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BUG: KCSAN: data-race in pcpu_alloc / pcpu_balance_workfn

write to 0xffffffff892d3b5c of 4 bytes by task 5 on cpu 0:
 pcpu_update_empty_pages mm/percpu.c:549 [inline]
 pcpu_chunk_populated mm/percpu.c:1455 [inline]
 pcpu_balance_workfn+0xc2b/0xeb0 mm/percpu.c:1898
 process_one_work+0x3e1/0x9a0 kernel/workqueue.c:2269
 worker_thread+0x665/0xbe0 kernel/workqueue.c:2415
 kthread+0x20d/0x230 kernel/kthread.c:291
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:293

read to 0xffffffff892d3b5c of 4 bytes by task 9120 on cpu 1:
 pcpu_alloc+0xb0f/0x10e0 mm/percpu.c:1706
 __alloc_percpu_gfp+0x22/0x30 mm/percpu.c:1760
 fib_nh_common_init+0x45/0x170 net/ipv4/fib_semantics.c:580
 fib6_nh_init+0xb6d/0x11e0 net/ipv6/route.c:3509
 ip6_route_info_create+0x80d/0xc50 net/ipv6/route.c:3696
 addrconf_f6i_alloc+0x157/0x1d0 net/ipv6/route.c:4459
 ipv6_add_addr+0x2f5/0xc80 net/ipv6/addrconf.c:1087
 inet6_addr_add+0x3ca/0x9c0 net/ipv6/addrconf.c:2916
 inet6_rtm_newaddr+0x52f/0x1310 net/ipv6/addrconf.c:4840
 rtnetlink_rcv_msg+0x71b/0x7b0 net/core/rtnetlink.c:5460
 netlink_rcv_skb+0x13e/0x240 net/netlink/af_netlink.c:2469
 rtnetlink_rcv+0x18/0x20 net/core/rtnetlink.c:5478
 netlink_unicast_kernel net/netlink/af_netlink.c:1303 [inline]
 netlink_unicast+0x5e2/0x6b0 net/netlink/af_netlink.c:1329
 netlink_sendmsg+0x6d8/0x7a0 net/netlink/af_netlink.c:1918
 sock_sendmsg_nosec net/socket.c:652 [inline]
 sock_sendmsg net/socket.c:672 [inline]
 __sys_sendto+0x2ad/0x380 net/socket.c:1995
 __do_sys_sendto net/socket.c:2007 [inline]
 __se_sys_sendto net/socket.c:2003 [inline]
 __x64_sys_sendto+0x74/0x90 net/socket.c:2003
 do_syscall_64+0x51/0xb0 arch/x86/entry/common.c:359
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 9120 Comm: syz-executor.4 Not tainted 5.8.0-rc3-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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