linux-kernel - Re: [PATCH] virtio-net: fix a race on 32bit arches

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Date:	Wed, 06 Jun 2012 15:10:10 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Jason Wang <jasowang@...hat.com>, netdev@...r.kernel.org,
	rusty@...tcorp.com.au, linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	Stephen Hemminger <shemminger@...tta.com>
Subject: Re: [PATCH] virtio-net: fix a race on 32bit arches

On Wed, 2012-06-06 at 14:13 +0300, Michael S. Tsirkin wrote:

> We currently do all stats either on napi callback or from
> start_xmit callback.
> This makes them safe, yes?

Hmm, then _bh() variant is needed in virtnet_stats(), as explained in
include/linux/u64_stats_sync.h section 6)

 * 6) If counter might be written by an interrupt, readers should block interrupts.
 *    (On UP, there is no seqcount_t protection, a reader allowing interrupts could
 *     read partial values)

Yes, its tricky...

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 5214b1e..705aaa7 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -703,12 +703,12 @@ static struct rtnl_link_stats64 *virtnet_stats(struct net_device *dev,
 		u64 tpackets, tbytes, rpackets, rbytes;
 
 		do {
-			start = u64_stats_fetch_begin(&stats->syncp);
+			start = u64_stats_fetch_begin_bh(&stats->syncp);
 			tpackets = stats->tx_packets;
 			tbytes   = stats->tx_bytes;
 			rpackets = stats->rx_packets;
 			rbytes   = stats->rx_bytes;
-		} while (u64_stats_fetch_retry(&stats->syncp, start));
+		} while (u64_stats_fetch_retry_bh(&stats->syncp, start));
 
 		tot->rx_packets += rpackets;
 		tot->tx_packets += tpackets;


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