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.

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);