linux-kernel - [PATCH net 1/2] netpoll: Use rcu_access_pointer() in __netpoll

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Message-Id: <20241118-netpoll_rcu-v1-1-a1888dcb4a02@debian.org>
Date: Mon, 18 Nov 2024 03:15:17 -0800
From: Breno Leitao <leitao@...ian.org>
To: "David S. Miller" <davem@...emloft.net>, 
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
 Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, 
 Herbert Xu <herbert@...dor.apana.org.au>, 
 Stephen Hemminger <stephen@...workplumber.org>
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
 Breno Leitao <leitao@...ian.org>, paulmck@...nel.org
Subject: [PATCH net 1/2] netpoll: Use rcu_access_pointer() in
 __netpoll_setup

The ndev->npinfo pointer in __netpoll_setup() 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.

Signed-off-by: Breno Leitao <leitao@...ian.org>
Fixes: 8fdd95ec162a ("netpoll: Allow netpoll_setup/cleanup recursion")
---
 net/core/netpoll.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/core/netpoll.c b/net/core/netpoll.c
index aa49b92e9194babab17b2e039daf092a524c5b88..45fb60bc4803958eb07d4038028269fc0c19622e 100644
--- a/net/core/netpoll.c
+++ b/net/core/netpoll.c
@@ -626,7 +626,7 @@ int __netpoll_setup(struct netpoll *np, struct net_device *ndev)
 		goto out;
 	}

-	if (!ndev->npinfo) {
+	if (!rcu_access_pointer(ndev->npinfo)) {
 		npinfo = kmalloc(sizeof(*npinfo), GFP_KERNEL);
 		if (!npinfo) {
 			err = -ENOMEM;

-- 
2.43.5