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BUG: KCSAN: data-race in pcpu_alloc / pcpu_balance_workfn

write to 0xffffffff8952301c of 4 bytes by task 51 on cpu 1:
 pcpu_update_empty_pages mm/percpu.c:558 [inline]
 pcpu_chunk_populated mm/percpu.c:1492 [inline]
 __pcpu_balance_workfn mm/percpu.c:2033 [inline]
 pcpu_balance_workfn+0xc6c/0xe30 mm/percpu.c:2069
 process_one_work+0x3e1/0x9a0 kernel/workqueue.c:2269
 worker_thread+0x665/0xbe0 kernel/workqueue.c:2415
 kthread+0x20d/0x230 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

read to 0xffffffff8952301c of 4 bytes by task 8714 on cpu 0:
 pcpu_alloc+0xb36/0x1370 mm/percpu.c:1835
 __alloc_percpu+0x20/0x30 mm/percpu.c:1907
 wg_packet_percpu_multicore_worker_alloc drivers/net/wireguard/queueing.c:13 [inline]
 wg_packet_queue_init+0x154/0x360 drivers/net/wireguard/queueing.c:36
 wg_newlink+0x333/0x580 drivers/net/wireguard/device.c:344
 __rtnl_newlink net/core/rtnetlink.c:3441 [inline]
 rtnl_newlink+0xdaf/0x1340 net/core/rtnetlink.c:3500
 rtnetlink_rcv_msg+0x71b/0x7b0 net/core/rtnetlink.c:5563
 netlink_rcv_skb+0x13e/0x240 net/netlink/af_netlink.c:2470
 rtnetlink_rcv+0x18/0x20 net/core/rtnetlink.c:5581
 netlink_unicast_kernel net/netlink/af_netlink.c:1304 [inline]
 netlink_unicast+0x5e2/0x6b0 net/netlink/af_netlink.c:1330
 netlink_sendmsg+0x6d8/0x7a0 net/netlink/af_netlink.c:1919
 sock_sendmsg_nosec net/socket.c:651 [inline]
 sock_sendmsg net/socket.c:671 [inline]
 __sys_sendto+0x2ae/0x380 net/socket.c:1992
 __do_sys_sendto net/socket.c:2004 [inline]
 __se_sys_sendto net/socket.c:2000 [inline]
 __x64_sys_sendto+0x74/0x90 net/socket.c:2000
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 8714 Comm: syz-executor.3 Not tainted 5.9.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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