Subject: Re: [PATCH net-next v2 4/7] net: openvswitch: fix load tearing with u64_stats

From: David Laight <[email-masked]>
Date: Mon, 26 Jan 2026 18:29:28 +0000

On Sat, 24 Jan 2026 00:21:36 +0800
David Yang <[email-masked]> 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 <[email-masked]>
> ---
>  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;