overhaul posix semaphores to fix destructability race

[musl] / src / thread / sem_trywait.c

diff --git a/src/thread/sem_trywait.c b/src/thread/sem_trywait.c
index dd8f57e..55d9007 100644 (file)
--- a/src/thread/sem_trywait.c
+++ b/src/thread/sem_trywait.c
@@ -4,7 +4,10 @@
 int sem_trywait(sem_t *sem)
 {
        int val = sem->__val[0];
-       if (val>0 && a_cas(sem->__val, val, val-1)==val) return 0;
+       if (val>0) {
+               int new = val-1-(val==1 && sem->__val[1]);
+               if (a_cas(sem->__val, val, new)==val) return 0;
+       }
        errno = EAGAIN;
        return -1;
 }