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BUG: KCSAN: data-race in pcpu_alloc / pcpu_free_area

write to 0xffffffff88f3cccc of 4 bytes by task 8403 on cpu 0:
 pcpu_update_empty_pages mm/percpu.c:559 [inline]
 pcpu_block_update_hint_free mm/percpu.c:994 [inline]
 pcpu_free_area+0x578/0x620 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:3263
 __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 0xffffffff88f3cccc of 4 bytes by task 8409 on cpu 1:
 pcpu_alloc+0xaed/0x1090 mm/percpu.c:1835
 __alloc_percpu+0x20/0x30 mm/percpu.c:1907
 xt_percpu_counter_alloc+0x6b/0x110 net/netfilter/x_tables.c:1841
 find_check_entry net/ipv4/netfilter/arp_tables.c:412 [inline]
 translate_table+0xd18/0x10f0 net/ipv4/netfilter/arp_tables.c:577
 do_replace net/ipv4/netfilter/arp_tables.c:980 [inline]
 do_arpt_set_ctl+0xda4/0x1360 net/ipv4/netfilter/arp_tables.c:1418
 nf_setsockopt+0x1a6/0x1c0 net/netfilter/nf_sockopt.c:101
 ip_setsockopt+0x2725/0x2b50 net/ipv4/ip_sockglue.c:1435
 tcp_setsockopt+0x8c/0xa0 net/ipv4/tcp.c:3643
 sock_common_setsockopt+0x5d/0x70 net/core/sock.c:3263
 __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

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 8409 Comm: syz-executor.3 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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