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BUG: KCSAN: data-race in pcpu_alloc / pcpu_free_area

write to 0xffffffff8455f74c of 4 bytes by task 8 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
 sock_inuse_exit_net+0x39/0x40 net/core/sock.c:3384
 ops_exit_list net/core/net_namespace.c:175 [inline]
 cleanup_net+0x445/0x900 net/core/net_namespace.c:595
 process_one_work+0x3e9/0x8f0 kernel/workqueue.c:2276
 worker_thread+0x636/0xae0 kernel/workqueue.c:2422
 kthread+0x1d0/0x1f0 kernel/kthread.c:313
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

read to 0xffffffff8455f74c of 4 bytes by task 20551 on cpu 1:
 pcpu_alloc+0xa33/0x1200 mm/percpu.c:1835
 __alloc_percpu+0x20/0x30 mm/percpu.c:1907
 sock_inuse_init_net+0x1c/0xa0 net/core/sock.c:3366
 ops_init+0x1e7/0x230 net/core/net_namespace.c:140
 setup_net+0x1cb/0x700 net/core/net_namespace.c:333
 copy_net_ns+0x2a9/0x450 net/core/net_namespace.c:474
 create_new_namespaces+0x231/0x560 kernel/nsproxy.c:110
 unshare_nsproxy_namespaces+0xe2/0x120 kernel/nsproxy.c:226
 ksys_unshare+0x381/0x710 kernel/fork.c:2996
 __do_sys_unshare kernel/fork.c:3064 [inline]
 __se_sys_unshare kernel/fork.c:3062 [inline]
 __x64_sys_unshare+0x1b/0x20 kernel/fork.c:3062
 do_syscall_64+0x4a/0x90 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae

value changed: 0x0000004d -> 0x0000004e

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 20551 Comm: syz-executor.0 Not tainted 5.13.0-rc5-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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