Add a "fork" of musl as //fusl.

fusl/src/thread/synccall.c

Index: fusl/src/thread/synccall.c
diff --git a/fusl/src/thread/synccall.c b/fusl/src/thread/synccall.c
new file mode 100644
index 0000000000000000000000000000000000000000..000ec4e353a474c930cea0c68ad922d6908f7309
--- /dev/null
+++ b/fusl/src/thread/synccall.c
@@ -0,0 +1,178 @@
+#include "pthread_impl.h"
+#include <semaphore.h>
+#include <unistd.h>
+#include <dirent.h>
+#include <string.h>
+#include <ctype.h>
+#include "futex.h"
+#include "atomic.h"
+#include "../dirent/__dirent.h"
+
+static struct chain {
+	struct chain *next;
+	int tid;
+	sem_t target_sem, caller_sem;
+} *volatile head;
+
+static volatile int synccall_lock[2];
+static volatile int target_tid;
+static void (*callback)(void *), *context;
+static volatile int dummy = 0;
+weak_alias(dummy, __block_new_threads);
+
+static void handler(int sig)
+{
+	struct chain ch;
+	int old_errno = errno;
+
+	sem_init(&ch.target_sem, 0, 0);
+	sem_init(&ch.caller_sem, 0, 0);
+
+	ch.tid = __syscall(SYS_gettid);
+
+	do ch.next = head;
+	while (a_cas_p(&head, ch.next, &ch) != ch.next);
+
+	if (a_cas(&target_tid, ch.tid, 0) == (ch.tid | 0x80000000))
+		__syscall(SYS_futex, &target_tid, FUTEX_UNLOCK_PI|FUTEX_PRIVATE);
+
+	sem_wait(&ch.target_sem);
+	callback(context);
+	sem_post(&ch.caller_sem);
+	sem_wait(&ch.target_sem);
+
+	errno = old_errno;
+}
+
+void __synccall(void (*func)(void *), void *ctx)
+{
+	sigset_t oldmask;
+	int cs, i, r, pid, self;;
+	DIR dir = {0};
+	struct dirent *de;
+	struct sigaction sa = { .sa_flags = 0, .sa_handler = handler };
+	struct chain *cp, *next;
+	struct timespec ts;
+
+	/* Blocking signals in two steps, first only app-level signals
+	 * before taking the lock, then all signals after taking the lock,
+	 * is necessary to achieve AS-safety. Blocking them all first would
+	 * deadlock if multiple threads called __synccall. Waiting to block
+	 * any until after the lock would allow re-entry in the same thread
+	 * with the lock already held. */
+	__block_app_sigs(&oldmask);
+	LOCK(synccall_lock);
+	__block_all_sigs(0);
+	pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, &cs);
+
+	head = 0;
+
+	if (!libc.threaded) goto single_threaded;
+
+	callback = func;
+	context = ctx;
+
+	/* This atomic store ensures that any signaled threads will see the
+	 * above stores, and prevents more than a bounded number of threads,
+	 * those already in pthread_create, from creating new threads until
+	 * the value is cleared to zero again. */
+	a_store(&__block_new_threads, 1);
+
+	/* Block even implementation-internal signals, so that nothing
+	 * interrupts the SIGSYNCCALL handlers. The main possible source
+	 * of trouble is asynchronous cancellation. */
+	memset(&sa.sa_mask, -1, sizeof sa.sa_mask);
+	__libc_sigaction(SIGSYNCCALL, &sa, 0);
+
+	pid = __syscall(SYS_getpid);
+	self = __syscall(SYS_gettid);
+
+	/* Since opendir is not AS-safe, the DIR needs to be setup manually
+	 * in automatic storage. Thankfully this is easy. */
+	dir.fd = open("/proc/self/task", O_RDONLY|O_DIRECTORY|O_CLOEXEC);
+	if (dir.fd < 0) goto out;
+
+	/* Initially send one signal per counted thread. But since we can't
+	 * synchronize with thread creation/exit here, there could be too
+	 * few signals. This initial signaling is just an optimization, not
+	 * part of the logic. */
+	for (i=libc.threads_minus_1; i; i--)
+		__syscall(SYS_kill, pid, SIGSYNCCALL);
+
+	/* Loop scanning the kernel-provided thread list until it shows no
+	 * threads that have not already replied to the signal. */
+	for (;;) {
+		int miss_cnt = 0;
+		while ((de = readdir(&dir))) {
+			if (!isdigit(de->d_name[0])) continue;
+			int tid = atoi(de->d_name);
+			if (tid == self || !tid) continue;
+
+			/* Set the target thread as the PI futex owner before
+			 * checking if it's in the list of caught threads. If it
+			 * adds itself to the list after we check for it, then
+			 * it will see its own tid in the PI futex and perform
+			 * the unlock operation. */
+			a_store(&target_tid, tid);
+
+			/* Thread-already-caught is a success condition. */
+			for (cp = head; cp && cp->tid != tid; cp=cp->next);
+			if (cp) continue;
+
+			r = -__syscall(SYS_tgkill, pid, tid, SIGSYNCCALL);
+
+			/* Target thread exit is a success condition. */
+			if (r == ESRCH) continue;
+
+			/* The FUTEX_LOCK_PI operation is used to loan priority
+			 * to the target thread, which otherwise may be unable
+			 * to run. Timeout is necessary because there is a race
+			 * condition where the tid may be reused by a different
+			 * process. */
+			clock_gettime(CLOCK_REALTIME, &ts);
+			ts.tv_nsec += 10000000;
+			if (ts.tv_nsec >= 1000000000) {
+				ts.tv_sec++;
+				ts.tv_nsec -= 1000000000;
+			}
+			r = -__syscall(SYS_futex, &target_tid,
+				FUTEX_LOCK_PI|FUTEX_PRIVATE, 0, &ts);
+
+			/* Obtaining the lock means the thread responded. ESRCH
+			 * means the target thread exited, which is okay too. */
+			if (!r || r == ESRCH) continue;
+
+			miss_cnt++;
+		}
+		if (!miss_cnt) break;
+		rewinddir(&dir);
+	}
+	close(dir.fd);
+
+	/* Serialize execution of callback in caught threads. */
+	for (cp=head; cp; cp=cp->next) {
+		sem_post(&cp->target_sem);
+		sem_wait(&cp->caller_sem);
+	}
+
+	sa.sa_handler = SIG_IGN;
+	__libc_sigaction(SIGSYNCCALL, &sa, 0);
+
+single_threaded:
+	func(ctx);
+
+	/* Only release the caught threads once all threads, including the
+	 * caller, have returned from the callback function. */
+	for (cp=head; cp; cp=next) {
+		next = cp->next;
+		sem_post(&cp->target_sem);
+	}
+
+out:
+	a_store(&__block_new_threads, 0);
+	__wake(&__block_new_threads, -1, 1);
+
+	pthread_setcancelstate(cs, 0);
+	UNLOCK(synccall_lock);
+	__restore_sigs(&oldmask);
+}