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

Vladimir Oltean
2021-02-10T11:04:38Z

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);
}