c
index c5c5c00..27d259d 100644
--- a/sys/cddl/contrib/opensolaris/uts/common/fs/gfs.c
+++ b/sys/cddl/contrib/opensolaris/uts/common/fs/gfs.c
@@ -442,19 +442,9 @@ gfs_lookup_dot(vnode_t **vpp, vnode_t *dvp, vnode_t *pvp, const char *nm)
 		*vpp = dvp;
 		return (0);
 	} else if (strcmp(nm, "..") == 0) {
-		if (pvp == NULL) {
-			ASSERT(dvp->v_flag & VROOT);
-			VN_HOLD(dvp);
-			*vpp = dvp;
-			ASSERT_VOP_ELOCKED(dvp, "gfs_lookup_dot: non-locked dvp");
-		} else {
-			ltype = VOP_ISLOCKED(dvp);
-			VOP_UNLOCK(dvp, 0);
-			VN_HOLD(pvp);
-			*vpp = pvp;
-			vn_lock(*vpp, LK_EXCLUSIVE | LK_RETRY);
-			vn_lock(dvp, ltype | LK_RETRY);
-		}
+		ASSERT(pvp != NULL);
+		VN_HOLD(pvp);
+		*vpp = pvp;
 		return (0);
 	}
 
diff --git a/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_ctldir.c b/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_ctldir.c
index 6404b77..ec4518d 100644
--- a/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_ctldir.c
+++ b/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_ctldir.c
@@ -987,6 +987,11 @@ zfsctl_snapdir_lookup(ap)
 
 	ZFS_ENTER(zfsvfs);
 	if (gfs_lookup_dot(vpp, dvp, zfsvfs->z_ctldir, nm) == 0) {
+		if (nm[0] == '.' && nm[1] == '.' && nm[2] =='\0') {
+			VOP_UNLOCK(dvp, 0);
+			VERIFY0(vn_lock(*vpp, LK_EXCLUSIVE));
+			VERIFY0(vn_lock(dvp, LK_EXCLUSIVE));
+		}
 		ZFS_EXIT(zfsvfs);
 		return (0);
 	}
@@ -1151,6 +1156,11 @@ zfsctl_shares_lookup(ap)
 	strlcpy(nm, cnp->cn_nameptr, cnp->cn_namelen + 1);
 
 	if (gfs_lookup_dot(vpp, dvp, zfsvfs->z_ctldir, nm) == 0) {
+		if (nm[0] == '.' && nm[1] == '.' && nm[2] =='\0') {
+			VOP_UNLOCK(dvp, 0);
+			VERIFY0(vn_lock(*vpp, LK_EXCLUSIVE));
+			VERIFY0(vn_lock(dvp, LK_EXCLUSIVE));
+		}
 		ZFS_EXIT(zfsvfs);
 		return (0);
 	}
@@ -1488,7 +1498,6 @@ zfsctl_snapshot_reclaim(ap)
 
 	VERIFY(gfs_dir_lookup(vp, "..", &dvp, cr, 0, NULL, NULL) == 0);
 	sdp = dvp->v_data;
-	VOP_UNLOCK(dvp, 0);
 	/* this may already have been unmounted */
 	if (sdp == NULL) {
 		VN_RELE(dvp);