netdev - Re: [PATCH 1/1 v3 bpf-next] bpf: increment and use correct thread iterator

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Date:   Fri, 11 Dec 2020 16:28:09 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Jonathan Lemon <jonathan.lemon@...il.com>
Cc:     Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Yonghong Song <yhs@...com>, bpf <bpf@...r.kernel.org>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH 1/1 v3 bpf-next] bpf: increment and use correct thread iterator

On Fri, Dec 11, 2020 at 3:01 PM Jonathan Lemon <jonathan.lemon@...il.com> wrote:
>
> On Fri, Dec 11, 2020 at 12:23:34PM -0800, Andrii Nakryiko wrote:
> > > @@ -164,7 +164,7 @@ task_file_seq_get_next(struct bpf_iter_seq_task_file_info *info)
> > >                 curr_files = get_files_struct(curr_task);
> > >                 if (!curr_files) {
> > >                         put_task_struct(curr_task);
> > > -                       curr_tid = ++(info->tid);
> > > +                       curr_tid = curr_tid + 1;
> >
> > Yonghong might know definitively, but it seems like we need to update
> > info->tid here as well:
> >
> > info->tid = curr_tid;
> >
> > If the search eventually yields no task, then info->tid will stay at
> > some potentially much smaller value, and we'll keep re-searching tasks
> > from the same TID on each subsequent read (if user keeps reading the
> > file). So corner case, but good to have covered.
>
> That applies earlier as well:
>
>                 curr_task = task_seq_get_next(ns, &curr_tid, true);
>                 if (!curr_task) {
>                         info->task = NULL;
>                         info->files = NULL;
>                         return NULL;
>                 }
>

True, info->tid = curr_tid + 1; seems to be needed here?

> The logic seems to be "if task == NULL, then return NULL and stop".
> Is the seq_iterator allowed to continue/restart if seq_next returns NULL?

I don't think we allow seeking, so no restarts. But nothing will
prevent the user to keep calling read() after it returns 0 byte, so
yes, continuation is possible.

> --
> Jonathan