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
