linux-kernel - Re: [PATCH net-next 2/5] netdev-genl: Dump napi_defer_hard

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Message-ID: <ZtGMl25LaopZk1So@LQ3V64L9R2>
Date: Fri, 30 Aug 2024 10:10:47 +0100
From: Joe Damato <jdamato@...tly.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: netdev@...r.kernel.org, edumazet@...gle.com, amritha.nambiar@...el.com,
	sridhar.samudrala@...el.com, sdf@...ichev.me, bjorn@...osinc.com,
	hch@...radead.org, willy@...radead.org,
	willemdebruijn.kernel@...il.com, skhawaja@...gle.com,
	Martin Karsten <mkarsten@...terloo.ca>,
	Donald Hunter <donald.hunter@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Paolo Abeni <pabeni@...hat.com>,
	Jesper Dangaard Brouer <hawk@...nel.org>,
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
	Daniel Jurgens <danielj@...dia.com>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next 2/5] netdev-genl: Dump napi_defer_hard_irqs

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.