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

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Message-ID: <CAE4R7bDLSOTutnJ65nkFbHF-gC6euGTFwjuK2f-AXfjCZuEqqw@mail.gmail.com>
Date:	Fri, 16 Oct 2015 09:20:55 -0700
From:	Scott Feldman <sfeldma@...il.com>
To:	Jiri Pirko <jiri@...nulli.us>
Cc:	John Fastabend <john.fastabend@...il.com>,
	Netdev <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Ido Schimmel <idosch@...lanox.com>,
	Elad Raz <eladr@...lanox.com>,
	Florian Fainelli <f.fainelli@...il.com>,
	Guenter Roeck <linux@...ck-us.net>,
	Vivien Didelot <vivien.didelot@...oirfairelinux.com>,
	"andrew@...n.ch" <andrew@...n.ch>,
	David Laight <David.Laight@...lab.com>,
	"stephen@...workplumber.org" <stephen@...workplumber.org>
Subject: Re: [patch net-next v5 3/8] switchdev: allow caller to explicitly
 request attr_set as deferred

On Fri, Oct 16, 2015 at 1:23 AM, Jiri Pirko <jiri@...nulli.us> 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.
>
>
>>
>>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?

Doesn't HW already see things in the right order since items are
dequeued from the deferred list in the order queued?
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