Subject: [PATCH net-next 2/3] net: manage napi add/del idempotence explicitly

From: Jakub Kicinski <[email-masked]>
Date: Wed, 9 Sep 2020 10:37:52 -0700

To RCUify napi->dev_list we need to replace list_del_init()
with list_del_rcu(). There is no _init() version for RCU for
obvious reasons. Up until now netif_napi_del() was idempotent
so to make sure it remains such add a bit which is set when
NAPI is listed, and cleared when it removed. Since we don't
expect multiple calls to netif_napi_add() to be correct,
add a warning on that side.

Now that napi_hash_add / napi_hash_del are only called by
napi_add / del we can actually steal its bit. We just need
to make sure hash node is initialized correctly.

Signed-off-by: Jakub Kicinski <[email-masked]>
---
 include/linux/netdevice.h |  4 ++--
 net/core/dev.c            | 13 +++++++++----
 2 files changed, 11 insertions(+), 6 deletions(-)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 74ed95215091..157e0242e9ee 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -355,7 +355,7 @@ enum {
 	NAPI_STATE_MISSED,	/* reschedule a napi */
 	NAPI_STATE_DISABLE,	/* Disable pending */
 	NAPI_STATE_NPSVC,	/* Netpoll - don't dequeue from poll_list */
-	NAPI_STATE_HASHED,	/* In NAPI hash (busy polling possible) */
+	NAPI_STATE_LISTED,	/* NAPI added to system lists */
 	NAPI_STATE_NO_BUSY_POLL,/* Do not add in napi_hash, no busy polling */
 	NAPI_STATE_IN_BUSY_POLL,/* sk_busy_loop() owns this NAPI */
 };
@@ -365,7 +365,7 @@ enum {
 	NAPIF_STATE_MISSED	 = BIT(NAPI_STATE_MISSED),
 	NAPIF_STATE_DISABLE	 = BIT(NAPI_STATE_DISABLE),
 	NAPIF_STATE_NPSVC	 = BIT(NAPI_STATE_NPSVC),
-	NAPIF_STATE_HASHED	 = BIT(NAPI_STATE_HASHED),
+	NAPIF_STATE_LISTED	 = BIT(NAPI_STATE_LISTED),
 	NAPIF_STATE_NO_BUSY_POLL = BIT(NAPI_STATE_NO_BUSY_POLL),
 	NAPIF_STATE_IN_BUSY_POLL = BIT(NAPI_STATE_IN_BUSY_POLL),
 };
diff --git a/net/core/dev.c b/net/core/dev.c
index 205b7339d4ae..e0a1be986824 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -6533,8 +6533,7 @@ EXPORT_SYMBOL(napi_busy_loop);
 
 static void napi_hash_add(struct napi_struct *napi)
 {
-	if (test_bit(NAPI_STATE_NO_BUSY_POLL, &napi->state) ||
-	    test_and_set_bit(NAPI_STATE_HASHED, &napi->state))
+	if (test_bit(NAPI_STATE_NO_BUSY_POLL, &napi->state))
 		return;
 
 	spin_lock(&napi_hash_lock);
@@ -6559,8 +6558,7 @@ static void napi_hash_del(struct napi_struct *napi)
 {
 	spin_lock(&napi_hash_lock);
 
-	if (test_and_clear_bit(NAPI_STATE_HASHED, &napi->state))
-		hlist_del_rcu(&napi->napi_hash_node);
+	hlist_del_init_rcu(&napi->napi_hash_node);
 
 	spin_unlock(&napi_hash_lock);
 }
@@ -6595,7 +6593,11 @@ static void init_gro_hash(struct napi_struct *napi)
 void netif_napi_add(struct net_device *dev, struct napi_struct *napi,
 		    int (*poll)(struct napi_struct *, int), int weight)
 {
+	if (WARN_ON(test_and_set_bit(NAPI_STATE_LISTED, &napi->state)))
+		return;
+
 	INIT_LIST_HEAD(&napi->poll_list);
+	INIT_HLIST_NODE(&napi->napi_hash_node);
 	hrtimer_init(&napi->timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL_PINNED);
 	napi->timer.function = napi_watchdog;
 	init_gro_hash(napi);
@@ -6650,6 +6652,9 @@ static void flush_gro_hash(struct napi_struct *napi)
 /* Must be called in process context */
 void __netif_napi_del(struct napi_struct *napi)
 {
+	if (!test_and_clear_bit(NAPI_STATE_LISTED, &napi->state))
+		return;
+
 	napi_hash_del(napi);
 	list_del_init(&napi->dev_list);
 	napi_free_frags(napi);
-- 
2.26.2