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Date:   Wed, 13 May 2020 22:45:17 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        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 3:42 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> 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++;
>       |    ^~~~~~

welp... thanks, fixing to prefix form

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