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BUG: KCSAN: data-race in pcpu_balance_workfn / pcpu_free_area

write to 0xffffffff86da8fc0 of 4 bytes by task 7893 on cpu 1:
 pcpu_update_empty_pages mm/percpu.c:548 [inline]
 pcpu_block_update_hint_free mm/percpu.c:983 [inline]
 pcpu_free_area+0x44b/0x6b0 mm/percpu.c:1238
 free_percpu mm/percpu.c:1946 [inline]
 free_percpu+0x14f/0x440 mm/percpu.c:1926
 xt_percpu_counter_free+0x78/0x90 net/netfilter/x_tables.c:1862
 cleanup_entry+0x12f/0x160 net/ipv6/netfilter/ip6_tables.c:672
 __do_replace+0x438/0x540 net/ipv6/netfilter/ip6_tables.c:1102
 do_replace net/ipv6/netfilter/ip6_tables.c:1157 [inline]
 do_ip6t_set_ctl+0x277/0x333 net/ipv6/netfilter/ip6_tables.c:1681
 nf_sockopt net/netfilter/nf_sockopt.c:106 [inline]
 nf_setsockopt+0x67/0xa0 net/netfilter/nf_sockopt.c:115
 ipv6_setsockopt net/ipv6/ipv6_sockglue.c:949 [inline]
 ipv6_setsockopt+0x118/0x130 net/ipv6/ipv6_sockglue.c:933
 tcp_setsockopt net/ipv4/tcp.c:3165 [inline]
 tcp_setsockopt+0x6c/0xb0 net/ipv4/tcp.c:3159
 sock_common_setsockopt+0x5d/0x80 net/core/sock.c:3149
 __sys_setsockopt+0x1d6/0x380 net/socket.c:2130
 __do_sys_setsockopt net/socket.c:2146 [inline]
 __se_sys_setsockopt net/socket.c:2143 [inline]
 __x64_sys_setsockopt+0x6b/0x80 net/socket.c:2143
 do_syscall_64+0xc7/0x390 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffffffff86da8fc0 of 4 bytes by task 7891 on cpu 0:
 pcpu_balance_workfn+0xbea/0xc80 mm/percpu.c:1861
 process_one_work+0x424/0x930 kernel/workqueue.c:2264
 worker_thread+0x9a/0x7e0 kernel/workqueue.c:2410
 kthread+0x1cb/0x1f0 kernel/kthread.c:255
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 7891 Comm: kworker/0:3 Not tainted 5.6.0-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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