make do_notify_parent() __must_check, update the callers

Change other callers of do_notify_parent() to check the value it
returns, this makes the subsequent task_detached() unnecessary.
Mark do_notify_parent() as __must_check.

Use thread_group_leader() instead of !task_detached() to check
if we need to notify the real parent in wait_task_zombie().

Remove the stale comment in release_task(). "just for sanity" is
no longer true, we have to set EXIT_DEAD to avoid the races with
do_wait().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Tejun Heo <tj@kernel.org>

include/linux/sched.h

@@ -2160,7 +2160,7 @@ extern int kill_pid_info_as_uid(int, struct siginfo *, struct pid *, uid_t, uid_
 extern int kill_pgrp(struct pid *pid, int sig, int priv);
 extern int kill_pid(struct pid *pid, int sig, int priv);
 extern int kill_proc_info(int, struct siginfo *, pid_t);
-extern bool do_notify_parent(struct task_struct *, int);
+extern __must_check bool do_notify_parent(struct task_struct *, int);
 extern void __wake_up_parent(struct task_struct *p, struct task_struct *parent);
 extern void force_sig(int, struct task_struct *);
 extern int send_sig(int, struct task_struct *, int);

kernel/exit.c

@@ -190,21 +190,12 @@ repeat:
 	leader = p->group_leader;
 	if (leader != p && thread_group_empty(leader) && leader->exit_state == EXIT_ZOMBIE) {
 		BUG_ON(task_detached(leader));
-		do_notify_parent(leader, leader->exit_signal);
 		/*
 		 * If we were the last child thread and the leader has
 		 * exited already, and the leader's parent ignores SIGCHLD,
 		 * then we are the one who should release the leader.
-		 *
-		 * do_notify_parent() will have marked it self-reaping in
-		 * that case.
-		 */
-		zap_leader = task_detached(leader);
-
-		/*
-		 * This maintains the invariant that release_task()
-		 * only runs on a task in EXIT_DEAD, just for sanity.
 		 */
+		zap_leader = do_notify_parent(leader, leader->exit_signal);
 		if (zap_leader)
 			leader->exit_state = EXIT_DEAD;
 	}
@@ -766,8 +757,7 @@ static void reparent_leader(struct task_struct *father, struct task_struct *p,
 	/* If it has exited notify the new parent about this child's death. */
 	if (!p->ptrace &&
 	    p->exit_state == EXIT_ZOMBIE && thread_group_empty(p)) {
-		do_notify_parent(p, p->exit_signal);
-		if (task_detached(p)) {
+		if (do_notify_parent(p, p->exit_signal)) {
 			p->exit_state = EXIT_DEAD;
 			list_move_tail(&p->sibling, dead);
 		}
@@ -1351,16 +1341,13 @@ static int wait_task_zombie(struct wait_opts *wo, struct task_struct *p)
 		/* We dropped tasklist, ptracer could die and untrace */
 		ptrace_unlink(p);
 		/*
-		 * If this is not a detached task, notify the parent.
-		 * If it's still not detached after that, don't release
-		 * it now.
+		 * If this is not a sub-thread, notify the parent.
+		 * If parent wants a zombie, don't release it now.
 		 */
-		if (!task_detached(p)) {
-			do_notify_parent(p, p->exit_signal);
-			if (!task_detached(p)) {
-				p->exit_state = EXIT_ZOMBIE;
-				p = NULL;
-			}
+		if (thread_group_leader(p) &&
+		    !do_notify_parent(p, p->exit_signal)) {
+			p->exit_state = EXIT_ZOMBIE;
+			p = NULL;
 		}
 		write_unlock_irq(&tasklist_lock);
 	}