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/[email protected]
Signed-off-by: Petr Mladek <[email protected]>
Reviewed-by: "Eric W. Biederman" <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Marco Elver <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: pmladek

kernel/kthread.c

@@ -340,7 +340,7 @@ static int kthread(void *_create)
 
 	self = to_kthread(current);
 
-	/* If user was SIGKILLed, I release the structure. */
+	/* Release the structure when caller killed by a fatal signal. */
 	done = xchg(&create->done, NULL);
 	if (!done) {
 		kfree(create);
@@ -398,7 +398,7 @@ static void create_kthread(struct kthread_create_info *create)
 	/* We want our own signal handler (we take no signals by default). */
 	pid = kernel_thread(kthread, create, CLONE_FS | CLONE_FILES | SIGCHLD);
 	if (pid < 0) {
-		/* If user was SIGKILLed, I release the structure. */
+		/* Release the structure when caller killed by a fatal signal. */
 		struct completion *done = xchg(&create->done, NULL);
 
 		if (!done) {
@@ -440,9 +440,9 @@ struct task_struct *__kthread_create_on_node(int (*threadfn)(void *data),
 	 */
 	if (unlikely(wait_for_completion_killable(&done))) {
 		/*
-		 * If I was SIGKILLed before kthreadd (or new kernel thread)
-		 * calls complete(), leave the cleanup of this structure to
-		 * that thread.
+		 * If I was killed by a fatal signal before kthreadd (or new
+		 * kernel thread) calls complete(), leave the cleanup of this
+		 * structure to that thread.
 		 */
 		if (xchg(&create->done, NULL))
 			return ERR_PTR(-EINTR);
@@ -876,7 +876,7 @@ __kthread_create_worker(int cpu, unsigned int flags,
  *
  * Returns a pointer to the allocated worker on success, ERR_PTR(-ENOMEM)
  * when the needed structures could not get allocated, and ERR_PTR(-EINTR)
- * when the worker was SIGKILLed.
+ * when the caller was killed by a fatal signal.
  */
 struct kthread_worker *
 kthread_create_worker(unsigned int flags, const char namefmt[], ...)
@@ -925,7 +925,7 @@ EXPORT_SYMBOL(kthread_create_worker);
  * Return:
  * The pointer to the allocated worker on success, ERR_PTR(-ENOMEM)
  * when the needed structures could not get allocated, and ERR_PTR(-EINTR)
- * when the worker was SIGKILLed.
+ * when the caller was killed by a fatal signal.
  */
 struct kthread_worker *
 kthread_create_worker_on_cpu(int cpu, unsigned int flags,