[Bridge] [PATCH net-next 1/2] bridge: use spin_lock_bh() in br_multicast_set_hash_max

Cong Wang cwang at twopensource.com
Mon Jan 6 19:11:45 UTC 2014


On Mon, Jan 6, 2014 at 11:00 AM, Scott Feldman
<sfeldma at cumulusnetworks.com> wrote:
> From: Curt Brune <curt at cumulusnetworks.com>
>
> 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
>


I think this should probably go to net instead of net-next,
and -stable too.


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