restore lock-skipping for processes that return to single-threaded state

the design used here relies on the barrier provided by the first lock
operation after the process returns to single-threaded state to
synchronize with actions by the last thread that exited. by storing
the intent to change modes in the same object used to detect whether
locking is needed, it's possible to avoid an extra (possibly costly)
memory load after the lock is taken.

src/internal/libc.h

@@ -21,6 +21,7 @@ struct __libc {
 	char can_do_threads;
 	char threaded;
 	char secure;
+	volatile signed char need_locks;
 	int threads_minus_1;
 	size_t *auxv;
 	struct tls_module *tls_head;

src/malloc/malloc.c

@@ -26,8 +26,11 @@ int __malloc_replaced;
 
 static inline void lock(volatile int *lk)
 {
-	if (libc.threaded)
+	int need_locks = libc.need_locks;
+	if (need_locks) {
 		while(a_swap(lk, 1)) __wait(lk, lk+1, 1, 1);
+		if (need_locks < 0) libc.need_locks = 0;
+	}
 }
 
 static inline void unlock(volatile int *lk)

src/thread/__lock.c

@@ -18,9 +18,11 @@
 
 void __lock(volatile int *l)
 {
-	if (!libc.threaded) return;
+	int need_locks = libc.need_locks;
+	if (!need_locks) return;
 	/* fast path: INT_MIN for the lock, +1 for the congestion */
 	int current = a_cas(l, 0, INT_MIN + 1);
+	if (need_locks < 0) libc.need_locks = 0;
 	if (!current) return;
 	/* A first spin loop, for medium congestion. */
 	for (unsigned i = 0; i < 10; ++i) {

src/thread/pthread_create.c

@@ -118,8 +118,8 @@ _Noreturn void __pthread_exit(void *result)
 	 * until the lock is released, which only happens after SYS_exit
 	 * has been called, via the exit futex address pointing at the lock.
 	 * This needs to happen after any possible calls to LOCK() that might
-	 * skip locking if libc.threads_minus_1 is zero. */
-	libc.threads_minus_1--;
+	 * skip locking if process appears single-threaded. */
+	if (!--libc.threads_minus_1) libc.need_locks = -1;
 	self->next->prev = self->prev;
 	self->prev->next = self->next;
 	self->prev = self->next = self;
@@ -339,7 +339,7 @@ int __pthread_create(pthread_t *restrict res, const pthread_attr_t *restrict att
 		~(1UL<<((SIGCANCEL-1)%(8*sizeof(long))));
 
 	__tl_lock();
-	libc.threads_minus_1++;
+	if (!libc.threads_minus_1++) libc.need_locks = 1;
 	ret = __clone((c11 ? start_c11 : start), stack, flags, args, &new->tid, TP_ADJ(new), &__thread_list_lock);
 
 	/* All clone failures translate to EAGAIN. If explicit scheduling
@@ -363,7 +363,7 @@ int __pthread_create(pthread_t *restrict res, const pthread_attr_t *restrict att
 		new->next->prev = new;
 		new->prev->next = new;
 	} else {
-		libc.threads_minus_1--;
+		if (!--libc.threads_minus_1) libc.need_locks = 0;
 	}
 	__tl_unlock();
 	__restore_sigs(&set);