net: dsa: xrs700x: Use irqsave variant for u64 stats update

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: ee00b24f ("net: dsa: add Arrow SpeedChips XRS700x driver")
Cc: Andrew Lunn <andrew@lunn.ch>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: George McCollister <george.mccollister@gmail.com>
Cc: Vivien Didelot <vivien.didelot@gmail.com>
Cc: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: George McCollister <george.mccollister@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

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);
 }