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

[David Miller <davem@davemloft.net>]

From: Cong Wang <cwang@twopensource.com>
Date: Mon, 6 Jan 2014 11:11:45 -0800

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

Agreed, applied to 'net' and queued up for -stable.