exit(3): make it thread-safe

It was explained by Rich Felker <[email protected]> on libc-coord.
See https://austingroupbugs.net/view.php?id=1845.

Reviewed by:	imp, markj
Tested by:	antoine (exp-run)
Sponsored by:	The FreeBSD Foundation
MFC after:	1 month
Differential revision:	https://reviews.freebsd.org/D46108

lib/libc/stdlib/exit.3

@@ -29,7 +29,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd August 5, 2021
+.Dd July 24, 2024
 .Dt EXIT 3
 .Os
 .Sh NAME
@@ -102,6 +102,23 @@ values described in
 .Xr sysexits 3
 may be used to provide more information to the parent process.
 .Pp
+Calls to the
+.Fn exit
+function are serialized.
+All functions registered by
+.Xr atexit 3
+are executed in the first thread that called
+.Nm exit .
+If any other thread of the process calls
+.Nm exit
+before all registered functions have completed or before the process
+terminates, the thread is blocked until the process terminates.
+The exit status of the process is the
+.Fa status
+argument of the first
+.Nm exit
+call which thread proceeds the atexit handlers.
+.Pp
 Note that
 .Fn exit
 does nothing to prevent bottomless recursion should a function registered

lib/libc/stdlib/exit.c

@@ -31,6 +31,7 @@
 
 #include "namespace.h"
 #include <stdlib.h>
+#include <pthread.h>
 #include <unistd.h>
 #include "un-namespace.h"
 
@@ -48,6 +49,20 @@ void (*__cleanup)(void);
  */
 int	__isthreaded	= 0;
 
+static pthread_mutex_t exit_mutex;
+static pthread_once_t exit_mutex_once = PTHREAD_ONCE_INIT;
+
+static void
+exit_mutex_init_once(void)
+{
+	pthread_mutexattr_t ma;
+
+	_pthread_mutexattr_init(&ma);
+	_pthread_mutexattr_settype(&ma, PTHREAD_MUTEX_RECURSIVE);
+	_pthread_mutex_init(&exit_mutex, &ma);
+	_pthread_mutexattr_destroy(&ma);
+}
+
 /*
  * Exit, flushing stdio buffers if necessary.
  */
@@ -59,6 +74,12 @@ exit(int status)
 
 	_thread_autoinit_dummy_decl = 1;
 
+	/* Make exit(3) thread-safe */
+	if (__isthreaded) {
+		_once(&exit_mutex_once, exit_mutex_init_once);
+		_pthread_mutex_lock(&exit_mutex);
+	}
+
 	/*
 	 * We're dealing with cleaning up thread_local destructors in the case of
 	 * the process termination through main() exit.