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BUG: KCSAN: data-race in pcpu_alloc / pcpu_free_area

write to 0xffffffff88f3dccc of 4 bytes by task 16345 on cpu 1:
 pcpu_update_empty_pages mm/percpu.c:559 [inline]
 pcpu_block_update_hint_free mm/percpu.c:994 [inline]
 pcpu_free_area+0x597/0x640 mm/percpu.c:1254
 free_percpu+0x1cb/0x740 mm/percpu.c:2102
 xt_percpu_counter_free+0x5f/0x70 net/netfilter/x_tables.c:1861
 cleanup_entry+0x170/0x1a0 net/ipv6/netfilter/ip6_tables.c:672
 __do_replace+0x46e/0x590 net/ipv6/netfilter/ip6_tables.c:1101
 do_replace net/ipv6/netfilter/ip6_tables.c:1156 [inline]
 do_ip6t_set_ctl+0x145f/0x1710 net/ipv6/netfilter/ip6_tables.c:1636
 nf_setsockopt+0x1a6/0x1c0 net/netfilter/nf_sockopt.c:101
 ipv6_setsockopt+0x1c38/0x37c0 net/ipv6/ipv6_sockglue.c:1008
 tcp_setsockopt+0x8c/0xa0 net/ipv4/tcp.c:3643
 sock_common_setsockopt+0x5d/0x70 net/core/sock.c:3257
 __sys_setsockopt+0x2e8/0x460 net/socket.c:2117
 __do_sys_setsockopt net/socket.c:2128 [inline]
 __se_sys_setsockopt net/socket.c:2125 [inline]
 __x64_sys_setsockopt+0x62/0x70 net/socket.c:2125
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffffffff88f3dccc of 4 bytes by task 29334 on cpu 0:
 pcpu_alloc+0xb11/0x10a0 mm/percpu.c:1835
 __alloc_percpu+0x20/0x30 mm/percpu.c:1907
 mpls_add_dev net/mpls/af_mpls.c:1460 [inline]
 mpls_dev_notify+0x2fc/0x9b0 net/mpls/af_mpls.c:1589
 notifier_call_chain kernel/notifier.c:83 [inline]
 raw_notifier_call_chain+0x53/0xb0 kernel/notifier.c:410
 call_netdevice_notifiers_info net/core/dev.c:2075 [inline]
 call_netdevice_notifiers_extack net/core/dev.c:2087 [inline]
 call_netdevice_notifiers net/core/dev.c:2101 [inline]
 register_netdevice+0xdf7/0x1080 net/core/dev.c:10239
 tun_set_iff drivers/net/tun.c:2743 [inline]
 __tun_chr_ioctl+0x2534/0x2ea0 drivers/net/tun.c:3019
 tun_chr_ioctl+0x23/0x30 drivers/net/tun.c:3304
 vfs_ioctl fs/ioctl.c:48 [inline]
 __do_sys_ioctl fs/ioctl.c:753 [inline]
 __se_sys_ioctl+0xcb/0x140 fs/ioctl.c:739
 __x64_sys_ioctl+0x3f/0x50 fs/ioctl.c:739
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 29334 Comm: syz-executor.4 Not tainted 5.12.0-rc6-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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