path: root/src/thread/sem_trywait.c
diff options
authorRich Felker <>2011-08-02 19:19:09 -0400
committerRich Felker <>2011-08-02 19:19:09 -0400
commit88c4e720317845a8e01aee03f142ba82674cd23d (patch)
tree54c92b29227e097c994e7d902e7c35dca60fe109 /src/thread/sem_trywait.c
parent88798393cab009ce78fe498051072db71ba9d035 (diff)
overhaul posix semaphores to fix destructability race
the race condition these changes address is described in glibc bug report number 12674: up until now, musl has shared the bug, and i had not been able to figure out how to eliminate it. in short, the problem is that it's not valid for sem_post to inspect the waiters count after incrementing the semaphore value, because another thread may have already successfully returned from sem_wait, (rightly) deemed itself the only remaining user of the semaphore, and chosen to destroy and free it (or unmap the shared memory it's stored in). POSIX is not explicit in blessing this usage, but it gives a very explicit analogous example with mutexes (which, in musl and glibc, also suffer from the same race condition bug) in the rationale for pthread_mutex_destroy. the new semaphore implementation augments the waiter count with a redundant waiter indication in the semaphore value itself, representing the presence of "last minute" waiters that may have arrived after sem_post read the waiter count. this allows sem_post to read the waiter count prior to incrementing the semaphore value, rather than after incrementing it, so as to avoid accessing the semaphore memory whatsoever after the increment takes place. a similar, but much simpler, fix should be possible for mutexes and other locking primitives whose usage rules are stricter than semaphores.
Diffstat (limited to 'src/thread/sem_trywait.c')
1 files changed, 4 insertions, 1 deletions
diff --git a/src/thread/sem_trywait.c b/src/thread/sem_trywait.c
index dd8f57e3..55d90075 100644
--- 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;