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

Jakub Kicinski
2020-12-09T00:18:33Z

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.