netdev - Re: [PATCH net-next 3/3] net: dsa: yt921x: Use u64_stats

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Message-ID: <20260204211252.71b8fa86@kernel.org>
Date: Wed, 4 Feb 2026 21:12:52 -0800
From: Jakub Kicinski <kuba@...nel.org>
To: David Yang <mmyangfl@...il.com>
Cc: netdev@...r.kernel.org, Andrew Lunn <andrew@...n.ch>, Vladimir Oltean
 <olteanv@...il.com>, "David S. Miller" <davem@...emloft.net>, Eric Dumazet
 <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 3/3] net: dsa: yt921x: Use u64_stats_t for MIB
 stats

On Wed,  4 Feb 2026 01:12:43 +0800 David Yang wrote:
> 64-bit variables might not be atomic on 32-bit architectures, thus
> cannot be unconditionally made lock-free. Use u64_stats_t so it would
> still be lock-free on 64-bit architectures.

IDK why we need to worry about lock free reading of those 64b values...

> @@ -771,22 +785,27 @@ yt921x_dsa_get_ethtool_stats(struct dsa_switch *ds, int port, uint64_t *data)
>  	struct yt921x_priv *priv = to_yt921x_priv(ds);
>  	struct yt921x_port *pp = &priv->ports[port];
>  	struct yt921x_mib *mib = &pp->mib;
> +	unsigned int start;
>  	size_t j;
>  
>  	mutex_lock(&priv->reg_lock);
>  	yt921x_read_mib(priv, port);
>  	mutex_unlock(&priv->reg_lock);

..when in all(?) the readers converted by this patch we take the mutex
to refresh the stats anyway. And presumably do expensive IO/reg reads
under that mutex. So just extend the mutex over the read section and
save us the retry logic, please, it will make no difference to perf.

> -	j = 0;
> -	for (size_t i = 0; i < ARRAY_SIZE(yt921x_mib_descs); i++) {
> -		const struct yt921x_mib_desc *desc = &yt921x_mib_descs[i];
> +	do {
> +		start = u64_stats_fetch_begin(&pp->syncp);
>  
> -		if (!desc->name)
> -			continue;
> +		j = 0;
> +		for (size_t i = 0; i < ARRAY_SIZE(yt921x_mib_descs); i++) {
> +			const struct yt921x_mib_desc *desc = &yt921x_mib_descs[i];
>  
> -		data[j] = ((u64 *)mib)[i];
> -		j++;
> -	}
> +			if (!desc->name)
> +				continue;
> +
> +			data[j] = u64_stats_read(&((u64_stats_t *)mib)[i]);
> +			j++;
> +		}
> +	} while (u64_stats_fetch_retry(&pp->syncp, start));
>  }