Re: [PATCH net-next 2/5] netdev-genl: Dump napi_defer_hard_irqs

Joe Damato
2024-08-30T09:10:52Z

On Thu, Aug 29, 2024 at 03:08:28PM -0700, Jakub Kicinski wrote:
> On Thu, 29 Aug 2024 13:11:58 +0000 Joe Damato wrote:
> > diff --git a/Documentation/netlink/specs/netdev.yaml b/Documentation/netlink/specs/netdev.yaml
> > index 959755be4d7f..ee4f99fd4574 100644
> > --- a/Documentation/netlink/specs/netdev.yaml
> > +++ b/Documentation/netlink/specs/netdev.yaml
> > @@ -244,6 +244,11 @@ attribute-sets:
> > threaded mode. If NAPI is not in threaded mode (i.e. uses normal
> > softirq context), the attribute will be absent.
> > type: u32
> > + -
> > + name: defer-hard-irqs
> > + doc: The number of consecutive empty polls before IRQ deferral ends
> > + and hardware IRQs are re-enabled.
> > + type: s32
>
> Why is this a signed value? 🤔️

In commit 6f8b12d661d0 ("net: napi: add hard irqs deferral
feature"), napi_defer_hard_irqs was added to struct net_device as an
int. I was trying to match that and thus made the field a signed int
in the napi struct, as well.

It looks like there was a possibility of overflow introduced in that
commit in change_napi_defer_hard_irqs maybe ?

If you'd prefer I could:
- submit a Fixes to change the net_device field to a u32 and then
change the netlink code to also be u32
- add an overflow check (val > U32_MAX) in
change_napi_defer_hard_irqs

Which would mean for the v2 of this series:
- drop the overflow check I added in Patch 1
- Change netlink to use u32 in this patch

What do you think?

> You can use:
>
> check:
> max: s32-max
>
> to have netlink validate the overflow if you switch to u32.
>
> > -
> > name: queue
> > attributes:
>
> > @@ -188,6 +189,10 @@ netdev_nl_napi_fill_one(struct sk_buff *rsp, struct napi_struct *napi,
> > goto nla_put_failure;
> > }
> >
> > + napi_defer_hard_irqs = napi_get_defer_hard_irqs(napi);
>
> Here, for example the READ_ONCE() wouldn't have been necessary, right?
> Cause we are holding rtnl_lock(), just a random thought, not really
> actionable.

That's right, yes.

I think it depends on where we land with the wrapper functions? I'll
reply with my thoughts about that in that thread.