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BUG: KCSAN: data-race in pcpu_alloc / pcpu_free_area

read-write to 0xffffffff88314a4c of 4 bytes by task 8454 on cpu 1:
 pcpu_update_empty_pages mm/percpu.c:558 [inline]
 pcpu_block_update_hint_free mm/percpu.c:993 [inline]
 pcpu_free_area+0x531/0x5d0 mm/percpu.c:1253
 free_percpu+0x1ca/0x710 mm/percpu.c:2101
 xt_percpu_counter_free+0x5f/0x70 net/netfilter/x_tables.c:1840
 cleanup_entry+0x170/0x1a0 net/ipv6/netfilter/ip6_tables.c:672
 __do_replace+0x41e/0x540 net/ipv6/netfilter/ip6_tables.c:1101
 do_replace net/ipv6/netfilter/ip6_tables.c:1156 [inline]
 do_ip6t_set_ctl+0x1422/0x16e0 net/ipv6/netfilter/ip6_tables.c:1636
 nf_setsockopt+0x1a6/0x1c0 net/netfilter/nf_sockopt.c:101
 ipv6_setsockopt+0x1b3a/0x3650 net/ipv6/ipv6_sockglue.c:1008
 tcp_setsockopt+0x8c/0xa0 net/ipv4/tcp.c:3597
 sock_common_setsockopt+0x5d/0x70 net/core/sock.c:3249
 __sys_setsockopt+0x2e8/0x460 net/socket.c:2115
 __do_sys_setsockopt net/socket.c:2126 [inline]
 __se_sys_setsockopt net/socket.c:2123 [inline]
 __x64_sys_setsockopt+0x62/0x70 net/socket.c:2123
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffffffff88314a4c of 4 bytes by task 8446 on cpu 0:
 pcpu_alloc+0xa9b/0x1020 mm/percpu.c:1834
 __alloc_percpu+0x20/0x30 mm/percpu.c:1906
 xt_percpu_counter_alloc+0x6e/0x110 net/netfilter/x_tables.c:1820
 find_check_entry net/ipv6/netfilter/ip6_tables.c:546 [inline]
 translate_table+0x9e2/0x1050 net/ipv6/netfilter/ip6_tables.c:734
 do_replace net/ipv6/netfilter/ip6_tables.c:1152 [inline]
 do_ip6t_set_ctl+0x13d2/0x16e0 net/ipv6/netfilter/ip6_tables.c:1636
 nf_setsockopt+0x1a6/0x1c0 net/netfilter/nf_sockopt.c:101
 ipv6_setsockopt+0x1b3a/0x3650 net/ipv6/ipv6_sockglue.c:1008
 tcp_setsockopt+0x8c/0xa0 net/ipv4/tcp.c:3597
 sock_common_setsockopt+0x5d/0x70 net/core/sock.c:3249
 __sys_setsockopt+0x2e8/0x460 net/socket.c:2115
 __do_sys_setsockopt net/socket.c:2126 [inline]
 __se_sys_setsockopt net/socket.c:2123 [inline]
 __x64_sys_setsockopt+0x62/0x70 net/socket.c:2123
 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: 8446 Comm: syz-executor.0 Not tainted 5.11.0-rc3-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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