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

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Message-ID: <a58e9615-036c-0431-4ea6-004af4988b27@nvidia.com>
Date:   Wed, 10 Feb 2021 12:52:33 +0200
From:   Nikolay Aleksandrov <nikolay@...dia.com>
To:     Vladimir Oltean <olteanv@...il.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 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.