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Date:   Wed, 13 May 2020 15:42:05 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Andrii Nakryiko <andriin@...com>
Cc:     bpf <bpf@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Kernel Team <kernel-team@...com>, Yonghong Song <yhs@...com>
Subject: Re: [PATCH bpf-next] bpf: fix bpf_iter's task iterator logic

On Wed, May 13, 2020 at 2:23 PM Andrii Nakryiko <andriin@...com> wrote:
>
> task_seq_get_next might stop prematurely if get_pid_task() fails to get
> task_struct. Failure to do so doesn't mean that there are no more tasks with
> higher pids. Procfs's iteration algorithm (see next_tgid in fs/proc/base.c)
> does a retry in such case. After this fix, instead of stopping prematurely
> after about 300 tasks on my server, bpf_iter program now returns >4000, which
> sounds much closer to reality.
>
> Cc: Yonghong Song <yhs@...com>
> Fixes: eaaacd23910f ("bpf: Add task and task/file iterator targets")
> Signed-off-by: Andrii Nakryiko <andriin@...com>
> ---
>  kernel/bpf/task_iter.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
> index a9b7264dda08..e1836def6738 100644
> --- a/kernel/bpf/task_iter.c
> +++ b/kernel/bpf/task_iter.c
> @@ -27,9 +27,15 @@ static struct task_struct *task_seq_get_next(struct pid_namespace *ns,
>         struct pid *pid;
>
>         rcu_read_lock();
> +retry:
>         pid = idr_get_next(&ns->idr, tid);
> -       if (pid)
> +       if (pid) {
>                 task = get_pid_task(pid, PIDTYPE_PID);
> +               if (!task) {
> +                       *tid++;

../kernel/bpf/task_iter.c: In function ‘task_seq_get_next’:
../kernel/bpf/task_iter.c:35:4: warning: value computed is not used
[-Wunused-value]
   35 |    *tid++;
      |    ^~~~~~

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