From 4abab5ad6c8465a7528ccdd5f49367da05f78bbd Mon Sep 17 00:00:00 2001
From: Vladimir Azarov <avm@intermediate-node.net>
Date: Tue, 1 Oct 2024 15:47:05 +0200
Subject: Initial version

---
 src/thread/synccall.c | 122 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 122 insertions(+)
 create mode 100644 src/thread/synccall.c

(limited to 'src/thread/synccall.c')

diff --git a/src/thread/synccall.c b/src/thread/synccall.c
new file mode 100644
index 0000000..3859725
--- /dev/null
+++ b/src/thread/synccall.c
@@ -0,0 +1,122 @@
+#include "pthread_impl.h"
+#include <semaphore.h>
+#include <string.h>
+
+static void dummy_0(void)
+{
+}
+
+weak_alias(dummy_0, __tl_lock);
+weak_alias(dummy_0, __tl_unlock);
+
+static int target_tid;
+static void (*callback)(void *), *context;
+static sem_t target_sem, caller_sem, exit_sem;
+
+static void dummy(void *p)
+{
+}
+
+static void handler(int sig)
+{
+	if (__pthread_self()->tid != target_tid) return;
+
+	int old_errno = errno;
+
+	/* Inform caller we have received signal and wait for
+	 * the caller to let us make the callback. */
+	sem_post(&caller_sem);
+	sem_wait(&target_sem);
+
+	callback(context);
+
+	/* Inform caller we've complered the callback and wait
+	 * for the caller to release us to return. */
+	sem_post(&caller_sem);
+	sem_wait(&exit_sem);
+
+	/* Inform caller we are returning and state is destroyable. */
+	sem_post(&caller_sem);
+
+	errno = old_errno;
+}
+
+void __synccall(void (*func)(void *), void *ctx)
+{
+	sigset_t oldmask;
+	int cs, i, r;
+	struct sigaction sa = { .sa_flags = SA_RESTART | SA_ONSTACK, .sa_handler = handler };
+	pthread_t self = __pthread_self(), td;
+	int count = 0;
+
+	/* 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);
+	__tl_lock();
+	__block_all_sigs(0);
+	pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, &cs);
+
+	sem_init(&target_sem, 0, 0);
+	sem_init(&caller_sem, 0, 0);
+	sem_init(&exit_sem, 0, 0);
+
+	if (!libc.threads_minus_1 || __syscall(SYS_gettid) != self->tid)
+		goto single_threaded;
+
+	callback = func;
+	context = ctx;
+
+	/* 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);
+
+
+	for (td=self->next; td!=self; td=td->next) {
+		target_tid = td->tid;
+		while ((r = -__syscall(SYS_tkill, td->tid, SIGSYNCCALL)) == EAGAIN);
+		if (r) {
+			/* If we failed to signal any thread, nop out the
+			 * callback to abort the synccall and just release
+			 * any threads already caught. */
+			callback = func = dummy;
+			break;
+		}
+		sem_wait(&caller_sem);
+		count++;
+	}
+	target_tid = 0;
+
+	/* Serialize execution of callback in caught threads, or just
+	 * release them all if synccall is being aborted. */
+	for (i=0; i<count; i++) {
+		sem_post(&target_sem);
+		sem_wait(&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 (i=0; i<count; i++)
+		sem_post(&exit_sem);
+	for (i=0; i<count; i++)
+		sem_wait(&caller_sem);
+
+	sem_destroy(&caller_sem);
+	sem_destroy(&target_sem);
+	sem_destroy(&exit_sem);
+
+	pthread_setcancelstate(cs, 0);
+	__tl_unlock();
+	__restore_sigs(&oldmask);
+}
-- 
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