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BUG: KCSAN: data-race in pcpu_alloc / pcpu_balance_workfn

read-write to 0xffffffff88315a4c of 4 bytes by task 34 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:2032 [inline]
 pcpu_balance_workfn+0xc05/0xdd0 mm/percpu.c:2068
 process_one_work+0x3e1/0x950 kernel/workqueue.c:2275
 worker_thread+0x64a/0xaa0 kernel/workqueue.c:2421
 kthread+0x1fd/0x220 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:296

read to 0xffffffff88315a4c of 4 bytes by task 8470 on cpu 0:
 pcpu_alloc+0xa9b/0x1020 mm/percpu.c:1834
 __alloc_percpu_gfp+0x22/0x30 mm/percpu.c:1893
 fib6_nh_init+0x568/0x10f0 net/ipv6/route.c:3520
 ip6_route_info_create+0x7ac/0xc20 net/ipv6/route.c:3701
 addrconf_f6i_alloc+0x157/0x1c0 net/ipv6/route.c:4464
 fixup_permanent_addr net/ipv6/addrconf.c:3422 [inline]
 addrconf_permanent_addr net/ipv6/addrconf.c:3461 [inline]
 addrconf_notify+0x711/0x1eb0 net/ipv6/addrconf.c:3529
 notifier_call_chain kernel/notifier.c:83 [inline]
 raw_notifier_call_chain+0x53/0xb0 kernel/notifier.c:410
 __dev_notify_flags+0x1d3/0x3a0 net/core/dev.c:2040
 dev_change_flags+0xa2/0xc0 net/core/dev.c:8554
 do_setlink+0x741/0x21d0 net/core/rtnetlink.c:2708
 __rtnl_newlink net/core/rtnetlink.c:3376 [inline]
 rtnl_newlink+0xe58/0x12f0 net/core/rtnetlink.c:3491
 rtnetlink_rcv_msg+0x723/0x7c0 net/core/rtnetlink.c:5553
 netlink_rcv_skb+0x13e/0x240 net/netlink/af_netlink.c:2494
 rtnetlink_rcv+0x18/0x20 net/core/rtnetlink.c:5571
 netlink_unicast_kernel net/netlink/af_netlink.c:1304 [inline]
 netlink_unicast+0x5df/0x6b0 net/netlink/af_netlink.c:1330
 netlink_sendmsg+0x6f8/0x7c0 net/netlink/af_netlink.c:1919
 sock_sendmsg_nosec net/socket.c:652 [inline]
 sock_sendmsg net/socket.c:672 [inline]
 __sys_sendto+0x2a8/0x370 net/socket.c:1975
 __do_sys_sendto net/socket.c:1987 [inline]
 __se_sys_sendto net/socket.c:1983 [inline]
 __x64_sys_sendto+0x74/0x90 net/socket.c:1983
 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: 8470 Comm: syz-executor.5 Not tainted 5.11.0-rc4-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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