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Message-ID: <CANn89iLKxuBcriFNjtAS8DuhyLq2MPzGdvZxzijzhYdKM+Cw6w@mail.gmail.com>
Date: Wed, 17 Apr 2024 12:04:12 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Jason Xing <kerneljasonxing@...il.com>
Cc: kuba@...nel.org, pabeni@...hat.com, davem@...emloft.net, horms@...nel.org,
netdev@...r.kernel.org, Jason Xing <kernelxing@...cent.com>
Subject: Re: [PATCH net-next v2 2/3] net: rps: protect filter locklessly
On Wed, Apr 17, 2024 at 8:27 AM Jason Xing <kerneljasonxing@...il.com> wrote:
>
> From: Jason Xing <kernelxing@...cent.com>
>
> As we can see, rflow->filter can be written/read concurrently, so
> lockless access is needed.
>
> Signed-off-by: Jason Xing <kernelxing@...cent.com>
> ---
> I'm not very sure if the READ_ONCE in set_rps_cpu() is useful. I
> scaned/checked the codes and found no lock can prevent multiple
> threads from calling set_rps_cpu() and handling the same flow
> simultaneously. The same question still exists in patch [3/3].
> ---
> net/core/dev.c | 6 +++---
> 1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 2003b9a61e40..40a535158e45 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -4524,8 +4524,8 @@ set_rps_cpu(struct net_device *dev, struct sk_buff *skb,
> goto out;
> old_rflow = rflow;
> rflow = &flow_table->flows[flow_id];
> - rflow->filter = rc;
> - if (old_rflow->filter == rflow->filter)
> + WRITE_ONCE(rflow->filter, rc);
> + if (old_rflow->filter == READ_ONCE(rflow->filter))
You missed the obvious opportunity to use
if (old_rflow->filter == rc)
Here your code is going to force the compiler to read the memory right
after a prior write, adding a stall on some arches.
> old_rflow->filter = RPS_NO_FILTER;
> out:
> #endif
> @@ -4666,7 +4666,7 @@ bool rps_may_expire_flow(struct net_device *dev, u16 rxq_index,
> if (flow_table && flow_id <= flow_table->mask) {
> rflow = &flow_table->flows[flow_id];
> cpu = READ_ONCE(rflow->cpu);
> - if (rflow->filter == filter_id && cpu < nr_cpu_ids &&
> + if (READ_ONCE(rflow->filter) == filter_id && cpu < nr_cpu_ids &&
> ((int)(READ_ONCE(per_cpu(softnet_data, cpu).input_queue_head) -
> READ_ONCE(rflow->last_qtail)) <
> (int)(10 * flow_table->mask)))
> --
> 2.37.3
>
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