netdev - Re: [PATCH bpf 1/2] bpf: fix a rcu_sched stall issue with bpf task/task

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Date:   Tue, 18 Aug 2020 10:06:47 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Yonghong Song <yhs@...com>
Cc:     bpf <bpf@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>,
        "Paul E . McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH bpf 1/2] bpf: fix a rcu_sched stall issue with bpf
 task/task_file iterator

On Tue, Aug 18, 2020 at 9:24 AM Yonghong Song <yhs@...com> wrote:
> index f21b5e1e4540..885b14cab2c0 100644
> --- a/kernel/bpf/task_iter.c
> +++ b/kernel/bpf/task_iter.c
> @@ -27,6 +27,8 @@ static struct task_struct *task_seq_get_next(struct pid_namespace *ns,
>         struct task_struct *task = NULL;
>         struct pid *pid;
>
> +       cond_resched();
> +
>         rcu_read_lock();
>  retry:
>         pid = idr_get_next(&ns->idr, tid);
> @@ -137,6 +139,8 @@ task_file_seq_get_next(struct bpf_iter_seq_task_file_info *info,
>         struct task_struct *curr_task;
>         int curr_fd = info->fd;
>
> +       cond_resched();
> +

Instead of adding it to every *seq_get_next() it probably should be in
bpf_seq_read().
If cond_resched() is needed in task_file_seq_get_next() it should
probably be after 'again:'.