do_wait-wakeup-optimization: fix child_wait_callback()->eligible_child() usage

child_wait_callback()->eligible_child() is not right, we can miss the
wakeup if the task was detached before __wake_up_parent() and the caller
of do_wait() didn't use __WALL.

Move ->wo_pid checks from eligible_child() to the new helper,
eligible_pid(), and change child_wait_callback() to use it instead of
eligible_child().

Note: actually I think it would be better to fix the __WCLONE check in
eligible_child(), it doesn't look exactly right.  But it is not clear what
is the supposed behaviour, and any change is user-visible.
Reported-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Tested-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/exit.c

@@ -1111,13 +1111,16 @@ static struct pid *task_pid_type(struct task_struct *task, enum pid_type type)
 	return pid;
 }
 
+static inline int eligible_pid(struct wait_opts *wo, struct task_struct *p)
+{
+	return	wo->wo_type == PIDTYPE_MAX ||
+		task_pid_type(p, wo->wo_type) == wo->wo_pid;
+}
+
 static int eligible_child(struct wait_opts *wo, struct task_struct *p)
 {
-	if (wo->wo_type < PIDTYPE_MAX) {
-		if (task_pid_type(p, wo->wo_type) != wo->wo_pid)
+	if (!eligible_pid(wo, p))
 		return 0;
-	}
-
 	/* Wait for all children (clone and not) if __WALL is set;
 	 * otherwise, wait for clone children *only* if __WCLONE is
 	 * set; otherwise, wait for non-clone children *only*.  (Note:
@@ -1578,7 +1581,7 @@ static int child_wait_callback(wait_queue_t *wait, unsigned mode,
 						child_wait);
 	struct task_struct *p = key;
 
-	if (!eligible_child(wo, p))
+	if (!eligible_pid(wo, p))
 		return 0;
 
 	if ((wo->wo_flags & __WNOTHREAD) && wait->private != p->parent)