Re: [GIT PULL] pidfd fixes

[Christian Brauner <christian@brauner.io>]

On Tue, Jul 23, 2019 at 12:12:49PM +0200, Oleg Nesterov wrote:
> 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.

Yeah, that was my thinking exactly.

> 
> 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.

You mind sending that as a proper patch?
I also have a few more cleanups for other parts I intend to send later
this week. I'd pick this one up too.

> 
> 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;

In case you don't mind the length of it how about:

bool autoreap, leading_empty_thread_group;

It's not the prettiest names but having rather descriptive var names in
these codepaths seems like a good idea to me.
It also reads very naturally further below:

if (leading_empty_thread_group || unlikely(tsk->ptrace)) {
 	int sig = leading_empty_thread_group && !ptrace_reparented(tsk)
 		? tsk->exit_signal : SIGCHLD;

>  	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.
>