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

[Jakub Kicinski <kuba@kernel.org>]

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
v1: https://lore.kernel.org/20260918210419.4088201-1-kuba@kernel.org
---
 drivers/net/loopback.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/net/loopback.c b/drivers/net/loopback.c
index 1fb6ce6843ad..6187fcca82d7 100644
--- a/drivers/net/loopback.c
+++ b/drivers/net/loopback.c
@@ -201,6 +201,8 @@ static void loopback_setup(struct net_device *dev)
 {
 	gen_lo_setup(dev, (64 * 1024), &loopback_ethtool_ops, &eth_header_ops,
 		     &loopback_ops, loopback_dev_free);
+
+	dev->request_ops_lock	= true;
 }
 
 /* Setup and register the loopback device. */
-- 
2.55.0