[PATCH 3/5] wait: use EXIT_TRACE only if thread_group_leader(zombie)

[Oleg Nesterov <oleg@redhat.com>]

wait_task_zombie() always uses EXIT_TRACE/ptrace_unlink() if
ptrace_reparented(). This is suboptimal and a bit confusing:
we do not need do_notify_parent(p) if !thread_group_leader(p)
and in this case we also do not need ptrace_unlink(), we can
rely on ptrace_release_task().

Change wait_task_zombie() to check thread_group_leader() along
with ptrace_reparented() and simplify the final p->exit_state
transition.

Tested-by: Michal Schmidt <mschmidt@redhat.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/exit.c |   17 +++++++----------
 1 files changed, 7 insertions(+), 10 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index c702824..aaad08d 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -1042,7 +1042,7 @@ static int wait_task_zombie(struct wait_opts *wo, struct task_struct *p)
 	/*
 	 * Move the task's state to DEAD/TRACE, only one thread can do this.
 	 */
-	state = traced ? EXIT_TRACE : EXIT_DEAD;
+	state = traced && thread_group_leader(p) ? EXIT_TRACE : EXIT_DEAD;
 	if (cmpxchg(&p->exit_state, EXIT_ZOMBIE, state) != EXIT_ZOMBIE)
 		return 0;
 	/*
@@ -1142,18 +1142,15 @@ static int wait_task_zombie(struct wait_opts *wo, struct task_struct *p)
 	if (!retval)
 		retval = pid;
 
-	if (traced) {
+	if (state == EXIT_TRACE) {
 		write_lock_irq(&tasklist_lock);
 		/* We dropped tasklist, ptracer could die and untrace */
 		ptrace_unlink(p);
-		/*
-		 * If this is not a sub-thread, notify the parent.
-		 * If parent wants a zombie, don't release it now.
-		 */
-		state = EXIT_DEAD;
-		if (thread_group_leader(p) &&
-		    !do_notify_parent(p, p->exit_signal))
-			state = EXIT_ZOMBIE;
+
+		/* If parent wants a zombie, don't release it now */
+		state = EXIT_ZOMBIE;
+		if (do_notify_parent(p, p->exit_signal))
+			state = EXIT_DEAD;
 		p->exit_state = state;
 		write_unlock_irq(&tasklist_lock);
 	}
-- 
1.5.5.1