--- x/security/landlock/tsync.c
+++ y/security/landlock/tsync.c
@@ -432,12 +432,17 @@ static void cancel_tsync_works(struct ts
 int landlock_restrict_sibling_threads(const struct cred *old_cred,
 				      const struct cred *new_cred)
 {
+	static int concur = 0;
 	int err;
 	struct tsync_shared_context shared_ctx;
 	struct tsync_works works = {};
 	size_t newly_discovered_threads;
 	bool found_more_threads;
 
+	if (concur++) {
+		concur--;
+		return -EBUSY;
+	}
 	atomic_set(&shared_ctx.preparation_error, 0);
 	init_completion(&shared_ctx.all_prepared);
 	init_completion(&shared_ctx.ready_to_commit);
@@ -556,6 +561,7 @@ int landlock_restrict_sibling_threads(co
 		wait_for_completion(&shared_ctx.all_finished);
 
 	tsync_works_release(&works);
+	concur--;
 
 	return atomic_read(&shared_ctx.preparation_error);
 }