simplify the callers of mnt_unhold_writers()

The logics in cleanup on failure in mount_setattr_prepare() is simplified
by having the mnt_hold_writers() failure followed by advancing m to the
next node in the tree before leaving the loop.

And since all calls are preceded by the same check that flag has been set
and the function is inlined, let's just shift the check into it.

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

Author: Al Viro

fs/namespace.c

@@ -714,13 +714,14 @@ static inline int mnt_hold_writers(struct mount *mnt)
  * Stop preventing write access to @mnt allowing callers to gain write access
  * to @mnt again.
  *
- * This function can only be called after a successful call to
- * mnt_hold_writers().
+ * This function can only be called after a call to mnt_hold_writers().
  *
  * Context: This function expects lock_mount_hash() to be held.
  */
 static inline void mnt_unhold_writers(struct mount *mnt)
 {
+	if (!(mnt->mnt_flags & MNT_WRITE_HOLD))
+		return;
 	/*
 	 * MNT_READONLY must become visible before ~MNT_WRITE_HOLD, so writers
 	 * that become unheld will see MNT_READONLY.
@@ -4768,34 +4769,20 @@ static int mount_setattr_prepare(struct mount_kattr *kattr, struct mount *mnt)
 
 		if (!mnt_allow_writers(kattr, m)) {
 			err = mnt_hold_writers(m);
-			if (err)
+			if (err) {
+				m = next_mnt(m, mnt);
 				break;
+			}
 		}
 
 		if (!(kattr->kflags & MOUNT_KATTR_RECURSE))
 			return 0;
 	}
 
 	if (err) {
-		struct mount *p;
-
-		/*
-		 * If we had to call mnt_hold_writers() MNT_WRITE_HOLD will
-		 * be set in @mnt_flags. The loop unsets MNT_WRITE_HOLD for all
-		 * mounts and needs to take care to include the first mount.
-		 */
-		for (p = mnt; p; p = next_mnt(p, mnt)) {
-			/* If we had to hold writers unblock them. */
-			if (p->mnt.mnt_flags & MNT_WRITE_HOLD)
-				mnt_unhold_writers(p);
-
-			/*
-			 * We're done once the first mount we changed got
-			 * MNT_WRITE_HOLD unset.
-			 */
-			if (p == m)
-				break;
-		}
+		/* undo all mnt_hold_writers() we'd done */
+		for (struct mount *p = mnt; p != m; p = next_mnt(p, mnt))
+			mnt_unhold_writers(p);
 	}
 	return err;
 }
@@ -4826,8 +4813,7 @@ static void mount_setattr_commit(struct mount_kattr *kattr, struct mount *mnt)
 		WRITE_ONCE(m->mnt.mnt_flags, flags);
 
 		/* If we had to hold writers unblock them. */
-		if (m->mnt.mnt_flags & MNT_WRITE_HOLD)
-			mnt_unhold_writers(m);
+		mnt_unhold_writers(m);
 
 		if (kattr->propagation)
 			change_mnt_propagation(m, kattr->propagation);