netdev - Re: [PATCH net-next v2 3/3] net: rps: locklessly access rflow->cpu

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Message-ID: <CAL+tcoCWT5PQ9BG697+AAxhxge2R=XsHgu-GEQjdDxYgLJn3aA@mail.gmail.com>
Date: Wed, 17 Apr 2024 20:24:53 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com, 
	davem@...emloft.net, horms@...nel.org
Cc: netdev@...r.kernel.org, Jason Xing <kernelxing@...cent.com>
Subject: Re: [PATCH net-next v2 3/3] net: rps: locklessly access rflow->cpu

On Wed, Apr 17, 2024 at 2:27 PM Jason Xing <kerneljasonxing@...il.com> wrote:
>
> From: Jason Xing <kernelxing@...cent.com>
>
> This is the last member in struct rps_dev_flow which should be
> protected locklessly. So finish it.
>
> Signed-off-by: Jason Xing <kernelxing@...cent.com>
> ---
>  net/core/dev.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 40a535158e45..aeb45025e2bc 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -4533,7 +4533,7 @@ set_rps_cpu(struct net_device *dev, struct sk_buff *skb,
>                 rps_input_queue_tail_save(&rflow->last_qtail, head);
>         }
>
> -       rflow->cpu = next_cpu;
> +       WRITE_ONCE(rflow->cpu, next_cpu);
>         return rflow;
>  }
>
> @@ -4597,7 +4597,7 @@ static int get_rps_cpu(struct net_device *dev, struct sk_buff *skb,
>                  * we can look at the local (per receive queue) flow table
>                  */
>                 rflow = &flow_table->flows[hash & flow_table->mask];
> -               tcpu = rflow->cpu;
> +               tcpu = READ_ONCE(rflow->cpu);

Hello Eric,

I think I don't need this one either, right? Only protecting the
writer side in set_rps_cpu() and the reader side in
rps_may_expire_flow() is enough.

Thanks,
Jason

>
>                 /*
>                  * If the desired CPU (where last recvmsg was done) is
> --
> 2.37.3
>