make thread killlock async-signal-safe for pthread_kill

pthread_kill is required to be AS-safe. that requirement can't be met
if the target thread's killlock can be taken in contexts where
application-installed signal handlers can run.

block signals around use of this lock in all pthread_* functions which
target a tid, and reorder blocking/unblocking of signals in
pthread_exit so that they're blocked whenever the killlock is held.

src/thread/pthread_create.c

@@ -72,12 +72,13 @@ _Noreturn void __pthread_exit(void *result)
 	/* Access to target the exiting thread with syscalls that use
 	 * its kernel tid is controlled by killlock. For detached threads,
 	 * any use past this point would have undefined behavior, but for
-	 * joinable threads it's a valid usage that must be handled. */
+	 * joinable threads it's a valid usage that must be handled.
+	 * Signals must be blocked since pthread_kill must be AS-safe. */
+	__block_app_sigs(&set);
 	LOCK(self->killlock);
 
-	/* The thread list lock must be AS-safe, and thus requires
-	 * application signals to be blocked before it can be taken. */
-	__block_app_sigs(&set);
+	/* The thread list lock must be AS-safe, and thus depends on
+	 * application signals being blocked above. */
 	__tl_lock();
 
 	/* If this is the only thread in the list, don't proceed with
@@ -85,8 +86,8 @@ _Noreturn void __pthread_exit(void *result)
 	 * signal state to prepare for exit to call atexit handlers. */
 	if (self->next == self) {
 		__tl_unlock();
-		__restore_sigs(&set);
 		UNLOCK(self->killlock);
+		__restore_sigs(&set);
 		exit(0);
 	}
 

src/thread/pthread_getschedparam.c

@@ -4,6 +4,8 @@
 int pthread_getschedparam(pthread_t t, int *restrict policy, struct sched_param *restrict param)
 {
 	int r;
+	sigset_t set;
+	__block_app_sigs(&set);
 	LOCK(t->killlock);
 	if (!t->tid) {
 		r = ESRCH;
@@ -14,5 +16,6 @@ int pthread_getschedparam(pthread_t t, int *restrict policy, struct sched_param
 		}
 	}
 	UNLOCK(t->killlock);
+	__restore_sigs(&set);
 	return r;
 }

src/thread/pthread_kill.c

@@ -4,9 +4,12 @@
 int pthread_kill(pthread_t t, int sig)
 {
 	int r;
+	sigset_t set;
+	__block_app_sigs(&set);
 	LOCK(t->killlock);
 	r = t->tid ? -__syscall(SYS_tkill, t->tid, sig)
 		: (sig+0U >= _NSIG ? EINVAL : 0);
 	UNLOCK(t->killlock);
+	__restore_sigs(&set);
 	return r;
 }

src/thread/pthread_setschedparam.c

@@ -4,8 +4,11 @@
 int pthread_setschedparam(pthread_t t, int policy, const struct sched_param *param)
 {
 	int r;
+	sigset_t set;
+	__block_app_sigs(&set);
 	LOCK(t->killlock);
 	r = !t->tid ? ESRCH : -__syscall(SYS_sched_setscheduler, t->tid, policy, param);
 	UNLOCK(t->killlock);
+	__restore_sigs(&set);
 	return r;
 }

src/thread/pthread_setschedprio.c

@@ -4,8 +4,11 @@
 int pthread_setschedprio(pthread_t t, int prio)
 {
 	int r;
+	sigset_t set;
+	__block_app_sigs(&set);
 	LOCK(t->killlock);
 	r = !t->tid ? ESRCH : -__syscall(SYS_sched_setparam, t->tid, &prio);
 	UNLOCK(t->killlock);
+	__restore_sigs(&set);
 	return r;
 }