path: root/src/internal/libc.h
diff options
authorRich Felker <>2013-09-20 02:00:27 -0400
committerRich Felker <>2013-09-20 02:00:27 -0400
commite803829e6b087c0ed91adc11f87185109bc59b31 (patch)
treead2fdda58fbba2c542785d9cc8babcd5f7d001ec /src/internal/libc.h
parentd8e283df58eb8bff1aa2f8a99347e294c7f67cb9 (diff)
fix potential deadlock bug in libc-internal locking logic
if a multithreaded program became non-multithreaded (i.e. all other threads exited) while one thread held an internal lock, the remaining thread would fail to release the lock. the the program then became multithreaded again at a later time, any further attempts to obtain the lock would deadlock permanently. the underlying cause is that the value of libc.threads_minus_1 at unlock time might not match the value at lock time. one solution would be returning a flag to the caller indicating whether the lock was taken and needs to be unlocked, but there is a simpler solution: using the lock itself as such a flag. note that this flag is not needed anyway for correctness; if the lock is not held, the unlock code is harmless. however, the memory synchronization properties associated with a_store are costly on some archs, so it's best to avoid executing the unlock code when it is unnecessary.
Diffstat (limited to 'src/internal/libc.h')
1 files changed, 2 insertions, 2 deletions
diff --git a/src/internal/libc.h b/src/internal/libc.h
index 3350b3d1..d625b56a 100644
--- a/src/internal/libc.h
+++ b/src/internal/libc.h
@@ -53,8 +53,8 @@ void __lock(volatile int *) ATTR_LIBC_VISIBILITY;
void __unlock(volatile int *) ATTR_LIBC_VISIBILITY;
int __lockfile(FILE *) ATTR_LIBC_VISIBILITY;
void __unlockfile(FILE *) ATTR_LIBC_VISIBILITY;
-#define LOCK(x) (libc.threads_minus_1 ? (__lock(x),1) : ((void)(x),1))
-#define UNLOCK(x) (libc.threads_minus_1 ? (__unlock(x),1) : ((void)(x),1))
+#define LOCK(x) __lock(x)
+#define UNLOCK(x) __unlock(x)
void __synccall(void (*)(void *), void *);
int __setxid(int, int, int, int);