linux-kernel - Re: [GIT PULL] pidfd fixes

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Date:   Tue, 23 Jul 2019 12:12:49 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Christian Brauner <christian@...uner.io>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [GIT PULL] pidfd fixes

On 07/22, Linus Torvalds wrote:
>
> So if we set EXIT_ZOMBIE early, then I think we should change the
> EXIT_DEAD case too. IOW, do something like this on top:
> 
>   --- a/kernel/exit.c
>   +++ b/kernel/exit.c
>   @@ -734,9 +734,10 @@ static void exit_notify(struct task_struct
> *tsk, int group_dead)
>                 autoreap = true;
>         }
> 
>   -     tsk->exit_state = autoreap ? EXIT_DEAD : EXIT_ZOMBIE;
>   -     if (tsk->exit_state == EXIT_DEAD)
>   +     if (autoreap) {
>   +             tsk->exit_state = EXIT_DEAD;
>                 list_add(&tsk->ptrace_entry, &dead);
>   +     }

Yes, this needs cleanups. Actually I was going to suggest another change
below, this way do_notify_pidfd() is only called when it is really needed.
But then I decided a trivial one-liner makes more sense for the start.

I'll try to think. Perhaps we should also change do_notify_parent() to set
p->exit_state, at least if autoreap. Then the early exit_state = EXIT_ZOMBIE
won't look so confusing and we can do more (minor) cleanups.

Oleg.

--- x/kernel/exit.c
+++ x/kernel/exit.c
@@ -182,6 +182,13 @@ static void delayed_put_task_struct(struct rcu_head *rhp)
 	put_task_struct(tsk);
 }
 
+static void do_notify_pidfd(struct task_struct *task)
+{
+	struct pid *pid;
+
+	pid = task_pid(task);
+	wake_up_all(&pid->wait_pidfd);
+}
 
 void release_task(struct task_struct *p)
 {
@@ -218,6 +225,8 @@ void release_task(struct task_struct *p)
 		zap_leader = do_notify_parent(leader, leader->exit_signal);
 		if (zap_leader)
 			leader->exit_state = EXIT_DEAD;
+
+		do_notify_pidfd(leader);
 	}
 
 	write_unlock_irq(&tasklist_lock);
@@ -710,7 +719,7 @@ static void forget_original_parent(struct task_struct *father,
  */
 static void exit_notify(struct task_struct *tsk, int group_dead)
 {
-	bool autoreap;
+	bool autoreap, xxx;
 	struct task_struct *p, *n;
 	LIST_HEAD(dead);
 
@@ -720,23 +729,22 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
 	if (group_dead)
 		kill_orphaned_pgrp(tsk->group_leader, NULL);
 
-	if (unlikely(tsk->ptrace)) {
-		int sig = thread_group_leader(tsk) &&
-				thread_group_empty(tsk) &&
-				!ptrace_reparented(tsk) ?
-			tsk->exit_signal : SIGCHLD;
+	autoreap = true;
+	xxx = thread_group_leader(tsk) && thread_group_empty(tsk);
+
+	if (xxx || unlikely(tsk->ptrace)) {
+		int sig = xxx && !ptrace_reparented(tsk)
+			? tsk->exit_signal : SIGCHLD;
 		autoreap = do_notify_parent(tsk, sig);
-	} else if (thread_group_leader(tsk)) {
-		autoreap = thread_group_empty(tsk) &&
-			do_notify_parent(tsk, tsk->exit_signal);
-	} else {
-		autoreap = true;
 	}
 
 	tsk->exit_state = autoreap ? EXIT_DEAD : EXIT_ZOMBIE;
 	if (tsk->exit_state == EXIT_DEAD)
 		list_add(&tsk->ptrace_entry, &dead);
 
+	if (xxx)
+		do_notify_pidfd(tsk);
+
 	/* mt-exec, de_thread() is waiting for group leader */
 	if (unlikely(tsk->signal->notify_count < 0))
 		wake_up_process(tsk->signal->group_exit_task);
--- x/kernel/signal.c
+++ x/kernel/signal.c
@@ -1881,14 +1881,6 @@ int send_sigqueue(struct sigqueue *q, struct pid *pid, enum pid_type type)
 	return ret;
 }
 
-static void do_notify_pidfd(struct task_struct *task)
-{
-	struct pid *pid;
-
-	pid = task_pid(task);
-	wake_up_all(&pid->wait_pidfd);
-}
-
 /*
  * Let a parent know about the death of a child.
  * For a stopped/continued status change, use do_notify_parent_cldstop instead.
@@ -1912,9 +1904,6 @@ bool do_notify_parent(struct task_struct *tsk, int sig)
 	BUG_ON(!tsk->ptrace &&
 	       (tsk->group_leader != tsk || !thread_group_empty(tsk)));
 
-	/* Wake up all pidfd waiters */
-	do_notify_pidfd(tsk);
-
 	if (sig != SIGCHLD) {
 		/*
 		 * This is only possible if parent == real_parent.