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Miller" , David Ahern , Jakub Kicinski , Paolo Abeni , Simon Horman , David Ahern , netdev@vger.kernel.org, Jiayuan Chen , linux-kernel@vger.kernel.org, Fei Liu Subject: Re: [PATCH net] ipv6: fix data race in fib6_metric_set() using cmpxchg Message-ID: References: <20260326-b4-fib6_metric_set-kmemleak-v1-1-c89fc1b312c0@gmail.com> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Thu, Mar 26, 2026 at 05:05:57AM -0700, Eric Dumazet wrote: > > diff --git a/net/ipv6/ip6_fib.c b/net/ipv6/ip6_fib.c > > index dd26657b6a4a..64de761f40d5 100644 > > --- a/net/ipv6/ip6_fib.c > > +++ b/net/ipv6/ip6_fib.c > > @@ -730,14 +730,16 @@ 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; > > struct dst_metrics *p = kzalloc_obj(*p, GFP_ATOMIC); > > > > if (!p) > > return; > > > > refcount_set(&p->refcnt, 1); > > - f6i->fib6_metrics = p; > > + if (cmpxchg(&f6i->fib6_metrics, dflt, p) != dflt) > > + kfree(p); > > } > > > > The following line should happen before the cmpxchg(), > ->metrics[X] accesses also need READ_ONCE()/WRITE_ONCE() annotations. Hi Eric, Jiayuan also suggested to using READ_ONCE()/WRITE_ONCE() for metrics[X] accesses. But I don't get why this line should happen before the cmpxchg(), Would you please help explain? > > > > f6i->fib6_metrics->metrics[metric - 1] = val; > > Thanks Hangbin