[PATCH net-next v2 3/4] net: page_pool: avoid false positive warning if NAPI was never added

[Jakub Kicinski <kuba@kernel.org>]

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.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
---
v2: new patch
v1: https://lore.kernel.org/20250205190131.564456-1-kuba@kernel.org

CC: hawk@kernel.org
CC: ilias.apalodimas@linaro.org
CC: jdamato@fastly.com
---
 net/core/dev.h       | 12 ++++++++++++
 net/core/page_pool.c |  7 ++-----
 2 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/net/core/dev.h b/net/core/dev.h
index a5b166bbd169..caa13e431a6b 100644
--- a/net/core/dev.h
+++ b/net/core/dev.h
@@ -299,6 +299,18 @@ void xdp_do_check_flushed(struct napi_struct *napi);
 static inline void xdp_do_check_flushed(struct napi_struct *napi) { }
 #endif
 
+/* 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);
+}
+
 void kick_defer_list_purge(struct softnet_data *sd, unsigned int cpu);
 
 #define XMIT_RECURSION_LIMIT	8
diff --git a/net/core/page_pool.c b/net/core/page_pool.c
index f5e908c9e7ad..b940c40ea2f2 100644
--- a/net/core/page_pool.c
+++ b/net/core/page_pool.c
@@ -25,6 +25,7 @@
 
 #include <trace/events/page_pool.h>
 
+#include "dev.h"
 #include "mp_dmabuf_devmem.h"
 #include "netmem_priv.h"
 #include "page_pool_priv.h"
@@ -1140,11 +1141,7 @@ 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);
-- 
2.48.1