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    SubjectRe: [PATCH] virtio-net: fix a race on 32bit arches
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    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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