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BUG: KCSAN: data-race in pcpu_balance_workfn / pcpu_free_area

read-write to 0xffffffff8830fa4c 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+0x15a/0x180 net/ipv4/netfilter/ip_tables.c:655
 __do_replace+0x41e/0x540 net/ipv6/netfilter/ip6_tables.c:1101
 do_replace net/ipv4/netfilter/ip_tables.c:1139 [inline]
 do_ipt_set_ctl+0x138e/0x1650 net/ipv4/netfilter/ip_tables.c:1627
 nf_setsockopt+0x1a6/0x1c0 net/netfilter/nf_sockopt.c:101
 ip_setsockopt+0x2691/0x2ac0 net/ipv4/ip_sockglue.c:1435
 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 0xffffffff8830fa4c of 4 bytes by task 9753 on cpu 0:
 __pcpu_balance_workfn mm/percpu.c:2001 [inline]
 pcpu_balance_workfn+0x8f1/0xdd0 mm/percpu.c:2068
 process_one_work+0x3e1/0x950 kernel/workqueue.c:2275
 worker_thread+0x635/0xb90 kernel/workqueue.c:2421
 kthread+0x1fd/0x220 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:296

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 9753 Comm: kworker/0:4 Not tainted 5.11.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: events pcpu_balance_workfn
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