netdev - Re: [RFC PATCH net-next 05/13] net: bonding: hold the netdev lists lock when retrieving device statistics

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Date:   Tue, 8 Dec 2020 16:17:37 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Vladimir Oltean <vladimir.oltean@....com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Jiri Benc <jbenc@...hat.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Eric Dumazet <edumazet@...gle.com>,
        George McCollister <george.mccollister@...il.com>,
        Oleksij Rempel <o.rempel@...gutronix.de>,
        Jay Vosburgh <j.vosburgh@...il.com>,
        Veaceslav Falico <vfalico@...il.com>,
        Andy Gospodarek <andy@...yhouse.net>
Subject: Re: [RFC PATCH net-next 05/13] net: bonding: hold the netdev lists
 lock when retrieving device statistics

On Wed, 9 Dec 2020 00:03:56 +0000 Vladimir Oltean wrote:
> On Tue, Dec 08, 2020 at 03:57:44PM -0800, Jakub Kicinski wrote:
> > On Mon, 7 Dec 2020 01:00:40 +0000 Vladimir Oltean wrote:  
> > > - ensuring through convention that user space always takes
> > >   net->netdev_lists_lock when calling dev_get_stats, and documenting
> > >   that, and therefore making it unnecessary to lock in bonding.  
> >
> > This seems like the better option to me. Makes the locking rules pretty
> > clear.  
> 
> It is non-obvious to me that top-level callers of dev_get_stats should
> hold a lock as specific as the one protecting the lists of network
> interfaces. In the vast majority of implementations of dev_get_stats,
> that lock would not actually protect anything, which would lead into
> just one more lock that is used for more than it should be. In my tree I
> had actually already switched over to mutex_lock_nested. Nonetheless, I
> am still open if you want to make the case that simplicity should prevail
> over specificity.

What are the locking rules you have in mind then? Caller may hold RTNL
or ifc mutex?

> But in that case, maybe we should just keep on using the RTNL mutex.

That's a wasted opportunity, RTNL lock is pretty busy.