diff --git a/src/thread/sem_trywait.c b/src/thread/sem_trywait.c
index 55d90075e274a5e0a82359ffc96e6b45a279e286..04edf46b524de72a575b1db7687f56d93fc1e291 100644
--- a/src/thread/sem_trywait.c
+++ b/src/thread/sem_trywait.c
@@ -3,8 +3,8 @@
 
 int sem_trywait(sem_t *sem)
 {
-	int val = sem->__val[0];
-	if (val>0) {
+	int val;
+	while ((val=sem->__val[0]) > 0) {
 		int new = val-1-(val==1 && sem->__val[1]);
 		if (a_cas(sem->__val, val, new)==val) return 0;
 	}