futex: Decrease the waiter count before the unlock operation

To support runtime resizing of the process private hash, it's required
to not use the obtained hash bucket once the reference count has been
dropped. The reference will be dropped after the unlock of the hash
bucket.
The amount of waiters is decremented after the unlock operation. There
is no requirement that this needs to happen after the unlock. The
increment happens before acquiring the lock to signal early that there
will be a waiter. The waiter can avoid blocking on the lock if it is
known that there will be no waiter.
There is no difference in terms of ordering if the decrement happens
before or after the unlock.

Decrease the waiter count before the unlock operation.

Signed-off-by: Sebastian Andrzej Siewior <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: Sebastian Andrzej Siewior

kernel/futex/core.c

@@ -537,8 +537,8 @@ void futex_q_lock(struct futex_q *q, struct futex_hash_bucket *hb)
 void futex_q_unlock(struct futex_hash_bucket *hb)
 	__releases(&hb->lock)
 {
-	spin_unlock(&hb->lock);
 	futex_hb_waiters_dec(hb);
+	spin_unlock(&hb->lock);
 }
 
 void __futex_queue(struct futex_q *q, struct futex_hash_bucket *hb,

kernel/futex/requeue.c

@@ -456,8 +456,8 @@ int futex_requeue(u32 __user *uaddr1, unsigned int flags1,
 			ret = futex_get_value_locked(&curval, uaddr1);
 
 			if (unlikely(ret)) {
-				double_unlock_hb(hb1, hb2);
 				futex_hb_waiters_dec(hb2);
+				double_unlock_hb(hb1, hb2);
 
 				ret = get_user(curval, uaddr1);
 				if (ret)
@@ -542,8 +542,8 @@ int futex_requeue(u32 __user *uaddr1, unsigned int flags1,
 				 * waiter::requeue_state is correct.
 				 */
 			case -EFAULT:
-				double_unlock_hb(hb1, hb2);
 				futex_hb_waiters_dec(hb2);
+				double_unlock_hb(hb1, hb2);
 				ret = fault_in_user_writeable(uaddr2);
 				if (!ret)
 					goto retry;
@@ -556,8 +556,8 @@ int futex_requeue(u32 __user *uaddr1, unsigned int flags1,
 				 *   exit to complete.
 				 * - EAGAIN: The user space value changed.
 				 */
-				double_unlock_hb(hb1, hb2);
 				futex_hb_waiters_dec(hb2);
+				double_unlock_hb(hb1, hb2);
 				/*
 				 * Handle the case where the owner is in the middle of
 				 * exiting. Wait for the exit to complete otherwise
@@ -674,8 +674,8 @@ int futex_requeue(u32 __user *uaddr1, unsigned int flags1,
 		put_pi_state(pi_state);
 
 out_unlock:
-		double_unlock_hb(hb1, hb2);
 		futex_hb_waiters_dec(hb2);
+		double_unlock_hb(hb1, hb2);
 	}
 	wake_up_q(&wake_q);
 	return ret ? ret : task_count;