Index: lib/libc/include/thread_private.h
===================================================================
RCS file: /data/cvs-mirror/OpenBSD/src/lib/libc/include/thread_private.h,v
retrieving revision 1.36
diff -p -u -r1.36 thread_private.h
--- lib/libc/include/thread_private.h	6 Jan 2021 19:54:17 -0000	1.36
+++ lib/libc/include/thread_private.h	8 Dec 2022 04:28:45 -0000
@@ -228,7 +228,7 @@ __END_HIDDEN_DECLS
 	} while (0)
 #define _ATFORK_UNLOCK()						\
 	do {								\
-		if (__isthreaded)					\
+		if (_thread_cb.tc_atfork_unlock != NULL) 		\
 			_thread_cb.tc_atfork_unlock();			\
 	} while (0)

Index: regress/lib/libpthread/pthread_atfork_on_fork/Makefile
===================================================================
RCS file: regress/lib/libpthread/pthread_atfork_on_fork/Makefile
diff -N regress/lib/libpthread/pthread_atfork_on_fork/Makefile
--- /dev/null	1 Jan 1970 00:00:00 -0000
+++ regress/lib/libpthread/pthread_atfork_on_fork/Makefile	7 Dec 2022 04:38:39 -0000
@@ -0,0 +1,9 @@
+#	$OpenBSD$
+
+PROG=	pthread_atfork_on_fork
+
+REGRESS_TARGETS=	timeout
+timeout:
+	timeout 10s ./${PROG}
+
+.include <bsd.regress.mk>
Index: regress/lib/libpthread/pthread_atfork_on_fork/pthread_atfork_on_fork.c
===================================================================
RCS file: regress/lib/libpthread/pthread_atfork_on_fork/pthread_atfork_on_fork.c
diff -N regress/lib/libpthread/pthread_atfork_on_fork/pthread_atfork_on_fork.c
--- /dev/null	1 Jan 1970 00:00:00 -0000
+++ regress/lib/libpthread/pthread_atfork_on_fork/pthread_atfork_on_fork.c	7 Dec 2022 04:59:10 -0000
@@ -0,0 +1,94 @@
+/*	$OpenBSD$ */
+
+/*
+ * Copyright (c) 2022 Joel Knight <joel@openbsd.org>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+/*
+ * This test exercises atfork lock/unlock through multiple generations of
+ * forked child processes where each child also becomes multi-threaded.
+ */
+
+#include <pthread.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+#include <sys/wait.h>
+
+#define NUMBER_OF_GENERATIONS 4
+#define NUMBER_OF_TEST_THREADS 2
+
+#define SAY(...) do { \
+    fprintf(stderr, "pid %5i ", getpid()); \
+    fprintf(stderr, __VA_ARGS__); \
+} while (0)
+
+void
+prepare(void)
+{
+    /* Do nothing */
+}
+
+void *
+thread(void *arg)
+{
+    return (NULL);
+}
+
+int
+test(int fork_level)
+{
+    pid_t       proc_pid;
+    size_t      thread_index;
+    pthread_t   threads[NUMBER_OF_TEST_THREADS];
+
+    proc_pid = fork();
+    fork_level = fork_level - 1;
+
+    if (proc_pid == 0) {
+        SAY("generation %i\n", fork_level);
+        pthread_atfork(prepare, NULL, NULL);
+
+        for (thread_index = 0; thread_index < NUMBER_OF_TEST_THREADS; thread_index++) {
+            pthread_create(&threads[thread_index], NULL, thread, NULL);
+        }
+
+        for (thread_index = 0; thread_index < NUMBER_OF_TEST_THREADS; thread_index++) {
+            pthread_join(threads[thread_index], NULL);
+        }
+
+        if (fork_level > 0) {
+            test(fork_level);
+        }
+
+        SAY("exiting\n");
+        exit(0);
+    }
+    else {
+        SAY("parent waiting on child %i\n", proc_pid);
+        waitpid(proc_pid, 0, 0);
+    }
+
+    return (0);
+}
+
+int
+main(int argc, char *argv[])
+{
+    test(NUMBER_OF_GENERATIONS);
+
+    return (0);
+}