futex: Plug a pi_state leak and clarify the refcounting

@@ -2755,6 +2755,11 @@ static int futex_wait_requeue_pi(u32 __u
 		if (q.pi_state && (q.pi_state->owner != current)) {
 			spin_lock(q.lock_ptr);
 			ret = fixup_pi_state_owner(uaddr2, &q, current);
+			/*
+			 * Drop the reference to the pi state which
+			 * the requeue_pi() code acquired for us.
+			 */
+			free_pi_state(q.pi_state);
 			spin_unlock(q.lock_ptr);
 		}
 	} else {

@@ -725,9 +725,12 @@ static struct futex_pi_state * alloc_pi_
 }
 
 /*
+ * Drops a reference to the pi_state object and frees or caches it
+ * when the last reference is gone.
+ *
  * Must be called with the hb lock held.
  */
-static void free_pi_state(struct futex_pi_state *pi_state)
+static void put_pi_state(struct futex_pi_state *pi_state)
 {
 	if (!pi_state)
 		return;

@@ -1729,7 +1732,7 @@ static int futex_requeue(u32 __user *uad
 		case 0:
 			break;
 		case -EFAULT:
-			free_pi_state(pi_state);
+			put_pi_state(pi_state);
 			pi_state = NULL;
 			double_unlock_hb(hb1, hb2);
 			hb_waiters_dec(hb2);

@@ -1746,7 +1749,7 @@ static int futex_requeue(u32 __user *uad
 			 *   exit to complete.
 			 * - The user space value changed.
 			 */
-			free_pi_state(pi_state);
+			put_pi_state(pi_state);
 			pi_state = NULL;
 			double_unlock_hb(hb1, hb2);
 			hb_waiters_dec(hb2);

@@ -1815,7 +1818,7 @@ static int futex_requeue(u32 __user *uad
 			} else if (ret) {
 				/* -EDEADLK */
 				this->pi_state = NULL;
-				free_pi_state(pi_state);
+				put_pi_state(pi_state);
 				goto out_unlock;
 			}
 		}

@@ -1824,7 +1827,7 @@ static int futex_requeue(u32 __user *uad
 	}
 
 out_unlock:
-	free_pi_state(pi_state);
+	put_pi_state(pi_state);
 	double_unlock_hb(hb1, hb2);
 	wake_up_q(&wake_q);
 	hb_waiters_dec(hb2);

@@ -1973,7 +1976,7 @@ static void unqueue_me_pi(struct futex_q
 	__unqueue_futex(q);
 
 	BUG_ON(!q->pi_state);
-	free_pi_state(q->pi_state);
+	put_pi_state(q->pi_state);
 	q->pi_state = NULL;
 
 	spin_unlock(q->lock_ptr);

@@ -2759,7 +2762,7 @@ static int futex_wait_requeue_pi(u32 __u
 			 * Drop the reference to the pi state which
 			 * the requeue_pi() code acquired for us.
 			 */
-			free_pi_state(q.pi_state);
+			put_pi_state(q.pi_state);
 			spin_unlock(q.lock_ptr);
 		}
 	} else {

@@ -1709,6 +1709,8 @@ static int futex_requeue(u32 __user *uad
 		 * exist yet, look it up one more time to ensure we have a
 		 * reference to it. If the lock was taken, ret contains the
 		 * vpid of the top waiter task.
+		 * If the lock was not taken, we have pi_state and an initial
+		 * refcount on it. In case of an error we have nothing.
 		 */
 		if (ret > 0) {
 			WARN_ON(pi_state);

@@ -1724,12 +1726,16 @@ static int futex_requeue(u32 __user *uad
 			 * it. So we rather use the known value than
 			 * rereading and handing potential crap to
 			 * lookup_pi_state.
+			 *
+			 * If that call succeeds then we have pi_state
+			 * and an initial refcount on it.
 			 */
 			ret = lookup_pi_state(ret, hb2, &key2, &pi_state);
 		}
 
 		switch (ret) {
 		case 0:
+			/* We hold a reference on the pi state. */
 			break;
 		case -EFAULT:
 			put_pi_state(pi_state);

@@ -1804,19 +1810,38 @@ static int futex_requeue(u32 __user *uad
 		 * of requeue_pi if we couldn't acquire the lock atomically.
 		 */
 		if (requeue_pi) {
-			/* Prepare the waiter to take the rt_mutex. */
+			/*
+			 * Prepare the waiter to take the rt_mutex. Take a
+			 * refcount on the pi_state and store the pointer in
+			 * the futex_q object of the waiter.
+			 */
 			atomic_inc(&pi_state->refcount);
 			this->pi_state = pi_state;
 			ret = rt_mutex_start_proxy_lock(&pi_state->pi_mutex,
 							this->rt_waiter,
 							this->task);
 			if (ret == 1) {
-				/* We got the lock. */
+				/*
+				 * We got the lock. We do neither drop
+				 * the refcount on pi_state nor clear
+				 * this->pi_state because the waiter
+				 * needs the pi_state for cleaning up
+				 * the user space value. It will drop
+				 * the refcount after doing so.
+				 */
 				requeue_pi_wake_futex(this, &key2, hb2);
 				drop_count++;
 				continue;
 			} else if (ret) {
-				/* -EDEADLK */
+				/*
+				 * rt_mutex_start_proxy_lock()
+				 * detected a potential deadlock when
+				 * we tried to queue that waiter. Drop
+				 * the pi_state reference which we
+				 * took above and remove the pointer
+				 * to the state from the waiters
+				 * futex_q object.
+				 */
 				this->pi_state = NULL;
 				put_pi_state(pi_state);
 				goto out_unlock;

@@ -1827,6 +1852,11 @@ static int futex_requeue(u32 __user *uad
 	}
 
 out_unlock:
+	/*
+	 * We took an extra initial reference to the pi_state either
+	 * in futex_proxy_trylock_atomic() or in lookup_pi_state(). We
+	 * need to drop it here again.
+	 */
 	put_pi_state(pi_state);
 	double_unlock_hb(hb1, hb2);
 	wake_up_q(&wake_q);

@@ -1737,9 +1737,9 @@ static int futex_requeue(u32 __user *uad
 		case 0:
 			/* We hold a reference on the pi state. */
 			break;
+
+			/* If the above failed, then pi_state is NULL */
 		case -EFAULT:
-			put_pi_state(pi_state);
-			pi_state = NULL;
 			double_unlock_hb(hb1, hb2);
 			hb_waiters_dec(hb2);
 			put_futex_key(&key2);

@@ -1755,8 +1755,6 @@ static int futex_requeue(u32 __user *uad
 			 *   exit to complete.
 			 * - The user space value changed.
 			 */
-			put_pi_state(pi_state);
-			pi_state = NULL;
 			double_unlock_hb(hb1, hb2);
 			hb_waiters_dec(hb2);
 			put_futex_key(&key2);

@@ -1842,20 +1842,21 @@ static int futex_requeue(u32 __user *uad
 				 */
 				this->pi_state = NULL;
 				put_pi_state(pi_state);
-				goto out_unlock;
+				break;
 			}
 		}
 		requeue_futex(this, hb1, hb2, &key2);
 		drop_count++;
 	}
 
-out_unlock:
 	/*
 	 * We took an extra initial reference to the pi_state either
 	 * in futex_proxy_trylock_atomic() or in lookup_pi_state(). We
 	 * need to drop it here again.
 	 */
 	put_pi_state(pi_state);
+
+out_unlock:
 	double_unlock_hb(hb1, hb2);
 	wake_up_q(&wake_q);
 	hb_waiters_dec(hb2);

index ae83ea7..4c71c86 100644
@@ -706,7 +706,7 @@ static int refill_pi_state_cache(void)
 	INIT_LIST_HEAD(&pi_state->list);
 	/* pi_mutex gets initialized later */
 	pi_state->owner = NULL;
-	atomic_set(&pi_state->refcount, 1);
+	atomic_set(&pi_state->refcount, 0);
 	pi_state->key = FUTEX_KEY_INIT;
 
 	current->pi_state_cache = pi_state;

@@ -714,12 +714,21 @@ static int refill_pi_state_cache(void)
 	return 0;
 }
 
+/*
+ * Adds a reference to the pi_state object.
+ */
+static inline void get_pi_state(struct futex_pi_state *pi_state)
+{
+	atomic_inc(&pi_state->refcount);
+}
+
 static struct futex_pi_state * alloc_pi_state(void)
 {
 	struct futex_pi_state *pi_state = current->pi_state_cache;
 
 	WARN_ON(!pi_state);
 	current->pi_state_cache = NULL;
+	get_pi_state(pi_state);
 
 	return pi_state;
 }

@@ -756,10 +765,9 @@ static void put_pi_state(struct futex_pi_state *pi_state)
 		/*
 		 * pi_state->list is already empty.
 		 * clear pi_state->owner.
-		 * refcount is at 0 - put it back to 1.
+		 * refcount is already at 0.
 		 */
 		pi_state->owner = NULL;
-		atomic_set(&pi_state->refcount, 1);
 		current->pi_state_cache = pi_state;
 	}
 }

@@ -954,7 +962,7 @@ static int attach_to_pi_state(u32 uval, struct futex_pi_state *pi_state,
 	if (pid != task_pid_vnr(pi_state->owner))
 		return -EINVAL;
 out_state:
-	atomic_inc(&pi_state->refcount);
+	get_pi_state(pi_state);
 	*ps = pi_state;
 	return 0;
 }

@@ -1813,7 +1821,7 @@ retry_private:
 			 * refcount on the pi_state and store the pointer in
 			 * the futex_q object of the waiter.
 			 */
-			atomic_inc(&pi_state->refcount);
+			get_pi_state(pi_state);
 			this->pi_state = pi_state;
 			ret = rt_mutex_start_proxy_lock(&pi_state->pi_mutex,
 							this->rt_waiter,