netdev - Re: [PATCH net-next 0/4] u64_stats: Introduce u64_stats

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Message-ID: <aXELAazPe7G5M6am@krikkit>
Date: Wed, 21 Jan 2026 18:21:05 +0100
From: Sabrina Dubroca <sd@...asysnail.net>
To: Ido Schimmel <idosch@...dia.com>
Cc: David Yang <mmyangfl@...il.com>, netdev@...r.kernel.org,
	Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Nikolay Aleksandrov <razor@...ckwall.org>,
	Simon Horman <horms@...nel.org>, Mark Bloch <mbloch@...dia.com>,
	Petr Machata <petrm@...dia.com>,
	Stanislav Fomichev <sdf@...ichev.me>,
	Carolina Jubran <cjubran@...dia.com>,
	Breno Leitao <leitao@...ian.org>,
	Shigeru Yoshida <syoshida@...hat.com>, linux-kernel@...r.kernel.org,
	bridge@...ts.linux.dev
Subject: Re: [PATCH net-next 0/4] u64_stats: Introduce u64_stats_copy()

2026-01-21, 13:16:35 +0200, Ido Schimmel wrote:
> On Tue, Jan 20, 2026 at 05:21:28PM +0800, David Yang wrote:
> > On 64bit arches, struct u64_stats_sync is empty and provides no help
> > against load/store tearing. memcpy() should not be considered atomic
> > against u64 values. Use u64_stats_copy() instead.
> 
> The existing memcpy() does seem problematic (even if in practice it's
> not) and the proposed solution in patch #1 seems OK to me given that all
> the callers only pass structures containing 64 bit counters. Couldn't
> find any more instances of this pattern.

No direct instances using memcpy, but do we need to also full structs
copied within a u64_stats_fetch_begin/u64_stats_fetch_retry loop?


// net/mpls/af_mpls.c
static void mpls_get_stats(struct mpls_dev *mdev,
			   struct mpls_link_stats *stats)
{
[...]
	for_each_possible_cpu(i) {
		struct mpls_link_stats local;
		unsigned int start;

		p = per_cpu_ptr(mdev->stats, i);
		do {
			start = u64_stats_fetch_begin(&p->syncp);
			local = p->stats;
		} while (u64_stats_fetch_retry(&p->syncp, start));
[...]

// net/openvswitch/datapath.c
static void get_dp_stats(const struct datapath *dp, struct ovs_dp_stats *stats,
			 struct ovs_dp_megaflow_stats *mega_stats)
{
[...]
	for_each_possible_cpu(i) {
		const struct dp_stats_percpu *percpu_stats;
		struct dp_stats_percpu local_stats;
		unsigned int start;

		percpu_stats = per_cpu_ptr(dp->stats_percpu, i);

		do {
			start = u64_stats_fetch_begin(&percpu_stats->syncp);
			local_stats = *percpu_stats;
		} while (u64_stats_fetch_retry(&percpu_stats->syncp, start));
[...]


And if not: can't we just use the same pattern for those other cases
that this series is touching?

-- 
Sabrina