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>

1 files changed, 9 insertions, 6 deletions

diff --git a/kernel/exit.c b/kernel/exit.c
index 270a68b7f22f..3fb9a77863d5 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -1111,13 +1111,16 @@ static struct pid *task_pid_type(struct task_struct *task, enum pid_type type)
         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:
@@ -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)