always block signals for starting new threads, refactor start args

whether signals need to be blocked at thread start, and whether
unblocking is necessary in the entry point function, has historically
depended on intricacies of the cancellation design and on whether
there are scheduling operations to perform on the new thread before
its successful creation can be committed. future changes to track an
AS-safe list of live threads will require signals to be blocked
whenever changes are made to the list, so ...

prior to commits b8742f32602add243ee2ce74d804015463726899 and
40bae2d32fd6f3ffea437fa745ad38a1fe77b27e, a signal mask for the entry
function to restore was part of the pthread structure. it was removed
to trim down the size of the structure, which both saved a small
amount of stack space and improved code generation on archs where
small immediate displacements are less costly than arbitrary ones, by
limiting the range of offsets between the base of the thread
structure, its members, and the thread pointer. these commits moved
the saved mask to a special structure used only when special
scheduling was needed, in which case the pthread_create caller and new
thread had to synchronize with each other and could use this memory to
pass a mask.

this commit partially reverts the above two commits, but instead of
putting the mask back in the pthread structure, it moves all "start
argument" members out of the pthread structure, trimming it down
further, and puts them in a separate structure passed on the new
thread's stack. the code path for explicit scheduling of the new
thread is also changed to synchronize with the calling thread in such
a way to avoid spurious futex wakes.

src/internal/pthread_impl.h

@@ -29,15 +29,12 @@ struct pthread {
 	volatile int cancel;
 	volatile unsigned char canceldisable, cancelasync;
 	unsigned char tsd_used:1;
-	unsigned char unblock_cancel:1;
 	unsigned char dlerror_flag:1;
 	unsigned char *map_base;
 	size_t map_size;
 	void *stack;
 	size_t stack_size;
 	size_t guard_size;
-	void *start_arg;
-	void *(*start)(void *);
 	void *result;
 	struct __ptcb *cancelbuf;
 	void **tsd;
@@ -58,14 +55,6 @@ struct pthread {
 	uintptr_t *dtv_copy;
 };
 
-struct start_sched_args {
-	void *start_arg;
-	void *(*start_fn)(void *);
-	sigset_t mask;
-	pthread_attr_t *attr;
-	volatile int futex;
-};
-
 enum {
 	DT_EXITED = 0,
 	DT_EXITING,

src/thread/pthread_attr_setinheritsched.c

@@ -1,25 +1,6 @@
 #include "pthread_impl.h"
 #include "syscall.h"
 
-hidden void *__start_sched(void *p)
-{
-	struct start_sched_args *ssa = p;
-	void *start_arg = ssa->start_arg;
-	void *(*start_fn)(void *) = ssa->start_fn;
-	pthread_t self = __pthread_self();
-
-	int ret = -__syscall(SYS_sched_setscheduler, self->tid,
-		ssa->attr->_a_policy, &ssa->attr->_a_prio);
-	if (!ret) __restore_sigs(&ssa->mask);
-	a_store(&ssa->futex, ret);
-	__wake(&ssa->futex, 1, 1);
-	if (ret) {
-		self->detach_state = DT_DYNAMIC;
-		return 0;
-	}
-	return start_fn(start_arg);
-}
-
 int pthread_attr_setinheritsched(pthread_attr_t *a, int inherit)
 {
 	if (inherit > 1U) return EINVAL;

src/thread/pthread_create.c

@@ -16,12 +16,6 @@ weak_alias(dummy_0, __pthread_tsd_run_dtors);
 weak_alias(dummy_0, __do_orphaned_stdio_locks);
 weak_alias(dummy_0, __dl_thread_cleanup);
 
-static void *dummy_1(void *p)
-{
-	return 0;
-}
-weak_alias(dummy_1, __start_sched);
-
 _Noreturn void __pthread_exit(void *result)
 {
 	pthread_t self = __pthread_self();
@@ -135,21 +129,38 @@ void __do_cleanup_pop(struct __ptcb *cb)
 	__pthread_self()->cancelbuf = cb->__next;
 }
 
+struct start_args {
+	void *(*start_func)(void *);
+	void *start_arg;
+	pthread_attr_t *attr;
+	volatile int *perr;
+	unsigned long sig_mask[_NSIG/8/sizeof(long)];
+};
+
 static int start(void *p)
 {
-	pthread_t self = p;
-	if (self->unblock_cancel)
-		__syscall(SYS_rt_sigprocmask, SIG_UNBLOCK,
-			SIGPT_SET, 0, _NSIG/8);
-	__pthread_exit(self->start(self->start_arg));
+	struct start_args *args = p;
+	if (args->attr) {
+		pthread_t self = __pthread_self();
+		int ret = -__syscall(SYS_sched_setscheduler, self->tid,
+			args->attr->_a_policy, &args->attr->_a_prio);
+		if (a_swap(args->perr, ret)==-2)
+			__wake(args->perr, 1, 1);
+		if (ret) {
+			self->detach_state = DT_DYNAMIC;
+			__pthread_exit(0);
+		}
+	}
+	__syscall(SYS_rt_sigprocmask, SIG_SETMASK, &args->sig_mask, 0, _NSIG/8);
+	__pthread_exit(args->start_func(args->start_arg));
 	return 0;
 }
 
 static int start_c11(void *p)
 {
-	pthread_t self = p;
-	int (*start)(void*) = (int(*)(void*)) self->start;
-	__pthread_exit((void *)(uintptr_t)start(self->start_arg));
+	struct start_args *args = p;
+	int (*start)(void*) = (int(*)(void*)) args->start_func;
+	__pthread_exit((void *)(uintptr_t)start(args->start_arg));
 	return 0;
 }
 
@@ -182,9 +193,9 @@ int __pthread_create(pthread_t *restrict res, const pthread_attr_t *restrict att
 	unsigned flags = CLONE_VM | CLONE_FS | CLONE_FILES | CLONE_SIGHAND
 		| CLONE_THREAD | CLONE_SYSVSEM | CLONE_SETTLS
 		| CLONE_PARENT_SETTID | CLONE_CHILD_CLEARTID | CLONE_DETACHED;
-	int do_sched = 0;
 	pthread_attr_t attr = { 0 };
-	struct start_sched_args ssa;
+	sigset_t set;
+	volatile int err = -1;
 
 	if (!libc.can_do_threads) return ENOSYS;
 	self = __pthread_self();
@@ -257,8 +268,6 @@ int __pthread_create(pthread_t *restrict res, const pthread_attr_t *restrict att
 	new->stack = stack;
 	new->stack_size = stack - stack_limit;
 	new->guard_size = guard;
-	new->start = entry;
-	new->start_arg = arg;
 	new->self = new;
 	new->tsd = (void *)tsd;
 	new->locale = &libc.global_locale;
@@ -268,38 +277,48 @@ int __pthread_create(pthread_t *restrict res, const pthread_attr_t *restrict att
 	} else {
 		new->detach_state = DT_JOINABLE;
 	}
-	if (attr._a_sched) {
-		do_sched = 1;
-		ssa.futex = -1;
-		ssa.start_fn = new->start;
-		ssa.start_arg = new->start_arg;
-		ssa.attr = &attr;
-		new->start = __start_sched;
-		new->start_arg = &ssa;
-		__block_app_sigs(&ssa.mask);
-	}
 	new->robust_list.head = &new->robust_list.head;
-	new->unblock_cancel = self->cancel;
 	new->CANARY = self->CANARY;
 
+	/* Setup argument structure for the new thread on its stack. */
+	stack -= (uintptr_t)stack % sizeof(uintptr_t);
+	stack -= sizeof(struct start_args);
+	struct start_args *args = (void *)stack;
+	args->start_func = entry;
+	args->start_arg = arg;
+	if (attr._a_sched) {
+		args->attr = &attr;
+		args->perr = &err;
+	} else {
+		args->attr = 0;
+		args->perr = 0;
+	}
+
+	__block_app_sigs(&set);
+
+	/* Ensure SIGCANCEL is unblocked in new thread. This requires
+	 * working with a copy of the set so we can restore the
+	 * original mask in the calling thread. */
+	memcpy(&args->sig_mask, &set, sizeof args->sig_mask);
+	args->sig_mask[(SIGCANCEL-1)/8/sizeof(long)] &=
+		~(1UL<<((SIGCANCEL-1)%(8*sizeof(long))));
+
 	a_inc(&libc.threads_minus_1);
-	ret = __clone((c11 ? start_c11 : start), stack, flags, new, &new->tid, TP_ADJ(new), &new->detach_state);
+	ret = __clone((c11 ? start_c11 : start), stack, flags, args, &new->tid, TP_ADJ(new), &new->detach_state);
 
+	__restore_sigs(&set);
 	__release_ptc();
 
-	if (do_sched) {
-		__restore_sigs(&ssa.mask);
-	}
-
 	if (ret < 0) {
 		a_dec(&libc.threads_minus_1);
 		if (map) __munmap(map, size);
 		return EAGAIN;
 	}
 
-	if (do_sched) {
-		__futexwait(&ssa.futex, -1, 1);
-		ret = ssa.futex;
+	if (attr._a_sched) {
+		if (a_cas(&err, -1, -2)==-1)
+			__wait(&err, 0, -2, 1);
+		ret = err;
 		if (ret) return ret;
 	}
 

src/time/timer_create.c

@@ -27,7 +27,6 @@ static void cleanup_fromsig(void *p)
 	self->cancelbuf = 0;
 	self->canceldisable = 0;
 	self->cancelasync = 0;
-	self->unblock_cancel = 0;
 	__reset_tls();
 	longjmp(p, 1);
 }