Re: [PATCH net] ipv6: add READ_ONCE() annotations to fib6_metrics reader paths

[Paolo Abeni]

On 4/3/26 6:16 AM, Hangbin Liu wrote:
> diff --git a/include/net/ip6_fib.h b/include/net/ip6_fib.h
> index 9f8b6814a96a..2dfb04fab5da 100644
> --- a/include/net/ip6_fib.h
> +++ b/include/net/ip6_fib.h
> @@ -594,7 +594,9 @@ void fib6_update_sernum_stub(struct net *net, struct fib6_info *f6i);
>  void fib6_metric_set(struct fib6_info *f6i, int metric, u32 val);
>  static inline bool fib6_metric_locked(struct fib6_info *f6i, int metric)
>  {
> -	return !!(f6i->fib6_metrics->metrics[RTAX_LOCK - 1] & (1 << metric));
> +	struct dst_metrics *m = READ_ONCE(f6i->fib6_metrics);
> +
> +	return !!(m->metrics[RTAX_LOCK - 1] & (1 << metric));

Sashiko notes that here you may want to add an additional READ_ONCE() on
m->metrics[RTAX_LOCK - 1], which in turn looks like more a
follow-up/separate change than a change specific to this patch

>  }
>  void fib6_info_hw_flags_set(struct net *net, struct fib6_info *f6i,
>  			    bool offload, bool trap, bool offload_failed);
> diff --git a/net/ipv6/ip6_fib.c b/net/ipv6/ip6_fib.c
> index dd26657b6a4a..3c96580c2a03 100644
> --- a/net/ipv6/ip6_fib.c
> +++ b/net/ipv6/ip6_fib.c
> @@ -1144,9 +1144,11 @@ static int fib6_add_rt2node(struct fib6_node *fn, struct fib6_info *rt,
>  					iter->fib6_flags &= ~RTF_PREFIX_RT;
>  				}
>  
> -				if (rt->fib6_pmtu)
> +				u32 pmtu = READ_ONCE(rt->fib6_pmtu);

Here the READ_ONCE() on rt->metrics is still missing.

> @@ -1635,11 +1635,12 @@ __rt6_find_exception_rcu(struct rt6_exception_bucket **bucket,
>  static unsigned int fib6_mtu(const struct fib6_result *res)
>  {
>  	const struct fib6_nh *nh = res->nh;
> +	struct dst_metrics *m;
>  	unsigned int mtu;
>  
> -	if (res->f6i->fib6_pmtu) {
> -		mtu = res->f6i->fib6_pmtu;
> -	} else {
> +	m = READ_ONCE(res->f6i->fib6_metrics);
> +	mtu = READ_ONCE(m->metrics[RTAX_MTU - 1]);

After this patch there will be a single usage of the `fib6_pmtu` macro.
I think it would be better to entirely drop it and replace with an helper:

static inline unsigned int fib6_mtu(const struct fib6_info *f6i)
{
	const struct dst_metrics *m = READ_ONCE(f6i->fib6_metrics);
	return READ_ONCE(m->metrics[RTAX_MTU - 1]);
}

/P