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BUG: KCSAN: data-race in pcpu_alloc / pcpu_balance_workfn

write to 0xffffffff88f4cccc of 4 bytes by task 7 on cpu 0:
 pcpu_update_empty_pages mm/percpu.c:559 [inline]
 pcpu_chunk_populated mm/percpu.c:1493 [inline]
 __pcpu_balance_workfn mm/percpu.c:2033 [inline]
 pcpu_balance_workfn+0xc7c/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

read to 0xffffffff88f4cccc of 4 bytes by task 8395 on cpu 1:
 pcpu_alloc+0xaed/0x1090 mm/percpu.c:1835
 __alloc_percpu_gfp+0x22/0x30 mm/percpu.c:1894
 percpu_ref_init+0x2c/0x210 lib/percpu-refcount.c:72
 cgroup_bpf_inherit+0x5f/0x400 kernel/bpf/cgroup.c:297
 cgroup_create kernel/cgroup/cgroup.c:5186 [inline]
 cgroup_mkdir+0x6ed/0xe00 kernel/cgroup/cgroup.c:5303
 kernfs_iop_mkdir+0x118/0x1d0 fs/kernfs/dir.c:1127
 vfs_mkdir+0x135/0x1f0 fs/namei.c:3817
 do_mkdirat+0x147/0x230 fs/namei.c:3842
 __do_sys_mkdir fs/namei.c:3860 [inline]
 __se_sys_mkdir fs/namei.c:3858 [inline]
 __x64_sys_mkdir+0x32/0x40 fs/namei.c:3858
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 8395 Comm: syz-executor.4 Not tainted 5.12.0-rc2-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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