netdev - Re: [PATCH net 1/2] net: dsa: xrs700x: Use irqsave variant for u64 stats update

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Message-ID: <CAFSKS=Pes1XzvG0OkuR+KfOcBH0xwgWnZinSf9mExFjJ+2s3sw@mail.gmail.com>
Date:   Thu, 25 Aug 2022 06:51:59 -0500
From:   George McCollister <george.mccollister@...il.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     netdev <netdev@...r.kernel.org>, Jakub Kicinski <kuba@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Vladimir Oltean <olteanv@...il.com>
Subject: Re: [PATCH net 1/2] net: dsa: xrs700x: Use irqsave variant for u64
 stats update

On Thu, Aug 25, 2022 at 6:36 AM Sebastian Andrzej Siewior
<bigeasy@...utronix.de> wrote:
>
> xrs700x_read_port_counters() updates the stats from a worker using the
> u64_stats_update_begin() version. This is okay on 32-UP since on the
> reader side preemption is disabled.
> On 32bit-SMP the writer can be preempted by the reader at which point
> the reader will spin on the seqcount until writer continues and
> completes the update.
>
> Assigning the mib_mutex mutex to the underlying seqcount would ensure
> proper synchronisation. The API for that on the u64_stats_init() side
> isn't available. Since it is the only user, just use disable interrupts
> during the update.
>
> Use u64_stats_update_begin_irqsave() on the writer side to ensure an
> uninterrupted update.
>
> Fixes: ee00b24f32eb8 ("net: dsa: add Arrow SpeedChips XRS700x driver")
> Cc: Andrew Lunn <andrew@...n.ch>
> Cc: Florian Fainelli <f.fainelli@...il.com>
> Cc: George McCollister <george.mccollister@...il.com>
> Cc: Vivien Didelot <vivien.didelot@...il.com>
> Cc: Vladimir Oltean <olteanv@...il.com>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
>  drivers/net/dsa/xrs700x/xrs700x.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/net/dsa/xrs700x/xrs700x.c b/drivers/net/dsa/xrs700x/xrs700x.c
> index 3887ed33c5fe2..fa622639d6401 100644
> --- a/drivers/net/dsa/xrs700x/xrs700x.c
> +++ b/drivers/net/dsa/xrs700x/xrs700x.c
> @@ -109,6 +109,7 @@ static void xrs700x_read_port_counters(struct xrs700x *priv, int port)
>  {
>         struct xrs700x_port *p = &priv->ports[port];
>         struct rtnl_link_stats64 stats;
> +       unsigned long flags;
>         int i;
>
>         memset(&stats, 0, sizeof(stats));
> @@ -138,9 +139,9 @@ static void xrs700x_read_port_counters(struct xrs700x *priv, int port)
>          */
>         stats.rx_packets += stats.multicast;
>
> -       u64_stats_update_begin(&p->syncp);
> +       flags = u64_stats_update_begin_irqsave(&p->syncp);
>         p->stats64 = stats;
> -       u64_stats_update_end(&p->syncp);
> +       u64_stats_update_end_irqrestore(&p->syncp, flags);
>
>         mutex_unlock(&p->mib_mutex);
>  }
> --
> 2.37.2
>

Acked-by: George McCollister <george.mccollister@...il.com>

Thanks,
George