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    Subject[PATCH 3.18 11/21] net: fix race on decreasing number of TX queues
    Date
    3.18-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Jakub Kicinski <jakub.kicinski@netronome.com>


    [ Upstream commit ac5b70198adc25c73fba28de4f78adcee8f6be0b ]

    netif_set_real_num_tx_queues() can be called when netdev is up.
    That usually happens when user requests change of number of
    channels/rings with ethtool -L. The procedure for changing
    the number of queues involves resetting the qdiscs and setting
    dev->num_tx_queues to the new value. When the new value is
    lower than the old one, extra care has to be taken to ensure
    ordering of accesses to the number of queues vs qdisc reset.

    Currently the queues are reset before new dev->num_tx_queues
    is assigned, leaving a window of time where packets can be
    enqueued onto the queues going down, leading to a likely
    crash in the drivers, since most drivers don't check if TX
    skbs are assigned to an active queue.

    Fixes: e6484930d7c7 ("net: allocate tx queues in register_netdevice")
    Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    net/core/dev.c | 11 +++++++++--
    1 file changed, 9 insertions(+), 2 deletions(-)

    --- a/net/core/dev.c
    +++ b/net/core/dev.c
    @@ -2070,8 +2070,11 @@ EXPORT_SYMBOL(netif_set_xps_queue);
    */
    int netif_set_real_num_tx_queues(struct net_device *dev, unsigned int txq)
    {
    + bool disabling;
    int rc;

    + disabling = txq < dev->real_num_tx_queues;
    +
    if (txq < 1 || txq > dev->num_tx_queues)
    return -EINVAL;

    @@ -2087,15 +2090,19 @@ int netif_set_real_num_tx_queues(struct
    if (dev->num_tc)
    netif_setup_tc(dev, txq);

    - if (txq < dev->real_num_tx_queues) {
    + dev->real_num_tx_queues = txq;
    +
    + if (disabling) {
    + synchronize_net();
    qdisc_reset_all_tx_gt(dev, txq);
    #ifdef CONFIG_XPS
    netif_reset_xps_queues_gt(dev, txq);
    #endif
    }
    + } else {
    + dev->real_num_tx_queues = txq;
    }

    - dev->real_num_tx_queues = txq;
    return 0;
    }
    EXPORT_SYMBOL(netif_set_real_num_tx_queues);

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