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

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Message-ID: <CAE4R7bC4vik8-V9L5oTukQ4YhMoCGF_uOM_481fJgq3yc1LN=g@mail.gmail.com>
Date:	Mon, 12 Oct 2015 23:53:45 -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 v4 2/7] switchdev: allow caller to explicitly
 request attr_set as deferred

On Mon, Oct 12, 2015 at 11:21 PM, Jiri Pirko <jiri@...nulli.us> wrote:
> Tue, Oct 13, 2015 at 08:03:46AM CEST, john.fastabend@...il.com wrote:
>>On 15-10-12 10:44 PM, Jiri Pirko wrote:
>>> Tue, Oct 13, 2015 at 04:52:42AM CEST, sfeldma@...il.com 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.
>>>
>>> If SWITCHDEV_F_DEFER flag is set, there's only 0 of -ENOMEM.
>>>
>>>
>>>>
>>>>>                 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.
>>>
>>> Why would someone we call (driver) return the lock? In that case, he is
>>> buggy and should be fixed.
>>>
>>> This hunk only ensures we have rtnl_lock. If not, we take it here. We do
>>> not take it unconditionally because we may already have it, for example
>>> if caller of switchdev_flush_deferred holds rtnl_lock.
>>>
>>
>>This is where you lost me. How do you know another core doesn't happen
>>to have the lock when you hit this code path? Maybe someone is running
>>an ethtool command on another core or something.
>
> You are right. The same problem exists currently in switchdev_port_attr_set.

You are right as in you'll change this back to unconditional grabbing
of rtnl_lock?  I don't follow how this problem currently exists as
current code does an unconditional grab of rtnl_lock.
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