net: page_pool: avoid false positive warning if NAPI was never added

JIRA: https://issues.redhat.com/browse/RHEL-93737

Conflicts:
net/core/dev.h
- context conflict due to missing commits like b9495b5 ("net: move
  kick_defer_list_purge() to net/core/dev.h")
net/core/page_pool.c
- simple context conflict in headers area

commit c1e00bc
Author: Jakub Kicinski <kuba@kernel.org>
Date:   Thu Feb 6 14:56:37 2025 -0800

    net: page_pool: avoid false positive warning if NAPI was never added

    We expect NAPI to be in disabled state when page pool is torn down.
    But it is also legal if the NAPI is completely uninitialized.

    Reviewed-by: Mina Almasry <almasrymina@google.com>
    Link: https://patch.msgid.link/20250206225638.1387810-4-kuba@kernel.org
    Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Signed-off-by: Ivan Vecera <ivecera@redhat.com>

Author: ivecera

net/core/dev.h

@@ -236,4 +236,16 @@ static inline void netdev_set_gro_flush_timeout(struct net_device *netdev,
 
 int rps_cpumask_housekeeping(struct cpumask *mask);
 
+/* Best effort check that NAPI is not idle (can't be scheduled to run) */
+static inline void napi_assert_will_not_race(const struct napi_struct *napi)
+{
+	/* uninitialized instance, can't race */
+	if (!napi->poll_list.next)
+		return;
+
+	/* SCHED bit is set on disabled instances */
+	WARN_ON(!test_bit(NAPI_STATE_SCHED, &napi->state));
+	WARN_ON(READ_ONCE(napi->list_owner) != -1);
+}
+
 #endif

net/core/page_pool.c

@@ -23,6 +23,7 @@
 
 #include <trace/events/page_pool.h>
 
+#include "dev.h"
 #include "page_pool_priv.h"
 
 #define DEFER_TIME (msecs_to_jiffies(1000))
@@ -1076,11 +1077,7 @@ void page_pool_disable_direct_recycling(struct page_pool *pool)
 	if (!pool->p.napi)
 		return;
 
-	/* To avoid races with recycling and additional barriers make sure
-	 * pool and NAPI are unlinked when NAPI is disabled.
-	 */
-	WARN_ON(!test_bit(NAPI_STATE_SCHED, &pool->p.napi->state));
-	WARN_ON(READ_ONCE(pool->p.napi->list_owner) != -1);
+	napi_assert_will_not_race(pool->p.napi);
 
 	mutex_lock(&page_pools_lock);
 	WRITE_ONCE(pool->p.napi, NULL);