netpoll: Use rcu_access_pointer() in netpoll_poll_lock

The ndev->npinfo pointer in netpoll_poll_lock() is RCU-protected but is
being accessed directly for a NULL check. While no RCU read lock is held
in this context, we should still use proper RCU primitives for
consistency and correctness.

Replace the direct NULL check with rcu_access_pointer(), which is the
appropriate primitive when only checking for NULL without dereferencing
the pointer. This function provides the necessary ordering guarantees
without requiring RCU read-side protection.

Fixes: bea3348 ("[NET]: Make NAPI polling independent of struct net_device objects.")
Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Michal Kubiak <michal.kubiak@intel.com>
Link: https://patch.msgid.link/20241118-netpoll_rcu-v1-2-a1888dcb4a02@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: leitao

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)