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Miller" , David Ahern , Jakub Kicinski , Paolo Abeni , Simon Horman , David Ahern , netdev@vger.kernel.org, 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> <39c100fe-1f6f-46d2-8dd5-cf82320508ff@linux.dev> 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=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Thu, Mar 26, 2026 at 07:01:36AM -0700, Eric Dumazet wrote: > On Thu, Mar 26, 2026 at 6:44 AM Jiayuan Chen wrote: > > > > > > On 3/26/26 9:13 PM, Hangbin Liu wrote: > > > 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? > > > > > > I think what Eric means is something like this: > > > > > > ... > > struct dst_metrics *p = kzalloc_obj(*p, GFP_ATOMIC); > > > > if (!p) > > return; > > > > p->metrics[metric - 1] = val; > > refcount_set(&p->refcnt, 1); > > if (cmpxchg(&f6i->fib6_metrics, dflt, p) != dflt) > > kfree(p); > > else > > return; > > } > > } > > > > m = READ_ONCE(f6i->fib6_metrics); > > WRITE_ONCE(m->metrics[metric - 1], val); > > > > > > Since p is private data before being published via cmpxchg(), we can > > safely initialize its metrics beforehand. This way we don't need to > > worry about concurrent access to f6i->fib6_metrics->metrics[] during > > initialization. Right? > > Yes. Think about RCU (Read Copy Update) rules. > > We allocate an object, and populate it, then make sure changes are > committed, before publishing the new pointer. > > Othewise an other cpu could read a 0 metric, while we wanted something else. Ah, got it. Thanks for the explanation. Regards Hangbin