netdev - Re: [patch net-next v4 2/7] switchdev: allow caller to explicitly request attr

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Message-ID: <20151013054543.GB2242@nanopsycho.orion>
Date:	Tue, 13 Oct 2015 07:45:44 +0200
From:	Jiri Pirko <jiri@...nulli.us>
To:	John Fastabend <john.fastabend@...il.com>
Cc:	Scott Feldman <sfeldma@...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 v4 2/7] switchdev: allow caller to explicitly
 request attr_set as deferred

Tue, Oct 13, 2015 at 06:40:25AM CEST, john.fastabend@...il.com wrote:
>On 15-10-12 07:52 PM, Scott Feldman wrote:
>> On Mon, Oct 12, 2015 at 11:03 AM, Jiri Pirko <jiri@...nulli.us> 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.
>>>
>>> Signed-off-by: Jiri Pirko <jiri@...lanox.com>
>>> ---
>>>  include/net/switchdev.h   |   1 +
>>>  net/bridge/br_stp.c       |   3 +-
>>>  net/switchdev/switchdev.c | 107 ++++++++++++++++++++++++----------------------
>>>  3 files changed, 59 insertions(+), 52 deletions(-)
>>>
>>> diff --git a/include/net/switchdev.h b/include/net/switchdev.h
>>> index d2879f2..6b109e4 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,
>>>         };
>>>         int err;
>>>
>>>         p->state = state;
>>>         err = switchdev_port_attr_set(p->dev, &attr);
>>> -       if (err && err != -EOPNOTSUPP)
>>> +       if (err)
>> 
>> This looks like a problem as now all other non-switchdev ports will
>> get an WARN in the log when STP state changes.  We should only WARN if
>> there was an err and the err is not -EOPNOTSUPP.
>> 
>>>                 br_warn(p->br, "error setting offload STP state on port %u(%s)\n",
>>>                                 (unsigned int) p->port_no, p->dev->name);
>>>  }
>> 
>> <snip>
>> 
>>>  struct switchdev_attr_set_work {
>>>         struct work_struct work;
>>>         struct net_device *dev;
>>> @@ -183,14 +226,17 @@ static void switchdev_port_attr_set_work(struct work_struct *work)
>>>  {
>>>         struct switchdev_attr_set_work *asw =
>>>                 container_of(work, struct switchdev_attr_set_work, work);
>>> +       bool rtnl_locked = rtnl_is_locked();
>>>         int err;
>>>
>>> -       rtnl_lock();
>>> -       err = switchdev_port_attr_set(asw->dev, &asw->attr);
>>> +       if (!rtnl_locked)
>>> +               rtnl_lock();
>> 
>> I'm not following this change.  If someone else has rtnl_lock, we'll
>> not wait to grab it here ourselves, and proceed as if we have the
>> lock.  But what if that someone else releases the lock in the middle
>> of us doing switchdev_port_attr_set_now?  Seems we want to
>> unconditionally wait and grab the lock.  We need to block anything
>> from moving while we do the attr set.
>> 
>
>Also an additional race between setting rtnl_locked and the if stmt
>and then grabbing the lock. There seems to be a something of pattern
>around this where other subsystems use a rtnl_trylock and if it fails
>do a restart/re-queue operation to retry. Looks like how you handle
>it in the team driver at least.

No, this is for different case. This is for case someone calls
switchdev_flush_defererd holding the rtnl_lock.


>
>>> +       err = switchdev_port_attr_set_now(asw->dev, &asw->attr);
>>>         if (err && err != -EOPNOTSUPP)
>>>                 netdev_err(asw->dev, "failed (err=%d) to set attribute (id=%d)\n",
>>>                            err, asw->attr.id);
>>> -       rtnl_unlock();
>>> +       if (!rtnl_locked)
>>> +               rtnl_unlock();
>>>
>>>         dev_put(asw->dev);
>>>         kfree(work);
>>> @@ -211,7 +257,7 @@ static int switchdev_port_attr_set_defer(struct net_device *dev,
>>>         asw->dev = dev;
>>>         memcpy(&asw->attr, attr, sizeof(asw->attr));
>>>
>>> -       schedule_work(&asw->work);
>>> +       queue_work(switchdev_wq, &asw->work);
>>>
>>>         return 0;
>>>  }
>
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