Message-ID: <ea2a8856-59d8-1b5f-9428-b16efa8f6979@stressinduktion.org>
Date:   Thu, 1 Dec 2016 22:57:52 +0100
From:   Hannes Frederic Sowa <hannes@...essinduktion.org>
To:     Ido Schimmel <idosch@...sch.org>
Cc:     Jiri Pirko <jiri@...nulli.us>, netdev@...r.kernel.org,
        davem@...emloft.net, idosch@...lanox.com, eladr@...lanox.com,
        yotamg@...lanox.com, nogahf@...lanox.com, arkadis@...lanox.com,
        ogerlitz@...lanox.com, roopa@...ulusnetworks.com,
        dsa@...ulusnetworks.com, nikolay@...ulusnetworks.com,
        andy@...yhouse.net, vivien.didelot@...oirfairelinux.com,
        andrew@...n.ch, f.fainelli@...il.com, alexander.h.duyck@...el.com,
        kaber@...sh.net
Subject: Re: [patch net-next v3 11/12] mlxsw: spectrum_router: Request a dump
 of FIB tables during init

On 30.11.2016 19:22, Ido Schimmel wrote:
> On Wed, Nov 30, 2016 at 05:49:56PM +0100, Hannes Frederic Sowa wrote:
>> On 30.11.2016 17:32, Ido Schimmel wrote:
>>> On Wed, Nov 30, 2016 at 04:37:48PM +0100, Hannes Frederic Sowa wrote:
>>>> On 30.11.2016 11:09, Jiri Pirko wrote:
>>>>> From: Ido Schimmel <idosch@...lanox.com>
>>>>>
>>>>> Make sure the device has a complete view of the FIB tables by invoking
>>>>> their dump during module init.
>>>>>
>>>>> Signed-off-by: Ido Schimmel <idosch@...lanox.com>
>>>>> Signed-off-by: Jiri Pirko <jiri@...lanox.com>
>>>>> ---
>>>>>  .../net/ethernet/mellanox/mlxsw/spectrum_router.c  | 23 ++++++++++++++++++++++
>>>>>  1 file changed, 23 insertions(+)
>>>>>
>>>>> diff --git a/drivers/net/ethernet/mellanox/mlxsw/spectrum_router.c b/drivers/net/ethernet/mellanox/mlxsw/spectrum_router.c
>>>>> index 14bed1d..d176047 100644
>>>>> --- a/drivers/net/ethernet/mellanox/mlxsw/spectrum_router.c
>>>>> +++ b/drivers/net/ethernet/mellanox/mlxsw/spectrum_router.c
>>>>> @@ -2027,8 +2027,23 @@ static int mlxsw_sp_router_fib_event(struct notifier_block *nb,
>>>>>  	return NOTIFY_DONE;
>>>>>  }
>>>>>  
>>>>> +static void mlxsw_sp_router_fib_dump_flush(struct notifier_block *nb)
>>>>> +{
>>>>> +	struct mlxsw_sp *mlxsw_sp = container_of(nb, struct mlxsw_sp, fib_nb);
>>>>> +
>>>>> +	/* Flush pending FIB notifications and then flush the device's
>>>>> +	 * table before requesting another dump. Do that with RTNL held,
>>>>> +	 * as FIB notification block is already registered.
>>>>> +	 */
>>>>> +	mlxsw_core_flush_owq();
>>>>> +	rtnl_lock();
>>>>> +	mlxsw_sp_router_fib_flush(mlxsw_sp);
>>>>> +	rtnl_unlock();
>>>>> +}
>>>>> +
>>>>>  int mlxsw_sp_router_init(struct mlxsw_sp *mlxsw_sp)
>>>>>  {
>>>>> +	fib_dump_cb_t *cb = mlxsw_sp_router_fib_dump_flush;
>>>>>  	int err;
>>>>>  
>>>>>  	INIT_LIST_HEAD(&mlxsw_sp->router.nexthop_neighs_list);
>>>>> @@ -2048,8 +2063,16 @@ int mlxsw_sp_router_init(struct mlxsw_sp *mlxsw_sp)
>>>>>  
>>>>>  	mlxsw_sp->fib_nb.notifier_call = mlxsw_sp_router_fib_event;
>>>>>  	register_fib_notifier(&mlxsw_sp->fib_nb);
>>>>
>>>> Sorry to pick in here again:
>>>>
>>>> There is a race here. You need to protect the registration of the fib
>>>> notifier as well by the sequence counter. Updates here are not ordered
>>>> in relation to this code below.
>>>
>>> You mean updates that can be received after you registered the notifier
>>> and until the dump started? I'm aware of that and that's OK. This
>>> listener should be able to handle duplicates.
>>
>> I am not concerned about duplicates, but about ordering deletes and
>> getting an add from the RCU code you will add the node to hw while it is
>> deleted in the software path. You probably will ignore the delete
>> because nothing is installed in hw and later add the node which was
>> actually deleted but just reordered which happend on another CPU, no?
> 
> Are you referring to reordering in the workqueue? We already covered
> this using an ordered workqueue, which has one context of execution
> system-wide.

Ups, sorry, I missed that mail. Probably read it on the mobile phone and
it became invisible for me later on. Busy day... ;)

The reordering in the workqueue seems fine to me and also still necessary.

Basically, if you delete a node right now the kernel might simply do a
RCU_INIT_POINTER(ptr_location, NULL), which has absolutely no barriers
or synchronization with the reader side. Thus you might get a callback
from the notifier for a delete event on the one CPU and you end up
queueing this fib entry after the delete queue, because the RCU walk
isn't protected by any means.

Looking closer at this series again, I overlooked the fact that you
fetch fib_seq using a rtnl_lock and rtnl_unlock pair, which first of all
orders fetching of fib_seq and thus the RCU dumping after any concurrent
executing fib table update, also the mutex_lock and unlock provide
proper acquire and release fences, so the CPU indeed sees the effect of
a RCU_INIT_POINTER update done on another CPU, because they pair with
the rtnl_unlock which might happen on the other CPU.

My question is if this is a bit of luck and if we should make this
explicit by putting the registration itself under the protection of the
sequence counter. I favor the additional protection, e.g. if we some day
actually we optimize the fib_seq code? Otherwise we might probably
document this fact. :)

>>> I've a follow up patchset that introduces a new event in switchdev
>>> notification chain called SWITCHDEV_SYNC, which is sent when port
>>> netdevs are enslaved / released  from a master device (points in time
>>> where kernel<->device can get out of sync). It will invoke
>>> re-propagation of configuration from different parts of the stack
>>> (e.g. bridge driver, 8021q driver, fib/neigh code), which can result
>>> in duplicates.
>>
>> Okay, understood. I wonder how we can protect against accidentally abort
>> calls actually. E.g. if I start to inject routes into my routing domain
>> how can I make sure the box doesn't die after I try to insert enough
>> routes. Do we need to touch quagga etc?
> 
> The whole point of moving abort mechanism to the driver is that the
> system won't die, but instead routing will be done in the kernel. If you
> respect hardware limitations, then there's no reason for abort mechanism
> to kick in.

Quick follow-up question: How can I quickly find out the hw limitations
via the kernel api?

Thanks,
Hannes

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