Re: [PATCH V3 net-next 1/4] net: bridge: add fdb flag to extent locked port feature

[Nikolay Aleksandrov]

On 24/05/2022 19:21, Hans Schultz wrote:
>>
>> Hi Hans,
>> So this approach has a fundamental problem, f->dst is changed without any synchronization
>> you cannot rely on it and thus you cannot account for these entries properly. We must be very
>> careful if we try to add any new synchronization not to affect performance as well.
>> More below...
>>
>>> @@ -319,6 +326,9 @@ static void fdb_delete(struct net_bridge *br, struct net_bridge_fdb_entry *f,
>>>  	if (test_bit(BR_FDB_STATIC, &f->flags))
>>>  		fdb_del_hw_addr(br, f->key.addr.addr);
>>>  
>>> +	if (test_bit(BR_FDB_ENTRY_LOCKED, &f->flags) && !test_bit(BR_FDB_OFFLOADED, &f->flags))
>>> +		atomic_dec(&f->dst->locked_entry_cnt);
>>
>> Sorry but you cannot do this for multiple reasons:
>>  - f->dst can be NULL
>>  - f->dst changes without any synchronization
>>  - there is no synchronization between fdb's flags and its ->dst
>>
>> Cheers,
>>  Nik
> 
> Hi Nik,
> 
> if a port is decoupled from the bridge, the locked entries would of
> course be invalid, so maybe if adding and removing a port is accounted
> for wrt locked entries and the count of locked entries, would that not
> work?
> 
> Best,
> Hans

Hi Hans,
Unfortunately you need the correct amount of locked entries per-port if you want
to limit their number per-port, instead of globally. So you need a consistent
fdb view with all its attributes when changing its dst in this case, which would
require new locking because you have multiple dependent struct fields and it will
kill roaming/learning scalability. I don't think this use case is worth the complexity it
will bring, so I'd suggest an alternative - you can monitor the number of locked entries
per-port from a user-space agent and disable port learning or some similar solution that
doesn't require any complex kernel changes. Is the limit a requirement to add the feature?

I have an idea how to do it and to minimize the performance hit if it really is needed
but it'll add a lot of complexity which I'd like to avoid if possible.

Cheers,
 Nik