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

[Vladimir Oltean]

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);
	}