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BUG: KCSAN: data-race in pcpu_balance_workfn / pcpu_free_area

write to 0xffffffff88f41ccc of 4 bytes by task 8406 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+0x578/0x620 mm/percpu.c:1254
 free_percpu+0x1cb/0x740 mm/percpu.c:2102
 xt_percpu_counter_free+0x5f/0x70 net/netfilter/x_tables.c:1842
 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+0x145f/0x1710 net/ipv6/netfilter/ip6_tables.c:1636
 nf_setsockopt+0x1a6/0x1c0 net/netfilter/nf_sockopt.c:101
 ipv6_setsockopt+0x1c38/0x37c0 net/ipv6/ipv6_sockglue.c:1008
 tcp_setsockopt+0x8c/0xa0 net/ipv4/tcp.c:3643
 sock_common_setsockopt+0x5d/0x70 net/core/sock.c:3263
 __sys_setsockopt+0x2e8/0x460 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+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffffffff88f41ccc of 4 bytes by task 9542 on cpu 0:
 __pcpu_balance_workfn mm/percpu.c:2002 [inline]
 pcpu_balance_workfn+0x911/0xe40 mm/percpu.c:2069
 process_one_work+0x3e1/0x950 kernel/workqueue.c:2275
 worker_thread+0x616/0xa70 kernel/workqueue.c:2421
 kthread+0x20b/0x230 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 9542 Comm: kworker/0:6 Not tainted 5.12.0-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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usb 3-1: USB disconnect, device number 2
usb 3-1: ath9k_htc: USB layer deinitialized
usb 2-1: new high-speed USB device number 6 using dummy_hcd
usb 2-1: config 21 has an invalid descriptor of length 0, skipping remainder of the config
usb 2-1: config 21 has 1 interface, different from the descriptor's value: 2
usb 2-1: config 21 interface 0 altsetting 0 endpoint 0x81 has an invalid bInterval 0, changing to 7
usb 2-1: New USB device found, idVendor=0525, idProduct=a4a1, bcdDevice= 0.40
usb 2-1: New USB device strings: Mfr=1, Product=1, SerialNumber=236
usb 2-1: Product: syz
usb 2-1: Manufacturer: syz
usb 2-1: SerialNumber: syz
hub 2-1:21.0: bad descriptor, ignoring hub
hub: probe of 2-1:21.0 failed with error -5
cdc_wdm 2-1:21.0: cdc-wdm1: USB WDM device
usb 6-1: new high-speed USB device number 4 using dummy_hcd
usb 6-1: New USB device found, idVendor=0cf3, idProduct=9271, bcdDevice= 1.08
usb 6-1: New USB device strings: Mfr=1, Product=2, SerialNumber=3
usb 6-1: Product: syz
usb 6-1: Manufacturer: syz
usb 6-1: SerialNumber: syz
usb 6-1: ath9k_htc: Firmware ath9k_htc/htc_9271-1.4.0.fw requested
usb 6-1: ath9k_htc: Transferred FW: ath9k_htc/htc_9271-1.4.0.fw, size: 51008