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BUG: KCSAN: data-race in pcpu_balance_workfn / pcpu_free_area

write to 0xffffffff892d4b5c of 4 bytes by task 8798 on cpu 0:
 pcpu_update_empty_pages mm/percpu.c:549 [inline]
 pcpu_block_update_hint_free mm/percpu.c:984 [inline]
 pcpu_free_area+0x57c/0x700 mm/percpu.c:1239
 free_percpu+0x1b0/0x5b0 mm/percpu.c:1952
 xt_percpu_counter_free+0x5f/0x70 net/netfilter/x_tables.c:1858
 cleanup_entry+0x170/0x1a0 net/ipv6/netfilter/ip6_tables.c:672
 __do_replace+0x46e/0x590 net/ipv6/netfilter/ip6_tables.c:1102
 do_replace net/ipv6/netfilter/ip6_tables.c:1157 [inline]
 do_ip6t_set_ctl+0x20e/0x2b0 net/ipv6/netfilter/ip6_tables.c:1681
 nf_sockopt net/netfilter/nf_sockopt.c:106 [inline]
 nf_setsockopt+0x18f/0x1b0 net/netfilter/nf_sockopt.c:115
 ipv6_setsockopt+0x101/0x110 net/ipv6/ipv6_sockglue.c:905
 tcp_setsockopt+0x79/0x90 net/ipv4/tcp.c:3334
 sock_common_setsockopt+0x53/0x60 net/core/sock.c:3262
 __sys_setsockopt+0x27a/0x390 net/socket.c:2127
 __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+0x51/0xb0 arch/x86/entry/common.c:359
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffffffff892d4b5c of 4 bytes by task 17 on cpu 1:
 pcpu_balance_workfn+0x8c1/0xeb0 mm/percpu.c:1867
 process_one_work+0x3e1/0x9a0 kernel/workqueue.c:2269
 worker_thread+0x665/0xbe0 kernel/workqueue.c:2415
 kthread+0x20d/0x230 kernel/kthread.c:291
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:293

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 17 Comm: kworker/1:0 Not tainted 5.8.0-rc3-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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