printk: Don't WARN on kthread_run failure.

Since __kthread_create_on_node() returns -EINTR upon SIGKILL,
we should not use WARN_ON() in order to catch kthread_run() failure.

Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=1ebbc20f223b99446034
Fixes: 5f53ca3ff8 ("printk: Implement legacy printer kthread for PREEMPT_RT")
Fixes: 76f258bf3f ("printk: nbcon: Introduce printer kthreads")
Signed-off-by: Tetsuo Handa <[email protected]>
Reviewed-by: John Ogness <[email protected]>
Reviewed-by: Petr Mladek <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Petr Mladek <[email protected]>

kernel/printk/nbcon.c

@@ -1382,7 +1382,7 @@ bool nbcon_kthread_create(struct console *con)
 		return true;
 
 	kt = kthread_run(nbcon_kthread_func, con, "pr/%s%d", con->name, con->index);
-	if (WARN_ON(IS_ERR(kt))) {
+	if (IS_ERR(kt)) {
 		con_printk(KERN_ERR, con, "failed to start printing thread\n");
 		return false;
 	}

kernel/printk/printk.c

@@ -3732,7 +3732,7 @@ static bool legacy_kthread_create(void)
 	lockdep_assert_console_list_lock_held();
 
 	kt = kthread_run(legacy_kthread_func, NULL, "pr/legacy");
-	if (WARN_ON(IS_ERR(kt))) {
+	if (IS_ERR(kt)) {
 		pr_err("failed to start legacy printing thread\n");
 		return false;
 	}