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    Subject[PATCH 3.8 166/166] bridge: use spin_lock_bh() in br_multicast_set_hash_max
    Date
    3.8.13.16 -stable review patch.  If anyone has any objections, please let me know.

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

    From: Curt Brune <curt@cumulusnetworks.com>

    [ Upstream commit fe0d692bbc645786bce1a98439e548ae619269f5 ]

    br_multicast_set_hash_max() is called from process context in
    net/bridge/br_sysfs_br.c by the sysfs store_hash_max() function.

    br_multicast_set_hash_max() calls spin_lock(&br->multicast_lock),
    which can deadlock the CPU if a softirq that also tries to take the
    same lock interrupts br_multicast_set_hash_max() while the lock is
    held . This can happen quite easily when any of the bridge multicast
    timers expire, which try to take the same lock.

    The fix here is to use spin_lock_bh(), preventing other softirqs from
    executing on this CPU.

    Steps to reproduce:

    1. Create a bridge with several interfaces (I used 4).
    2. Set the "multicast query interval" to a low number, like 2.
    3. Enable the bridge as a multicast querier.
    4. Repeatedly set the bridge hash_max parameter via sysfs.

    # brctl addbr br0
    # brctl addif br0 eth1 eth2 eth3 eth4
    # brctl setmcqi br0 2
    # brctl setmcquerier br0 1

    # while true ; do echo 4096 > /sys/class/net/br0/bridge/hash_max; done

    Signed-off-by: Curt Brune <curt@cumulusnetworks.com>
    Signed-off-by: Scott Feldman <sfeldma@cumulusnetworks.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>
    Signed-off-by: Kamal Mostafa <kamal@canonical.com>
    ---
    net/bridge/br_multicast.c | 4 ++--
    1 file changed, 2 insertions(+), 2 deletions(-)

    diff --git a/net/bridge/br_multicast.c b/net/bridge/br_multicast.c
    index 4a51874..fda703b 100644
    --- a/net/bridge/br_multicast.c
    +++ b/net/bridge/br_multicast.c
    @@ -1812,7 +1812,7 @@ int br_multicast_set_hash_max(struct net_bridge *br, unsigned long val)
    u32 old;
    struct net_bridge_mdb_htable *mdb;

    - spin_lock(&br->multicast_lock);
    + spin_lock_bh(&br->multicast_lock);
    if (!netif_running(br->dev))
    goto unlock;

    @@ -1844,7 +1844,7 @@ rollback:
    }

    unlock:
    - spin_unlock(&br->multicast_lock);
    + spin_unlock_bh(&br->multicast_lock);

    return err;
    }
    --
    1.8.3.2


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