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

Nikolay Aleksandrov
2021-02-10T11:13:10Z

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.