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

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Message-ID: <20230825124115.GA13849@redhat.com>
Date:   Fri, 25 Aug 2023 14:41:15 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Yonghong Song <yhs@...com>, Kui-Feng Lee <kuifeng@...com>,
        Andrii Nakryiko <andrii@...nel.org>
Cc:     "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()

OK, it seems that you are not going to take these preparatory
cleanups ;)

I'll resend along with the s/next_thread/__next_thread/ change.
I was going to do the last change later, but this recent discussion
https://lore.kernel.org/all/20230824143112.GA31208@redhat.com/
makes me think we should do this right now.

On 08/21, 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.
>
> 2. next_thread() simply can't return NULL, kill the bogus "if (!next_task)"
>    check.
>
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>
> ---
>  kernel/bpf/task_iter.c | 7 -------
>  1 file changed, 7 deletions(-)
>
> diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
> index c4ab9d6cdbe9..4d1125108014 100644
> --- a/kernel/bpf/task_iter.c
> +++ b/kernel/bpf/task_iter.c
> @@ -75,15 +75,8 @@ static struct task_struct *task_group_seq_get_next(struct bpf_iter_seq_task_comm
>  		return NULL;
>
>  retry:
> -	if (!pid_alive(task)) {
> -		put_task_struct(task);
> -		return NULL;
> -	}
> -
>  	next_task = next_thread(task);
>  	put_task_struct(task);
> -	if (!next_task)
> -		return NULL;
>
>  	saved_tid = *tid;
>  	*tid = __task_pid_nr_ns(next_task, PIDTYPE_PID, common->ns);
> --
> 2.25.1.362.g51ebf55
>
>