timer_create: replace pthread barrier with semaphores for thread start

our pthread barrier implementation reportedly has bugs that are could
lead to malfunction or crash in timer_create. while this has not been
reviewed to confirm, there have been past reports of pthread barrier
bugs, and it seems likely that something is actually wrong.

pthread barriers are an obscure primitive, and timer_create is the
only place we are using them internally at present. even if they were
working correctly, this means we are imposing linking of otherwise
likely-dead code whenever timer_create is used.

a pair of semaphores functions identically to a 2-waiter barrier
except for destruction order properties. since the parent is
responsible for the argument structure (including semaphores)
lifetimes, the last operation on them in the timer thread must be
posting to the parent.

src/time/timer_create.c

@@ -1,6 +1,7 @@
 #include <time.h>
 #include <setjmp.h>
 #include <limits.h>
+#include <semaphore.h>
 #include "pthread_impl.h"
 #include "atomic.h"
 
@@ -12,7 +13,7 @@ struct ksigevent {
 };
 
 struct start_args {
-	pthread_barrier_t b;
+	sem_t sem1, sem2;
 	struct sigevent *sev;
 };
 
@@ -42,7 +43,14 @@ static void *start(void *arg)
 	void (*notify)(union sigval) = args->sev->sigev_notify_function;
 	union sigval val = args->sev->sigev_value;
 
-	pthread_barrier_wait(&args->b);
+	/* The two-way semaphore synchronization ensures that we see
+	 * self->cancel set by the parent if timer creation failed or
+	 * self->timer_id if it succeeded, and informs the parent that
+	 * we are done accessing the arguments so that the parent can
+	 * proceed past their block lifetime. */
+	while (sem_wait(&args->sem1));
+	sem_post(&args->sem2);
+
 	if (self->cancel)
 		return 0;
 	for (;;) {
@@ -99,7 +107,8 @@ int timer_create(clockid_t clk, struct sigevent *restrict evp, timer_t *restrict
 		else
 			pthread_attr_init(&attr);
 		pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED);
-		pthread_barrier_init(&args.b, 0, 2);
+		sem_init(&args.sem1, 0, 0);
+		sem_init(&args.sem2, 0, 0);
 		args.sev = evp;
 
 		__block_app_sigs(&set);
@@ -120,7 +129,8 @@ int timer_create(clockid_t clk, struct sigevent *restrict evp, timer_t *restrict
 			td->cancel = 1;
 		}
 		td->timer_id = timerid;
-		pthread_barrier_wait(&args.b);
+		sem_post(&args.sem1);
+		while (sem_wait(&args.sem2));
 		if (timerid < 0) return -1;
 		*res = (void *)(INTPTR_MIN | (uintptr_t)td>>1);
 		break;