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

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Date:   Wed, 10 Feb 2021 14:01:06 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     Nikolay Aleksandrov <nikolay@...dia.com>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        bridge@...ts.linux-foundation.org, Roopa Prabhu <roopa@...dia.com>,
        Jiri Pirko <jiri@...nulli.us>,
        Ido Schimmel <idosch@...sch.org>,
        Claudiu Manoil <claudiu.manoil@....com>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        UNGLinuxDriver@...rochip.com, Vadym Kochan <vkochan@...vell.com>,
        Taras Chornyi <tchornyi@...vell.com>,
        Grygorii Strashko <grygorii.strashko@...com>,
        Ioana Ciornei <ioana.ciornei@....com>,
        Ivan Vecera <ivecera@...hat.com>, linux-omap@...r.kernel.org
Subject: Re: [PATCH v3 net-next 00/11] Cleanup in brport flags switchdev
 offload for DSA

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