netdev - Re: [PATCH net 2/3] net: sched: multiq: don't call qdisc

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Date:   Wed, 18 Sep 2019 15:56:36 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Vlad Buslov <vladbu@...lanox.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>,
        David Miller <davem@...emloft.net>
Subject: Re: [PATCH net 2/3] net: sched: multiq: don't call qdisc_put() while
 holding tree lock

On Wed, Sep 18, 2019 at 12:32 AM Vlad Buslov <vladbu@...lanox.com> wrote:
> diff --git a/net/sched/sch_multiq.c b/net/sched/sch_multiq.c
> index e1087746f6a2..4cfa9a7bd29e 100644
> --- a/net/sched/sch_multiq.c
> +++ b/net/sched/sch_multiq.c
> @@ -187,18 +187,21 @@ static int multiq_tune(struct Qdisc *sch, struct nlattr *opt,
>
>         sch_tree_lock(sch);
>         q->bands = qopt->bands;
> +       sch_tree_unlock(sch);
> +
>         for (i = q->bands; i < q->max_bands; i++) {
>                 if (q->queues[i] != &noop_qdisc) {
>                         struct Qdisc *child = q->queues[i];
>
> +                       sch_tree_lock(sch);
>                         q->queues[i] = &noop_qdisc;
>                         qdisc_tree_flush_backlog(child);
> +                       sch_tree_unlock(sch);
> +
>                         qdisc_put(child);
>                 }
>         }

Repeatedly acquiring and releasing a spinlock in a loop
does not seem to be a good idea. Is it possible to save
those qdisc pointers to an array or something similar?

Thanks.