kthread: make it clear that kthread_create_on_node() might be terminated by any fatal signal

The comments in kernel/kthread.c create a feeling that only SIGKILL is
able to terminate the creation of kernel kthreads by
kthread_create()/_on_node()/_on_cpu() APIs.

In reality, wait_for_completion_killable() might be killed by any fatal
signal that does not have a custom handler:

	(!siginmask(signr, SIG_KERNEL_IGNORE_MASK|SIG_KERNEL_STOP_MASK) && \
	 (t)->sighand->action[(signr)-1].sa.sa_handler == SIG_DFL)

static inline void signal_wake_up(struct task_struct *t, bool resume)
{
	signal_wake_up_state(t, resume ? TASK_WAKEKILL : 0);
}

static void complete_signal(int sig, struct task_struct *p, enum pid_type type)
{
[...]
	/*
	 * Found a killable thread.  If the signal will be fatal,
	 * then start taking the whole group down immediately.
	 */
	if (sig_fatal(p, sig) ...) {
		if (!sig_kernel_coredump(sig)) {
		[...]
			do {
				task_clear_jobctl_pending(t, JOBCTL_PENDING_MASK);
				sigaddset(&t->pending.signal, SIGKILL);
				signal_wake_up(t, 1);
			} while_each_thread(p, t);
			return;
		}
	}
}

Update the comments in kernel/kthread.c to make this more obvious.

The motivation for this change was debugging why a module initialization
failed.  The module was being loaded from initrd.  It "magically" failed
when systemd was switching to the real root.  The clean up operations sent
SIGTERM to various pending processed that were started from initrd.

Link: https://lkml.kernel.org/r/20220315102444.2380-1-pmladek@suse.com


Signed-off-by: Petr Mladek <pmladek@suse.com>
Reviewed-by: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Cc: Kees Cook <keescook@chromium.org>
Cc: Marco Elver <elver@google.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>