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Message-ID: <CAE4R7bBk91rr8UoLtXH8eFK5V1TrSZ+mceZYdjQ8gb2E7Vo0JQ@mail.gmail.com>
Date: Tue, 13 Oct 2015 07:07:31 -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 Tue, Oct 13, 2015 at 12:30 AM, Jiri Pirko <jiri@...nulli.us> wrote:
> Tue, Oct 13, 2015 at 08:53:45AM CEST, sfeldma@...il.com wrote:
>>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.
>
> cpu1 cpu2
> rtnl_lock()
> switchdev_port_attr_set
> !rtnl_is_locked() == false
> switchdev_trans_init
> rtnl_unlock()
> __switchdev_port_attr_set
>
> now __switchdev_port_attr_set is called without rtnl_lock.
Got it. Another example of trying to guess context and getting it
wrong. This is why I like your DEFERRED option so caller can be
explicit.
> Would make sense to introduce rtnl_is_locked_by_me() or something.
Is it sufficient to simply call rtnl_lock() in your deferred context?
You can sleep there and that way there is no question who has the
lock.
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