[PATCH] pidfs: cleanup the usage of do_notify_pidfd()

[Oleg Nesterov <oleg@redhat.com>]

If a single-threaded process exits do_notify_pidfd() will be called twice,
from exit_notify() and right after that from do_notify_parent().

1. Change exit_notify() to call do_notify_pidfd() if the exiting task is
   not ptraced and it is not a group leader.

2. Change do_notify_parent() to call do_notify_pidfd() unconditionally.

   If tsk is not ptraced, do_notify_parent() will only be called when it
   is a group-leader and thread_group_empty() is true.

This means that if tsk is ptraced, do_notify_pidfd() will be called from
do_notify_parent() even if tsk is a delay_group_leader(). But this case is
less common, and apart from the unnecessary __wake_up() is harmless.

Granted, this unnecessary __wake_up() can be avoided, but I don't want to
do it in this patch because it's just a consequence of another historical
oddity: we notify the tracer even if !thread_group_empty(), but do_wait()
from debugger can't work until all other threads exit. With or without this
patch we should either eliminate do_notify_parent() in this case, or change
do_wait(WEXITED) to untrace the ptraced delay_group_leader() at least when
ptrace_reparented().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/exit.c   | 8 ++------
 kernel/signal.c | 8 +++-----
 2 files changed, 5 insertions(+), 11 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index 683766316a3d..d0ebccb9dec0 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -742,12 +742,6 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
 		kill_orphaned_pgrp(tsk->group_leader, NULL);
 
 	tsk->exit_state = EXIT_ZOMBIE;
-	/*
-	 * Ignore thread-group leaders that exited before all
-	 * subthreads did.
-	 */
-	if (!delay_group_leader(tsk))
-		do_notify_pidfd(tsk);
 
 	if (unlikely(tsk->ptrace)) {
 		int sig = thread_group_leader(tsk) &&
@@ -760,6 +754,8 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
 			do_notify_parent(tsk, tsk->exit_signal);
 	} else {
 		autoreap = true;
+		/* untraced sub-thread */
+		do_notify_pidfd(tsk);
 	}
 
 	if (autoreap) {
diff --git a/kernel/signal.c b/kernel/signal.c
index 027ad9e97417..1d8db0dabb71 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2179,11 +2179,9 @@ bool do_notify_parent(struct task_struct *tsk, int sig)
 
 	WARN_ON_ONCE(!tsk->ptrace &&
 	       (tsk->group_leader != tsk || !thread_group_empty(tsk)));
-	/*
-	 * Notify for thread-group leaders without subthreads.
-	 */
-	if (thread_group_empty(tsk))
-		do_notify_pidfd(tsk);
+
+	/* ptraced, or group-leader without sub-threads */
+	do_notify_pidfd(tsk);
 
 	if (sig != SIGCHLD) {
 		/*
-- 
2.25.1.362.g51ebf55