Linux Ethernet Bridging - May 2022 - [Bridge] [PATCH V3 net-next 1/4] net: bridge: add fdb flag to extent locked port feature

On ons, maj 25, 2022 at 11:06, Nikolay Aleksandrov <razor at
blackwall.org> wrote:
> 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
Hi Nik,
the used dst is a port structure, so it is per-port and not globally.

Best,
Hans
> 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

On 25/05/2022 11:34, Hans Schultz wrote:
> On ons, maj 25, 2022 at 11:06, Nikolay Aleksandrov <razor at
blackwall.org> wrote:
>> 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
> 
> Hi Nik,
> the used dst is a port structure, so it is per-port and not globally.
> 
> Best,
> Hans
> 
Yeah, I know. :) That's why I wrote it, if the limit is not a feature
requirement I'd suggest
dropping it altogether, it can be enforced externally (e.g. from user-space) if
needed.

By the way just fyi net-next is closed right now due to merge window. And one
more
thing please include a short log of changes between versions when you send a new
one.
I had to go look for v2 to find out what changed.
>> 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