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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-48066be7404sm7185725e9.1.2026.01.26.10.29.29 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 26 Jan 2026 10:29:29 -0800 (PST) Date: Mon, 26 Jan 2026 18:29:28 +0000 From: David Laight To: David Yang Cc: netdev@vger.kernel.org, Aaron Conole , Eelco Chaudron , Ilya Maximets , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , dev@openvswitch.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH net-next v2 4/7] net: openvswitch: fix load tearing with u64_stats Message-ID: <20260126182928.39ab1d58@pumpkin> In-Reply-To: <20260123162159.2877941-5-mmyangfl@gmail.com> References: <20260123162159.2877941-1-mmyangfl@gmail.com> <20260123162159.2877941-5-mmyangfl@gmail.com> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) 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-Transfer-Encoding: 7bit On Sat, 24 Jan 2026 00:21:36 +0800 David Yang wrote: > On 64bit arches, struct u64_stats_sync is empty and provides no help > against load/store tearing. struct copying should not be considered > tear-free. Use u64_stats_reads() instead. Except that the compiler doesn't ever generate 'tearing accesses' for aligned 64bit accesses on any 64bit architecture. Similarly memcpy() won't generate problematic accesses. The problem is purely theoretical - the C language lets the compiler split accesses, but it doesn't. David > > Signed-off-by: David Yang > --- > net/openvswitch/datapath.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/net/openvswitch/datapath.c b/net/openvswitch/datapath.c > index d5b6e2002bc1..8ba94df7f942 100644 > --- a/net/openvswitch/datapath.c > +++ b/net/openvswitch/datapath.c > @@ -770,7 +770,8 @@ static void get_dp_stats(const struct datapath *dp, struct ovs_dp_stats *stats, > > do { > start = u64_stats_fetch_begin(&percpu_stats->syncp); > - local_stats = *percpu_stats; > + u64_stats_reads(&local_stats, percpu_stats, > + sizeof(local_stats)); > } while (u64_stats_fetch_retry(&percpu_stats->syncp, start)); > > stats->n_hit += local_stats.n_hit;