change sem_trywait algorithm so it never has to call __wake

diff --git a/src/thread/sem_trywait.c b/src/thread/sem_trywait.c
index beb7aa5d3d359b793e11bf106ee21dda92feef73..dd8f57e36a0bd54a60a5b4c21516b9a1b6a8bba5 100644 (file)
--- a/src/thread/sem_trywait.c
+++ b/src/thread/sem_trywait.c
@@ -3,9 +3,8 @@
 
 int sem_trywait(sem_t *sem)
 {
-       if (a_fetch_add(sem->__val, -1) > 0) return 0;
-       if (!a_fetch_add(sem->__val, 1) && sem->__val[1])
-               __wake(sem->__val, 1, 0);
+       int val = sem->__val[0];
+       if (val>0 && a_cas(sem->__val, val, val-1)==val) return 0;
        errno = EAGAIN;
        return -1;
 }