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BUG: KCSAN: data-race in pcpu_alloc / pcpu_free_area

write to 0xffffffff8455f74c of 4 bytes by task 27845 on cpu 0:
 pcpu_update_empty_pages mm/percpu.c:559 [inline]
 pcpu_block_update_hint_free mm/percpu.c:994 [inline]
 pcpu_free_area+0x50c/0x5a0 mm/percpu.c:1254
 free_percpu+0x1d2/0x690 mm/percpu.c:2102
 xt_percpu_counter_free+0x5f/0x70 net/netfilter/x_tables.c:1889
 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+0x6ac/0x1630 net/ipv4/netfilter/ip_tables.c:1629
 nf_setsockopt+0x1a6/0x1c0 net/netfilter/nf_sockopt.c:101
 ip_setsockopt+0x2602/0x2a20 net/ipv4/ip_sockglue.c:1435
 tcp_setsockopt+0x8c/0xa0 net/ipv4/tcp.c:3655
 sock_common_setsockopt+0x5d/0x70 net/core/sock.c:3257
 __sys_setsockopt+0x18f/0x200 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+0x4a/0x90 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffffffff8455f74c of 4 bytes by task 11635 on cpu 1:
 pcpu_alloc+0xa33/0x1200 mm/percpu.c:1835
 __alloc_percpu+0x20/0x30 mm/percpu.c:1907
 __percpu_init_rwsem+0x15/0x80 kernel/locking/percpu-rwsem.c:15
 alloc_super+0x16d/0x520 fs/super.c:235
 sget_fc+0x1cf/0x4e0 fs/super.c:531
 vfs_get_super fs/super.c:1135 [inline]
 get_tree_keyed+0x3f/0x120 fs/super.c:1196
 mqueue_get_tree+0x32/0x40 ipc/mqueue.c:427
 vfs_get_tree+0x4a/0x1a0 fs/super.c:1498
 fc_mount+0x12/0x60 fs/namespace.c:993
 mq_create_mount ipc/mqueue.c:470 [inline]
 mq_init_ns+0x26d/0x310 ipc/mqueue.c:1685
 create_ipc_ns ipc/namespace.c:58 [inline]
 copy_ipcs+0x211/0x360 ipc/namespace.c:84
 create_new_namespaces+0x137/0x560 kernel/nsproxy.c:90
 copy_namespaces+0x111/0x160 kernel/nsproxy.c:178
 copy_process+0x1254/0x2e00 kernel/fork.c:2119
 kernel_clone+0x14b/0x680 kernel/fork.c:2503
 __do_sys_clone kernel/fork.c:2620 [inline]
 __se_sys_clone kernel/fork.c:2604 [inline]
 __x64_sys_clone+0xb4/0xe0 kernel/fork.c:2604
 do_syscall_64+0x4a/0x90 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 11635 Comm: syz-executor.3 Tainted: G        W         5.12.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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