linux-kernel - Re: [PATCH] bpf: task_group_seq_get_next: cleanup the usage of next

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Date:   Mon, 21 Aug 2023 20:34:43 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Kui-Feng Lee <sinquersw@...il.com>
Cc:     Yonghong Song <yhs@...com>, Kui-Feng Lee <kuifeng@...com>,
        Andrii Nakryiko <andrii@...nel.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Martin KaFai Lau <martin.lau@...nel.org>,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] bpf: task_group_seq_get_next: cleanup the usage of
 next_thread()

On 08/21, Kui-Feng Lee wrote:
>
>
> On 8/21/23 08:09, Oleg Nesterov wrote:
> >1. find_pid_ns() + get_pid_task() under rcu_read_lock() guarantees that we
> >    can safely iterate the task->thread_group list. Even if this task exits
> >    right after get_pid_task() (or goto retry) and pid_alive() returns 0 >
> >    Kill the unnecessary pid_alive() check.
>
> This function will return next_task holding a refcount, and release the
> refcount until the next time calling the same function. Meanwhile,
> the returned task A may be killed, and its next task B may be
> killed after A as well, before calling this function again.
> However, even task B is destroyed (free), A's next is still pointing to
> task B. When this function is called again for the same iterator,
> it doesn't promise that B is still there.

Not sure I understand...

OK, if we have a task pointer with incremented refcount and do not hold
rcu lock, then yes, you can't remove the pid_alive() check in this code:

	rcu_read_lock();
	if (pid_alive(task))
		do_something(next_thread(task));
	rcu_read_unlock();

because task and then task->next can exit and do call_rcu(delayed_put_task_struct)
before we take rcu_read_lock().

But if you do something like

	rcu_read_lock();

	task = find_task_in_some_rcu_protected_list();
	do_something(next_thread(task));

	rcu_read_unlock();

then next_thread(task) should be safe without pid_alive().

And iiuc task_group_seq_get_next() always does

	rcu_read_lock();	// the caller does lock/unlock

	task = get_pid_task(pid, PIDTYPE_PID);
	if (!task)
		return;
	
	next_task = next_thread(task);

	rcu_read_unlock();

Yes, both task and task->next can exit right after get_pid_task(), but since
can only happen after we took rcu_read_lock(), delayed_put_task_struct() can't
be called until we drop rcu lock.

What have I missed?

Oleg.