netdev - [PATCH net] udp: fix busy polling

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Message-ID: <20240118143504.3466830-1-edumazet@google.com>
Date: Thu, 18 Jan 2024 14:35:04 +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>, David Ahern <dsahern@...nel.org>, netdev@...r.kernel.org, 
	eric.dumazet@...il.com, Eric Dumazet <edumazet@...gle.com>
Subject: [PATCH net] udp: fix busy polling

Generic sk_busy_loop_end() only looks at sk->sk_receive_queue
for presence of packets.

Problem is that for UDP sockets after blamed commit, some packets
could be present in another queue: udp_sk(sk)->reader_queue

In some cases, a busy poller could spin until timeout expiration,
even if some packets are available in udp_sk(sk)->reader_queue.

Fixes: 2276f58ac589 ("udp: use a separate rx queue for packet reception")
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
 include/linux/skmsg.h |  6 ------
 include/net/sock.h    |  5 +++++
 net/core/sock.c       | 10 +++++++++-
 3 files changed, 14 insertions(+), 7 deletions(-)

diff --git a/include/linux/skmsg.h b/include/linux/skmsg.h
index 888a4b217829fd4d6baf52f784ce35e9ad6bd0ed..e65ec3fd27998a5b82fc2c4597c575125e653056 100644
--- a/include/linux/skmsg.h
+++ b/include/linux/skmsg.h
@@ -505,12 +505,6 @@ static inline bool sk_psock_strp_enabled(struct sk_psock *psock)
 	return !!psock->saved_data_ready;
 }
 
-static inline bool sk_is_udp(const struct sock *sk)
-{
-	return sk->sk_type == SOCK_DGRAM &&
-	       sk->sk_protocol == IPPROTO_UDP;
-}
-
 #if IS_ENABLED(CONFIG_NET_SOCK_MSG)
 
 #define BPF_F_STRPARSER	(1UL << 1)
diff --git a/include/net/sock.h b/include/net/sock.h
index a7f815c7cfdfdf1296be2967fd100efdb10cdd63..b1ceba8e179aa5cc4c90e98d353551b3a3e1ab86 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -2770,6 +2770,11 @@ static inline bool sk_is_tcp(const struct sock *sk)
 	return sk->sk_type == SOCK_STREAM && sk->sk_protocol == IPPROTO_TCP;
 }
 
+static inline bool sk_is_udp(const struct sock *sk)
+{
+	return sk->sk_type == SOCK_DGRAM && sk->sk_protocol == IPPROTO_UDP;
+}
+
 static inline bool sk_is_stream_unix(const struct sock *sk)
 {
 	return sk->sk_family == AF_UNIX && sk->sk_type == SOCK_STREAM;
diff --git a/net/core/sock.c b/net/core/sock.c
index 158dbdebce6a3693deb63e557e856d9cdd7500ae..e7e2435ed28681772bf3637b96ddd9334e6a639e 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -107,6 +107,7 @@
 #include <linux/interrupt.h>
 #include <linux/poll.h>
 #include <linux/tcp.h>
+#include <linux/udp.h>
 #include <linux/init.h>
 #include <linux/highmem.h>
 #include <linux/user_namespace.h>
@@ -4143,8 +4144,15 @@ subsys_initcall(proto_init);
 bool sk_busy_loop_end(void *p, unsigned long start_time)
 {
 	struct sock *sk = p;
+	bool packet_ready;
 
-	return !skb_queue_empty_lockless(&sk->sk_receive_queue) ||
+	packet_ready = !skb_queue_empty_lockless(&sk->sk_receive_queue);
+	if (!packet_ready && sk_is_udp(sk)) {
+		struct sk_buff_head *reader_queue = &udp_sk(sk)->reader_queue;
+
+		packet_ready = !skb_queue_empty_lockless(reader_queue);
+	}
+	return packet_ready ||
 	       sk_busy_loop_timeout(sk, start_time);
 }
 EXPORT_SYMBOL(sk_busy_loop_end);
-- 
2.43.0.381.gb435a96ce8-goog