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Miller" , David Ahern , Eric Dumazet , Paolo Abeni , Simon Horman , Jiayuan Chen , David Ahern , netdev@vger.kernel.org, linux-kernel@vger.kernel.org, Fei Liu Subject: Re: [PATCH net v2] ipv6: fix data race in fib6_metric_set() using cmpxchg Message-ID: References: <20260327-b4-fib6_metric_set-kmemleak-v2-1-366b2c78b5c2@gmail.com> <20260330174628.0cb1ebc6@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@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: <20260330174628.0cb1ebc6@kernel.org> 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