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

[Ido Schimmel]

On Wed, Feb 10, 2021 at 02:29:36PM +0200, Vladimir Oltean wrote:
> On Wed, Feb 10, 2021 at 02:21:05PM +0200, Ido Schimmel wrote:
> > On Wed, Feb 10, 2021 at 02:01:06PM +0200, Vladimir Oltean wrote:
> > > On Wed, Feb 10, 2021 at 01:05:57PM +0200, Nikolay Aleksandrov wrote:
> > > > 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.
> > > 
> > > What I just don't understand is how others can get away with doing
> > > sleepable work in atomic context but I can't make the notifier blocking
> > > by dropping a spinlock which isn't needed there, because it looks ugly :D
> > 
> > Can you please point to the bug? I'm not following
> 
> For example, mlxsw eventually calls mlxsw_sp_fid_flood_set from the
> SWITCHDEV_ATTR_ID_PORT_BRIDGE_FLAGS handling data path, and this
> function allocates memory with GFP_KERNEL.
> 
> Another example is prestera which eventually calls prestera_fw_send_req
> which takes a mutex_lock.
> 
> Yet another example are mv88e6xxx and b53 which use MDIO and SPI
> from their .port_egress_floods implementation, buses which have
> might_sleep() in them.

Right, but see the code:

```
	attr.id = SWITCHDEV_ATTR_ID_PORT_BRIDGE_FLAGS;
	attr.flags = SWITCHDEV_F_DEFER;
	attr.u.brport_flags = flags;

	err = switchdev_port_attr_set(p->dev, &attr);
```

And check how SWITCHDEV_F_DEFER is used.

We can squash SWITCHDEV_ATTR_ID_PORT_BRIDGE_FLAGS and
SWITCHDEV_ATTR_ID_PORT_PRE_BRIDGE_FLAGS into one blocking notification
by reducing the scope of the bridge lock like Nik suggested. Currently
it's just blindly taken around br_setport().