netdev - Re: [patch net-next v5 3/8] switchdev: allow caller to explicitly request attr

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Message-ID: <5621AE3B.2030601@gmail.com>
Date:	Fri, 16 Oct 2015 19:11:07 -0700
From:	John Fastabend <john.fastabend@...il.com>
To:	Jiri Pirko <jiri@...nulli.us>
CC:	netdev@...r.kernel.org, davem@...emloft.net, idosch@...lanox.com,
	eladr@...lanox.com, sfeldma@...il.com, f.fainelli@...il.com,
	linux@...ck-us.net, vivien.didelot@...oirfairelinux.com,
	andrew@...n.ch, David.Laight@...LAB.COM, stephen@...workplumber.org
Subject: Re: [patch net-next v5 3/8] switchdev: allow caller to explicitly
 request attr_set as deferred

On 15-10-16 01:23 AM, Jiri Pirko wrote:
> Thu, Oct 15, 2015 at 05:21:22PM CEST, john.fastabend@...il.com wrote:
>> On 15-10-14 10:40 AM, Jiri Pirko wrote:
>>> From: Jiri Pirko <jiri@...lanox.com>
>>>
>>> Caller should know if he can call attr_set directly (when holding RTNL)
>>> or if he has to defer the att_set processing for later.
>>>
>>> This also allows drivers to sleep inside attr_set and report operation
>>> status back to switchdev core. Switchdev core then warns if status is
>>> not ok, instead of silent errors happening in drivers.
>>>
>>> Benefit from newly introduced switchdev deferred ops infrastructure.
>>>
>>
>> A nit but the patch description should note your setting the defer bit
>> on the bridge set state.
>>
>>> Signed-off-by: Jiri Pirko <jiri@...lanox.com>
>>> ---
>>>  include/net/switchdev.h   |   1 +
>>>  net/bridge/br_stp.c       |   3 +-
>>>  net/switchdev/switchdev.c | 108 ++++++++++++++++++----------------------------
>>>  3 files changed, 46 insertions(+), 66 deletions(-)
>>>
>>> diff --git a/include/net/switchdev.h b/include/net/switchdev.h
>>> index d1c7f90..f7de6f8 100644
>>> --- a/include/net/switchdev.h
>>> +++ b/include/net/switchdev.h
>>> @@ -17,6 +17,7 @@
>>>  
>>>  #define SWITCHDEV_F_NO_RECURSE		BIT(0)
>>>  #define SWITCHDEV_F_SKIP_EOPNOTSUPP	BIT(1)
>>> +#define SWITCHDEV_F_DEFER		BIT(2)
>>>  
>>>  struct switchdev_trans_item {
>>>  	struct list_head list;
>>> diff --git a/net/bridge/br_stp.c b/net/bridge/br_stp.c
>>> index db6d243de..80c34d7 100644
>>> --- a/net/bridge/br_stp.c
>>> +++ b/net/bridge/br_stp.c
>>> @@ -41,13 +41,14 @@ void br_set_state(struct net_bridge_port *p, unsigned int state)
>>>  {
>>>  	struct switchdev_attr attr = {
>>>  		.id = SWITCHDEV_ATTR_ID_PORT_STP_STATE,
>>> +		.flags = SWITCHDEV_F_DEFER,
>>>  		.u.stp_state = state,
>>>  	};
>>
>>
>> This creates a possible race (with 6/8) I think, please check!
> 
> Wait. This patch does not change the previous behaviour. Patch 6 does,
> so I don't understand why you are asking here. Confusing.
> 

Sorry if its confusing I keyed of the addition of the SWITCHDEV_F_DEFER
here.

> 
>>
>> In del_nbp() we call br_stp_disable_port() to set the port state
>> to BR_STATE_DISABLE and disabling learning events. But with this
>> patch it can be deferred. Also note the STP agent may be in userspace
>> which actually seems more likely the case because you likely want to
>> run some more modern variant of STP than the kernel supports.
>>
>> So at some point in the future the driver will turn off learning. At
>> the same time we call br_fdb_delete_by_port which calls a deferred
>> set of fdb deletes.
>>
>> I don't see how you guarantee learning is off before you start doing
>> the deletes here and possibly learning new addresses after the software
>> side believes the port is down.
>>
>> So
>>
>>   br_stp_disable_port
>>                           br_fdb_delete_by_port
>>                           {fdb_del_external_learn}
>>   [hw learns a fdb]
>>   [hw disables learning]
>>
>> What stops this from happening?
> 
> Okay. This behaviour is the same as without the patchset. What would
> resolve the issue it to put switchdev_deferred_process() after
> br_stp_disable_port() and before br_fdb_delete_by_port() call.
> That would enforce stp change to happen in hw before fdbs are explicitly
> deleted. Sound good to you?

OK so putting the switchdev_deferred_process() between the disable_port
and the delete_by_port will enforce the stp change to happen in hw
before the fdbs are explicitly deleted. I think this is minimally
required. I don't like scattering these flush_workqueue() calls all
over the place but I don't have any better ideas right now so sounds
good enough.

But now I'm wondering if you can have a deferred fdb add in the rocker
driver (rocker_port_fdb_learn_work) running in parallel with this that
could happen after the delete and add a bogus fdb entry. I think you
also need to have a flush in rocker_port_stp_update() to handle this
case.

Also I agree these issues were not completely caused by your patches.

Thanks,
John
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