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Date: Tue, 23 Jan 2024 20:56:08 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Tycho Andersen <tycho@tycho.pizza>
Cc: Christian Brauner <brauner@kernel.org>, linux-kernel@vger.kernel.org,
	linux-api@vger.kernel.org, Tycho Andersen <tandersen@netflix.com>
Subject: Re: [PATCH v3 1/3] pidfd: allow pidfd_open() on non-thread-group
 leaders
Message-ID: <20240123195608.GB9978@redhat.com>
References: <20240123153452.170866-1-tycho@tycho.pizza>
 <20240123153452.170866-2-tycho@tycho.pizza>
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In-Reply-To: <20240123153452.170866-2-tycho@tycho.pizza>
User-Agent: Mutt/1.5.24 (2015-08-30)

Too late for me, but I don't understand this patch after a quick glance.
perhaps I missed something...

On 01/23, Tycho Andersen wrote:
>
> @@ -256,6 +256,17 @@ void release_task(struct task_struct *p)
>  	write_lock_irq(&tasklist_lock);
>  	ptrace_release_task(p);
>  	thread_pid = get_pid(p->thread_pid);
> +
> +	/*
> +	 * If we're not the leader, notify any waiters on our pidfds. Note that
> +	 * we don't want to notify the leader until /everyone/ in the thread
> +	 * group is dead, viz. the condition below.
> +	 *
> +	 * We have to do this here, since __exit_signal() will
> +	 * __unhash_processes(), and break do_notify_pidfd()'s lookup.
> +	 */
> +	if (!thread_group_leader(p))
> +		do_notify_pidfd(p);

This doesn't look consistent.

If the task is a group leader do_notify_pidfd() is called by exit_notify()
when it becomes a zombie (if no other threads), before it is reaped by its
parent (unless autoreap).

If it is a sub-thread, it is called by release_task() above. Note that a
sub-thread can become a zombie too if it is traced.

>  	__exit_signal(p);

and,  do_notify_pidfd() is called before __exit_signal() which does
__unhash_process() -> detach_pid(PIDTYPE_PID).

Doesn't this mean that pidfd_poll() can hang? thread_group_exited()
won't return true after do_notify_pidfd() above, not to mention that
thread_group_empty() is not possible if !thread_group_leader().

So. When do we want to do do_notify_pidfd() ? Whe the task (leader or not)
becomes a zombie (passes exit_notify) or when it is reaped by release_task?

Either way pidfd_poll() needs more changes with this patch and it can't
use thread_group_exited(). If do_notify_pidfd() is called by release_task()
after __exit_signal(), it can just check pid_has_task(PIDTYPE_PID).

Oleg.