Subject: Re: [PATCH net v2] ipv6: fix data race in fib6_metric_set() using cmpxchg

From: Hangbin Liu <[email-masked]>
Date: Tue, 31 Mar 2026 01:12:04 +0000

On Mon, Mar 30, 2026 at 05:46:28PM -0700, Jakub Kicinski wrote:
> On Fri, 27 Mar 2026 10:24:47 +0800 Hangbin Liu wrote:
> > --- a/net/ipv6/ip6_fib.c
> > +++ b/net/ipv6/ip6_fib.c
> > @@ -730,17 +730,24 @@ void fib6_metric_set(struct fib6_info *f6i, int metric, u32 val)
> >  	if (!f6i)
> >  		return;
> >  
> > -	if (f6i->fib6_metrics == &dst_default_metrics) {
> > +	if (READ_ONCE(f6i->fib6_metrics) == &dst_default_metrics) {
> > +		struct dst_metrics *dflt = (struct dst_metrics *)&dst_default_metrics;
> 
> Why does this exist? To cast away the const?

Yes, cause cmpxchg doesn't accept const type.

> 
> >  		struct dst_metrics *p = kzalloc_obj(*p, GFP_ATOMIC);
> >  
> >  		if (!p)
> >  			return;
> >  
> > +		p->metrics[metric - 1] = val;
> >  		refcount_set(&p->refcnt, 1);
> > -		f6i->fib6_metrics = p;
> > +		if (cmpxchg(&f6i->fib6_metrics, dflt, p) != dflt)
> > +			kfree(p);
> > +		else
> > +			return;
> >  	}
> >  
> > -	f6i->fib6_metrics->metrics[metric - 1] = val;
> > +	struct dst_metrics *m = READ_ONCE(f6i->fib6_metrics);
> 
> No variable declarations in the middle of a function please.

Oh, I thought it's OK now since kernel supports C99...

I will fix it.

Thanks
Hangbin