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BUG: KCSAN: data-race in pcpu_alloc / pcpu_free_area

write to 0xffffffff8455f74c of 4 bytes by task 1792 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+0x50c/0x5a0 mm/percpu.c:1254
 free_percpu+0x1d2/0x690 mm/percpu.c:2102
 xt_percpu_counter_free+0x5f/0x70 net/netfilter/x_tables.c:1889
 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+0x6b5/0x16d0 net/ipv6/netfilter/ip6_tables.c:1638
 nf_setsockopt+0x1a6/0x1c0 net/netfilter/nf_sockopt.c:101
 ipv6_setsockopt+0x1af1/0x3410 net/ipv6/ipv6_sockglue.c:1008
 tcp_setsockopt+0x8c/0xa0 net/ipv4/tcp.c:3656
 sock_common_setsockopt+0x5d/0x70 net/core/sock.c:3257
 __sys_setsockopt+0x18f/0x200 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+0x4a/0x90 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffffffff8455f74c of 4 bytes by task 8654 on cpu 0:
 pcpu_alloc+0xa33/0x1200 mm/percpu.c:1835
 __alloc_percpu+0x20/0x30 mm/percpu.c:1907
 alloc_vfsmnt+0xcf/0x2e0 fs/namespace.c:212
 clone_mnt+0x40/0x8e0 fs/namespace.c:1054
 copy_tree+0xb6/0x910 fs/namespace.c:1840
 copy_mnt_ns+0x11d/0x870 fs/namespace.c:3345
 create_new_namespaces+0x89/0x560 kernel/nsproxy.c:78
 unshare_nsproxy_namespaces+0xe2/0x120 kernel/nsproxy.c:226
 ksys_unshare+0x381/0x710 kernel/fork.c:2996
 __do_sys_unshare kernel/fork.c:3064 [inline]
 __se_sys_unshare kernel/fork.c:3062 [inline]
 __x64_sys_unshare+0x1b/0x20 kernel/fork.c:3062
 do_syscall_64+0x4a/0x90 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 8654 Comm: syz-executor.5 Not tainted 5.13.0-rc2-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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