wireguard: queueing: annotate intentional data race in cpu round robin

KCSAN reports a race in the CPU round robin function, which, as the
comment points out, is intentional:

    BUG: KCSAN: data-race in wg_packet_send_staged_packets / wg_packet_send_staged_packets

    read to 0xffff88811254eb28 of 4 bytes by task 3160 on cpu 1:
     wg_cpumask_next_online drivers/net/wireguard/queueing.h:127 [inline]
     wg_queue_enqueue_per_device_and_peer drivers/net/wireguard/queueing.h:173 [inline]
     wg_packet_create_data drivers/net/wireguard/send.c:320 [inline]
     wg_packet_send_staged_packets+0x60e/0xac0 drivers/net/wireguard/send.c:388
     wg_packet_send_keepalive+0xe2/0x100 drivers/net/wireguard/send.c:239
     wg_receive_handshake_packet drivers/net/wireguard/receive.c:186 [inline]
     wg_packet_handshake_receive_worker+0x449/0x5f0 drivers/net/wireguard/receive.c:213
     process_one_work kernel/workqueue.c:3248 [inline]
     process_scheduled_works+0x483/0x9a0 kernel/workqueue.c:3329
     worker_thread+0x526/0x720 kernel/workqueue.c:3409
     kthread+0x1d1/0x210 kernel/kthread.c:389
     ret_from_fork+0x4b/0x60 arch/x86/kernel/process.c:147
     ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244

    write to 0xffff88811254eb28 of 4 bytes by task 3158 on cpu 0:
     wg_cpumask_next_online drivers/net/wireguard/queueing.h:130 [inline]
     wg_queue_enqueue_per_device_and_peer drivers/net/wireguard/queueing.h:173 [inline]
     wg_packet_create_data drivers/net/wireguard/send.c:320 [inline]
     wg_packet_send_staged_packets+0x6e5/0xac0 drivers/net/wireguard/send.c:388
     wg_packet_send_keepalive+0xe2/0x100 drivers/net/wireguard/send.c:239
     wg_receive_handshake_packet drivers/net/wireguard/receive.c:186 [inline]
     wg_packet_handshake_receive_worker+0x449/0x5f0 drivers/net/wireguard/receive.c:213
     process_one_work kernel/workqueue.c:3248 [inline]
     process_scheduled_works+0x483/0x9a0 kernel/workqueue.c:3329
     worker_thread+0x526/0x720 kernel/workqueue.c:3409
     kthread+0x1d1/0x210 kernel/kthread.c:389
     ret_from_fork+0x4b/0x60 arch/x86/kernel/process.c:147
     ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244

    value changed: 0xffffffff -> 0x00000000

Mark this race as intentional by using READ/WRITE_ONCE().

Cc: [email protected]
Fixes: e7096c1 ("net: WireGuard secure network tunnel")
Signed-off-by: Jason A. Donenfeld <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: zx2c4

drivers/net/wireguard/queueing.h

@@ -124,10 +124,10 @@ static inline int wg_cpumask_choose_online(int *stored_cpu, unsigned int id)
  */
 static inline int wg_cpumask_next_online(int *last_cpu)
 {
-	int cpu = cpumask_next(*last_cpu, cpu_online_mask);
+	int cpu = cpumask_next(READ_ONCE(*last_cpu), cpu_online_mask);
 	if (cpu >= nr_cpu_ids)
 		cpu = cpumask_first(cpu_online_mask);
-	*last_cpu = cpu;
+	WRITE_ONCE(*last_cpu, cpu);
 	return cpu;
 }