[PATCH -mm 1/2] do_wait-wakeup-optimization: fix child_wait_callback()->eligible_child() usage

[Oleg Nesterov <oleg@redhat.com>]

(on top of do_wait-wakeup-optimization-child_wait_callback-check-__wnothread-case.patch,
 fixes do_wait-wakeup-optimization-change-__wake_up_parent-to-use-filtered-wakeup.patch)

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>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---

 kernel/exit.c |   15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

--- WAIT/kernel/exit.c~2_WAKE_PARENT_ELIGIBLE_FIX	2009-09-01 12:49:27.000000000 +0200
+++ WAIT/kernel/exit.c	2009-09-01 12:59:23.000000000 +0200
@@ -1106,13 +1106,16 @@ static struct pid *task_pid_type(struct 
 	return pid;
 }
 
-static int eligible_child(struct wait_opts *wo, struct task_struct *p)
+static inline int eligible_pid(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)
-			return 0;
-	}
+	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 (!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:
@@ -1564,7 +1567,7 @@ static int child_wait_callback(wait_queu
 						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)