netdev - [PATCH net-next 1/4] udp: annotate data-race in __udp_enqueue_schedule

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Message-ID: <20240328144032.1864988-2-edumazet@google.com>
Date: Thu, 28 Mar 2024 14:40:29 +0000
From: Eric Dumazet <edumazet@...gle.com>
To: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>
Cc: Willem de Bruijn <willemb@...gle.com>, netdev@...r.kernel.org, eric.dumazet@...il.com, 
	Eric Dumazet <edumazet@...gle.com>
Subject: [PATCH net-next 1/4] udp: annotate data-race in __udp_enqueue_schedule_skb()

sk->sk_rcvbuf is read locklessly twice, while other threads
could change its value.

Use a READ_ONCE() to annotate the race.

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
 net/ipv4/udp.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 661d0e0d273f616ad82746b69b2c76d056633017..f2736e8958187e132ef45d8e25ab2b4ea7bcbc3d 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -1492,13 +1492,14 @@ int __udp_enqueue_schedule_skb(struct sock *sk, struct sk_buff *skb)
 	struct sk_buff_head *list = &sk->sk_receive_queue;
 	int rmem, err = -ENOMEM;
 	spinlock_t *busy = NULL;
-	int size;
+	int size, rcvbuf;
 
-	/* try to avoid the costly atomic add/sub pair when the receive
-	 * queue is full; always allow at least a packet
+	/* Immediately drop when the receive queue is full.
+	 * Always allow at least one packet.
 	 */
 	rmem = atomic_read(&sk->sk_rmem_alloc);
-	if (rmem > sk->sk_rcvbuf)
+	rcvbuf = READ_ONCE(sk->sk_rcvbuf);
+	if (rmem > rcvbuf)
 		goto drop;
 
 	/* Under mem pressure, it might be helpful to help udp_recvmsg()
@@ -1507,7 +1508,7 @@ int __udp_enqueue_schedule_skb(struct sock *sk, struct sk_buff *skb)
 	 * - Less cache line misses at copyout() time
 	 * - Less work at consume_skb() (less alien page frag freeing)
 	 */
-	if (rmem > (sk->sk_rcvbuf >> 1)) {
+	if (rmem > (rcvbuf >> 1)) {
 		skb_condense(skb);
 
 		busy = busylock_acquire(sk);
-- 
2.44.0.396.g6e790dbe36-goog