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BUG: KCSAN: data-race in pcpu_balance_workfn / pcpu_free_area

write to 0xffffffff8952b01c of 4 bytes by task 8729 on cpu 0:
 pcpu_update_empty_pages mm/percpu.c:558 [inline]
 pcpu_block_update_hint_free mm/percpu.c:993 [inline]
 pcpu_free_area+0x58e/0x630 mm/percpu.c:1253
 free_percpu+0x1d1/0x7b0 mm/percpu.c:2102
 xt_percpu_counter_free+0x5f/0x70 net/netfilter/x_tables.c:1859
 cleanup_entry+0x15a/0x180 net/ipv4/netfilter/ip_tables.c:655
 __do_replace+0x45e/0x580 net/ipv4/netfilter/ip_tables.c:1084
 do_replace net/ipv4/netfilter/ip_tables.c:1139 [inline]
 do_ipt_set_ctl+0x13f5/0x1690 net/ipv4/netfilter/ip_tables.c:1627
 nf_setsockopt+0x1a6/0x1c0 net/netfilter/nf_sockopt.c:101
 ip_setsockopt+0x274e/0x2b90 net/ipv4/ip_sockglue.c:1436
 tcp_setsockopt+0x8c/0xa0 net/ipv4/tcp.c:3334
 sock_common_setsockopt+0x5d/0x70 net/core/sock.c:3247
 __sys_setsockopt+0x2dc/0x450 net/socket.c:2132
 __do_sys_setsockopt net/socket.c:2143 [inline]
 __se_sys_setsockopt net/socket.c:2140 [inline]
 __x64_sys_setsockopt+0x62/0x70 net/socket.c:2140
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffffffff8952b01c of 4 bytes by task 4776 on cpu 1:
 __pcpu_balance_workfn mm/percpu.c:2002 [inline]
 pcpu_balance_workfn+0x901/0xe30 mm/percpu.c:2069
 process_one_work+0x3e1/0x9a0 kernel/workqueue.c:2269
 worker_thread+0x665/0xbe0 kernel/workqueue.c:2415
 kthread+0x20d/0x230 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 4776 Comm: kworker/1:2 Not tainted 5.9.0-rc8-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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