Re: [PATCH net-next v2] net: opt loopback into instance locking

[Stanislav Fomichev <sdf.kernel@gmail.com>]

On 09/21, Jakub Kicinski wrote:
> netdev netlink iterates over devices when dumping queues/NAPIs/qstats
> etc. and takes rtnl_lock or instance lock depending on the underlying
> device. Currently even if all the "real" devices in the system are ops
> locked we still have to take rtnl_lock for lo (only to find out
> that it doesn't even have queues or NAPIs to report).
> Opt loopback into having control path under the ops lock.
> 
> This is a pretty obvious thing to do, I've held back this
> patch because we used to only apply ops locking on physical devices.
> netkit queue leasing made a precedent for (far more complex)
> SW devices enabling ops locking. Now adding it to lo should
> not create much new bug surface.
> 
> I considered an alternative of adding a "predicate" to the iteration
> primitive so that we can skip the devices which obviously don't
> support given API (eg qstat) without any locking. But it's more
> LoC and real_num_.x_queues is not currently WRITE_ONCE()ed so
> it doesn't work too well for queues.
> 
> Signed-off-by: Jakub Kicinski <kuba@kernel.org>
> ---
> v2:
>  - move setting the request flag from gen_lo_setup() to loopback_setup()
>    to avoid splats on the blackhole dev which we don't care about

Acked-by: Stanislav Fomichev <sdf@fomichev.me>

(mostly because dummy is also ops locked, don't think loopback should
bring any problems)