Read write_completions earlier

as it should be stable before cnt_and_flag is updated.

lib/isc/rwlock.c

@@ -564,6 +564,7 @@ isc_rwlock_downgrade(isc_rwlock_t *rwl) {
 isc_result_t
 isc_rwlock_unlock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
 	int32_t prev_cnt;
+	int_fast32_t write_completions;
 
 	REQUIRE(VALID_RWLOCK(rwl));
 
@@ -572,6 +573,8 @@ isc_rwlock_unlock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
 #endif /* ifdef ISC_RWLOCK_TRACE */
 
 	if (type == isc_rwlocktype_read) {
+		write_completions =
+			atomic_load_acquire(&rwl->write_completions);
 		prev_cnt = atomic_fetch_sub_release(&rwl->cnt_and_flag,
 						    READER_INCR);
 		/*
@@ -580,7 +583,7 @@ isc_rwlock_unlock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
 		 * FIFO order.
 		 */
 		if (prev_cnt == READER_INCR &&
-		    atomic_load_acquire(&rwl->write_completions) !=
+		    write_completions !=
 			    atomic_load_acquire(&rwl->write_requests))
 		{
 			LOCK(&rwl->lock);