[patch 3/5] futex: Document pi_state refcounting in requeue code

[Thomas Gleixner]

Documentation of the pi_state refcounting in the requeue code is non
existent. Add it.

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
---
 kernel/futex.c |   36 +++++++++++++++++++++++++++++++++---
 1 file changed, 33 insertions(+), 3 deletions(-)

--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -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);


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