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Date:   Wed, 26 Aug 2020 12:40:06 -0700
From:   Jakub Kicinski <>
        Jakub Kicinski <>, Rob Sherwood <>
Subject: [PATCH net 1/2] net: disable netpoll on fresh napis

napi_disable() makes sure to set the NAPI_STATE_NPSVC bit to prevent
netpoll from accessing rings before init is complete. However, the
same is not done for fresh napi instances in netif_napi_add(),
even though we expect NAPI instances to be added as disabled.

This causes crashes during driver reconfiguration (enabling XDP,
changing the channel count) - if there is any printk() after
netif_napi_add() but before napi_enable().

To ensure memory ordering is correct we need to use RCU accessors.

Reported-by: Rob Sherwood <>
Fixes: 2d8bff12699a ("netpoll: Close race condition between poll_one_napi and napi_disable")
Signed-off-by: Jakub Kicinski <>
 net/core/dev.c     | 3 ++-
 net/core/netpoll.c | 2 +-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index d42c9ea0c3c0..95ac7568f693 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -6612,12 +6612,13 @@ void netif_napi_add(struct net_device *dev, struct napi_struct *napi,
 		netdev_err_once(dev, "%s() called with weight %d\n", __func__,
 	napi->weight = weight;
-	list_add(&napi->dev_list, &dev->napi_list);
 	napi->dev = dev;
 	napi->poll_owner = -1;
 	set_bit(NAPI_STATE_SCHED, &napi->state);
+	set_bit(NAPI_STATE_NPSVC, &napi->state);
+	list_add_rcu(&napi->dev_list, &dev->napi_list);
diff --git a/net/core/netpoll.c b/net/core/netpoll.c
index 093e90e52bc2..2338753e936b 100644
--- a/net/core/netpoll.c
+++ b/net/core/netpoll.c
@@ -162,7 +162,7 @@ static void poll_napi(struct net_device *dev)
 	struct napi_struct *napi;
 	int cpu = smp_processor_id();
-	list_for_each_entry(napi, &dev->napi_list, dev_list) {
+	list_for_each_entry_rcu(napi, &dev->napi_list, dev_list) {
 		if (cmpxchg(&napi->poll_owner, -1, cpu) == -1) {
 			smp_store_release(&napi->poll_owner, -1);

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