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BUG: KCSAN: data-race in pcpu_alloc / pcpu_balance_workfn

write to 0xffffffff892d9b5c of 4 bytes by task 3363 on cpu 0:
 pcpu_update_empty_pages mm/percpu.c:549 [inline]
 pcpu_chunk_populated mm/percpu.c:1455 [inline]
 pcpu_balance_workfn+0xc2b/0xeb0 mm/percpu.c:1898
 process_one_work+0x3cb/0x990 kernel/workqueue.c:2269
 worker_thread+0x665/0xbe0 kernel/workqueue.c:2415
 kthread+0x210/0x220 kernel/kthread.c:291
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:293

read to 0xffffffff892d9b5c of 4 bytes by task 9137 on cpu 1:
 pcpu_alloc+0xb0f/0x10e0 mm/percpu.c:1706
 __alloc_percpu+0x20/0x30 mm/percpu.c:1773
 alloc_vfsmnt+0xcf/0x2d0 fs/namespace.c:192
 clone_mnt+0x40/0x740 fs/namespace.c:1039
 copy_tree+0x358/0x8c0 fs/namespace.c:1850
 copy_mnt_ns+0xca/0x7c0 fs/namespace.c:3298
 create_new_namespaces+0x84/0x520 kernel/nsproxy.c:78
 unshare_nsproxy_namespaces+0xe2/0x120 kernel/nsproxy.c:231
 ksys_unshare+0x36e/0x6a0 kernel/fork.c:2983
 __do_sys_unshare kernel/fork.c:3051 [inline]
 __se_sys_unshare kernel/fork.c:3049 [inline]
 __x64_sys_unshare+0x1b/0x20 kernel/fork.c:3049
 do_syscall_64+0x71/0x120 arch/x86/entry/common.c:359
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 9137 Comm: syz-executor.5 Not tainted 5.8.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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