Proper implementation of adaptive rwlocks

lib/isc/rwlock.c

@@ -397,8 +397,10 @@ isc__rwlock_lock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
 isc_result_t
 isc_rwlock_lock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
 	int32_t cnt = 0;
-	int32_t spins = atomic_load_acquire(&rwl->spins) * 2 + 10;
-	int32_t max_cnt = ISC_MAX(spins, RWLOCK_MAX_ADAPTIVE_COUNT);
+	int32_t cachedspins = atomic_load_acquire(&rwl->spins);
+	int32_t spins = cachedspins * 2 + 10;
+	int32_t max_cnt = ISC_MIN(spins, RWLOCK_MAX_ADAPTIVE_COUNT);
+
 	isc_result_t result = ISC_R_SUCCESS;
 
 	do {
@@ -408,8 +410,12 @@ isc_rwlock_lock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
 		}
 		isc_rwlock_pause();
 	} while (isc_rwlock_trylock(rwl, type) != ISC_R_SUCCESS);
-
-	atomic_fetch_add_release(&rwl->spins, (cnt - spins) / 8);
+	/*
+	 * C99 integer division rounds towards 0, but we want a real 'floor'
+	 * here - otherwise we will never drop to anything below 7.
+	 */
+	int32_t update = ((cnt - cachedspins + 9) / 8) - 1;
+	atomic_fetch_add_release(&rwl->spins, update);
 
 	return (result);
 }