netdev - Re: [PATCH v3 net-next 00/11] Cleanup in brport flags switchdev offload for DSA

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Date:   Wed, 10 Feb 2021 13:05:57 +0200
From:   Nikolay Aleksandrov <nikolay@...dia.com>
To:     Vladimir Oltean <olteanv@...il.com>
CC:     Jakub Kicinski <kuba@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <bridge@...ts.linux-foundation.org>,
        "Roopa Prabhu" <roopa@...dia.com>, Jiri Pirko <jiri@...nulli.us>,
        Ido Schimmel <idosch@...sch.org>,
        Claudiu Manoil <claudiu.manoil@....com>,
        "Alexandre Belloni" <alexandre.belloni@...tlin.com>,
        <UNGLinuxDriver@...rochip.com>, Vadym Kochan <vkochan@...vell.com>,
        Taras Chornyi <tchornyi@...vell.com>,
        Grygorii Strashko <grygorii.strashko@...com>,
        Ioana Ciornei <ioana.ciornei@....com>,
        Ivan Vecera <ivecera@...hat.com>, <linux-omap@...r.kernel.org>
Subject: Re: [PATCH v3 net-next 00/11] Cleanup in brport flags switchdev
 offload for DSA

On 10/02/2021 13:01, Vladimir Oltean wrote:
> On Wed, Feb 10, 2021 at 12:52:33PM +0200, Nikolay Aleksandrov wrote:
>> On 10/02/2021 12:45, Vladimir Oltean wrote:
>>> Hi Nikolay,
>>>
>>> On Wed, Feb 10, 2021 at 12:31:43PM +0200, Nikolay Aleksandrov wrote:
>>>> Hi Vladimir,
>>>> Let's take a step back for a moment and discuss the bridge unlock/lock sequences
>>>> that come with this set. I'd really like to avoid those as they're a recipe
>>>> for future problems. The only good way to achieve that currently is to keep
>>>> the PRE_FLAGS call and do that in unsleepable context but move the FLAGS call
>>>> after the flags have been changed (if they have changed obviously). That would
>>>> make the code read much easier since we'll have all our lock/unlock sequences
>>>> in the same code blocks and won't play games to get sleepable context.
>>>> Please let's think and work in that direction, rather than having:
>>>> +	spin_lock_bh(&p->br->lock);
>>>> +	if (err) {
>>>> +		netdev_err(p->dev, "%s\n", extack._msg);
>>>> +		return err;
>>>>  	}
>>>> +
>>>>
>>>> which immediately looks like a bug even though after some code checking we can
>>>> verify it's ok. WDYT?
>>>>
>>>> I plan to get rid of most of the br->lock since it's been abused for a very long
>>>> time because it's essentially STP lock, but people have started using it for other
>>>> things and I plan to fix that when I get more time.
>>>
>>> This won't make the sysfs codepath any nicer, will it?
>>>
>>
>> Currently we'll have to live with a hack that checks if the flags have changed. I agree
>> it won't be pretty, but we won't have to unlock and lock again in the middle of the 
>> called function and we'll have all our locking in the same place, easier to verify and
>> later easier to remove. Once I get rid of most of the br->lock usage we can revisit
>> the drop of PRE_FLAGS if it's a problem. The alternative is to change the flags, then
>> send the switchdev notification outside of the lock and revert the flags if it doesn't
>> go through which doesn't sound much better.
>> I'm open to any other suggestions, but definitely would like to avoid playing locking games.
>> Even if it means casing out flag setting from all other store_ functions for sysfs.
> 
> By casing out flag settings you mean something like this?
> 
> 
> #define BRPORT_ATTR(_name, _mode, _show, _store)		\
> const struct brport_attribute brport_attr_##_name = { 	        \
> 	.attr = {.name = __stringify(_name), 			\
> 		 .mode = _mode },				\
> 	.show	= _show,					\
> 	.store_unlocked	= _store,				\
> };
> 
> #define BRPORT_ATTR_FLAG(_name, _mask)				\
> static ssize_t show_##_name(struct net_bridge_port *p, char *buf) \
> {								\
> 	return sprintf(buf, "%d\n", !!(p->flags & _mask));	\
> }								\
> static int store_##_name(struct net_bridge_port *p, unsigned long v) \
> {								\
> 	return store_flag(p, v, _mask);				\
> }								\
> static BRPORT_ATTR(_name, 0644,					\
> 		   show_##_name, store_##_name)
> 
> static ssize_t brport_store(struct kobject *kobj,
> 			    struct attribute *attr,
> 			    const char *buf, size_t count)
> {
> 	...
> 
> 	} else if (brport_attr->store_unlocked) {
> 		val = simple_strtoul(buf, &endp, 0);
> 		if (endp == buf)
> 			goto out_unlock;
> 		ret = brport_attr->store_unlocked(p, val);
> 	}
> 

Yes, this can work but will need a bit more changes because of br_port_flags_change().
Then the netlink side can be modeled in a similar way.