netdev - Re: [PATCH net-next v2 2/3] net: rps: protect filter locklessly

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Message-ID: <CAL+tcoBZ0MCntKO2POZ9g6kZ7euMXZY94FWN85siH1tZ6w5Lrg@mail.gmail.com>
Date: Wed, 17 Apr 2024 19:03:10 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Eric Dumazet <edumazet@...gle.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 6:04 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> 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.

Thanks. I see. I will remove READ_ONCE() and then reuse 'rc'.

I would like to ask one relational question: could multiple threads
access the same rflow in set_rps_cpu() concurrently? Because I was
thinking a lot about whether I should use the READ_ONCE() here to
prevent another thread accessing/modifying this value concurrently.
The answer is probably yes?

Thanks,
Jason

>
> >                         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
> >