netdev - Re: [PATCH bpf-next 2/9] net: add SO_BUSY_POLL

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Date:   Thu, 12 Nov 2020 15:36:16 +0100
From:   Eric Dumazet <edumazet@...gle.com>
To:     Björn Töpel <bjorn.topel@...il.com>
Cc:     netdev <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Björn Töpel <bjorn.topel@...el.com>,
        magnus.karlsson@...el.com, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        maciej.fijalkowski@...el.com,
        "Samudrala, Sridhar" <sridhar.samudrala@...el.com>,
        Jesse Brandeburg <jesse.brandeburg@...el.com>,
        qi.z.zhang@...el.com, Jakub Kicinski <kuba@...nel.org>,
        Jonathan Lemon <jonathan.lemon@...il.com>, maximmi@...dia.com
Subject: Re: [PATCH bpf-next 2/9] net: add SO_BUSY_POLL_BUDGET socket option

On Thu, Nov 12, 2020 at 12:41 PM Björn Töpel <bjorn.topel@...il.com> wrote:
>
> From: Björn Töpel <bjorn.topel@...el.com>
>
> This option lets a user set a per socket NAPI budget for
> busy-polling. If the options is not set, it will use the default of 8.
>
> Signed-off-by: Björn Töpel <bjorn.topel@...el.com>
> ---
>

...

>  #else /* CONFIG_NET_RX_BUSY_POLL */
>  static inline unsigned long net_busy_loop_on(void)
> @@ -106,7 +108,8 @@ static inline void sk_busy_loop(struct sock *sk, int nonblock)
>
>         if (napi_id >= MIN_NAPI_ID)
>                 napi_busy_loop(napi_id, nonblock ? NULL : sk_busy_loop_end, sk,
> -                              READ_ONCE(sk->sk_prefer_busy_poll));
> +                              READ_ONCE(sk->sk_prefer_busy_poll),
> +                              sk->sk_busy_poll_budget ?: BUSY_POLL_BUDGET);

Please use :

       READ_ONCE(sk->sk_busy_poll_budget) ?: BUSY_POLL_BUDGET

Because sk_busy_loop() is usually called without socket lock being held.

This will prevent yet another KCSAN report.

>  #endif
>  }
>

...

> --- a/net/core/sock.c
> +++ b/net/core/sock.c
> @@ -1165,6 +1165,16 @@ int sock_setsockopt(struct socket *sock, int level, int optname,
>                 else
>                         sk->sk_prefer_busy_poll = valbool;
>                 break;
> +       case SO_BUSY_POLL_BUDGET:
> +               if (val > sk->sk_busy_poll_budget && !capable(CAP_NET_ADMIN)) {
> +                       ret = -EPERM;
> +               } else {
> +                       if (val < 0)

               if (val < 0 || val > (u16)~0)

> +                               ret = -EINVAL;
> +                       else
> +                               sk->sk_busy_poll_budget = val;


                               WRITE_ONCE(sk->sk_busy_poll_budget, val);

> +               }
> +               break;
>  #endif
>
>         case SO_MAX_PACING_RATE:
> --
> 2.27.0
>