posix-timers: Add proper comments in do_timer_create()

The comment about timer lifetime at the end of the function is misplaced
and uncomprehensible.

Make it understandable and put it at the right place. Add a new comment
about the visibility of the new timer ID to user space.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/r/20230425183313.619897296@linutronix.de

kernel/time/posix-timers.c

@@ -529,12 +529,17 @@ static int do_timer_create(clockid_t which_clock, struct sigevent *event,
 	new_timer->sigq->info.si_tid   = new_timer->it_id;
 	new_timer->sigq->info.si_code  = SI_TIMER;
 
-	if (copy_to_user(created_timer_id,
-			 &new_timer_id, sizeof (new_timer_id))) {
+	if (copy_to_user(created_timer_id, &new_timer_id, sizeof (new_timer_id))) {
 		error = -EFAULT;
 		goto out;
 	}
-
+	/*
+	 * After succesful copy out, the timer ID is visible to user space
+	 * now but not yet valid because new_timer::signal is still NULL.
+	 *
+	 * Complete the initialization with the clock specific create
+	 * callback.
+	 */
 	error = kc->timer_create(new_timer);
 	if (error)
 		goto out;
@@ -544,14 +549,11 @@ static int do_timer_create(clockid_t which_clock, struct sigevent *event,
 	WRITE_ONCE(new_timer->it_signal, current->signal);
 	list_add(&new_timer->list, &current->signal->posix_timers);
 	spin_unlock_irq(&current->sighand->siglock);
-
-	return 0;
 	/*
-	 * In the case of the timer belonging to another task, after
-	 * the task is unlocked, the timer is owned by the other task
-	 * and may cease to exist at any time.  Don't use or modify
-	 * new_timer after the unlock call.
+	 * After unlocking sighand::siglock @new_timer is subject to
+	 * concurrent removal and cannot be touched anymore
 	 */
+	return 0;
 out:
 	posix_timer_unhash_and_free(new_timer);
 	return error;