Kill CL_PROPAGATION, sanitize fs/pnode.c:get_source()

First of all, get_source() never results in CL_PROPAGATION
alone.  We either get CL_MAKE_SHARED (for the continuation
of peer group) or CL_SLAVE (slave that is not shared) or both
(beginning of peer group among slaves).  Massage the code to
make that explicit, kill CL_PROPAGATION test in clone_mnt()
(nothing sets CL_MAKE_SHARED without CL_PROPAGATION and in
clone_mnt() we are checking CL_PROPAGATION after we'd found
that there's no CL_SLAVE, so the check for CL_MAKE_SHARED
would do just as well).

Fix comments, while we are at it...
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/namespace.c

@@ -573,7 +573,7 @@ static struct vfsmount *clone_mnt(struct vfsmount *old, struct dentry *root,
 			mnt->mnt_master = old;
 			CLEAR_MNT_SHARED(mnt);
 		} else if (!(flag & CL_PRIVATE)) {
-			if ((flag & CL_PROPAGATION) || IS_MNT_SHARED(old))
+			if ((flag & CL_MAKE_SHARED) || IS_MNT_SHARED(old))
 				list_add(&mnt->mnt_share, &old->mnt_share);
 			if (IS_MNT_SLAVE(old))
 				list_add(&mnt->mnt_slave, &old->mnt_slave);

fs/pnode.c

@@ -86,7 +86,7 @@ static int do_make_slave(struct vfsmount *mnt)
 
 	/*
 	 * slave 'mnt' to a peer mount that has the
-	 * same root dentry. If none is available than
+	 * same root dentry. If none is available then
 	 * slave it to anything that is available.
 	 */
 	while ((peer_mnt = next_peer(peer_mnt)) != mnt &&
@@ -147,6 +147,11 @@ void change_mnt_propagation(struct vfsmount *mnt, int type)
  * get the next mount in the propagation tree.
  * @m: the mount seen last
  * @origin: the original mount from where the tree walk initiated
+ *
+ * Note that peer groups form contiguous segments of slave lists.
+ * We rely on that in get_source() to be able to find out if
+ * vfsmount found while iterating with propagation_next() is
+ * a peer of one we'd found earlier.
  */
 static struct vfsmount *propagation_next(struct vfsmount *m,
 					 struct vfsmount *origin)
@@ -186,10 +191,6 @@ static struct vfsmount *get_source(struct vfsmount *dest,
 {
 	struct vfsmount *p_last_src = NULL;
 	struct vfsmount *p_last_dest = NULL;
-	*type = CL_PROPAGATION;
-
-	if (IS_MNT_SHARED(dest))
-		*type |= CL_MAKE_SHARED;
 
 	while (last_dest != dest->mnt_master) {
 		p_last_dest = last_dest;
@@ -202,13 +203,18 @@ static struct vfsmount *get_source(struct vfsmount *dest,
 		do {
 			p_last_dest = next_peer(p_last_dest);
 		} while (IS_MNT_NEW(p_last_dest));
+		/* is that a peer of the earlier? */
+		if (dest == p_last_dest) {
+			*type = CL_MAKE_SHARED;
+			return p_last_src;
+		}
 	}
-
-	if (dest != p_last_dest) {
-		*type |= CL_SLAVE;
-		return last_src;
-	} else
-		return p_last_src;
+	/* slave of the earlier, then */
+	*type = CL_SLAVE;
+	/* beginning of peer group among the slaves? */
+	if (IS_MNT_SHARED(dest))
+		*type |= CL_MAKE_SHARED;
+	return last_src;
 }
 
 /*

fs/pnode.h

@@ -21,8 +21,7 @@
 #define CL_SLAVE     		0x02
 #define CL_COPY_ALL 		0x04
 #define CL_MAKE_SHARED 		0x08
-#define CL_PROPAGATION 		0x10
-#define CL_PRIVATE 		0x20
+#define CL_PRIVATE 		0x10
 
 static inline void set_mnt_shared(struct vfsmount *mnt)
 {