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

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Date:	Fri, 16 Oct 2015 10:23:48 +0200
From:	Jiri Pirko <jiri@...nulli.us>
To:	John Fastabend <john.fastabend@...il.com>
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

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?



>
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