Merge branch 'netpoll-use-rcu-primitives-for-npinfo-pointer-access'

Breno Leitao says:

====================
netpoll: Use RCU primitives for npinfo pointer access

The net_device->npinfo pointer is marked with __rcu, indicating it requires
proper RCU access primitives:

  struct net_device {
	...
	struct netpoll_info __rcu *npinfo;
	...
  };

Direct access to this pointer can lead to issues such as:
- Compiler incorrectly caching/reusing stale pointer values
- Missing memory ordering guarantees
- Non-atomic pointer loads

Replace direct NULL checks of npinfo with rcu_access_pointer(),
which provides the necessary memory ordering guarantees without the
overhead of a full RCU dereference, since we only need to verify
if the pointer is NULL.

In both cases, the RCU read lock is not held when the function is being
called. I checked that by using lockdep_assert_in_rcu_read_lock(), and
seeing the warning on both cases.
====================

Link: https://patch.msgid.link/20241118-netpoll_rcu-v1-0-a1888dcb4a02@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/linux/netpoll.h

@@ -72,7 +72,7 @@ static inline void *netpoll_poll_lock(struct napi_struct *napi)
 {
 	struct net_device *dev = napi->dev;
 
-	if (dev && dev->npinfo) {
+	if (dev && rcu_access_pointer(dev->npinfo)) {
 		int owner = smp_processor_id();
 
 		while (cmpxchg(&napi->poll_owner, -1, owner) != -1)

net/core/netpoll.c

@@ -626,7 +626,7 @@ int __netpoll_setup(struct netpoll *np, struct net_device *ndev)
 		goto out;
 	}
 
-	if (!ndev->npinfo) {
+	if (!rcu_access_pointer(ndev->npinfo)) {
 		npinfo = kmalloc(sizeof(*npinfo), GFP_KERNEL);
 		if (!npinfo) {
 			err = -ENOMEM;