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.
