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

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Message-ID: <CAAXyoMNUSQXNCdzLWNimvu7TPV8+60GVcQASFCS2-N3d9MAmsg@mail.gmail.com>
Date: Thu, 5 Feb 2026 13:32:29 +0800
From: David Yang <mmyangfl@...il.com>
To: Jakub Kicinski <kuba@...nel.org>
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 Thu, Feb 5, 2026 at 1:12 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> 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.

.get_stats64() is atomic, that's why everyone (mostly) is using
u64_stats_t for DSA stats.