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BUG: KCSAN: data-race in pcpu_alloc / pcpu_balance_workfn

write to 0xffffffff8952501c of 4 bytes by task 8323 on cpu 0:
 pcpu_update_empty_pages mm/percpu.c:558 [inline]
 pcpu_chunk_populated mm/percpu.c:1492 [inline]
 __pcpu_balance_workfn mm/percpu.c:2033 [inline]
 pcpu_balance_workfn+0xc6c/0xe30 mm/percpu.c:2069
 process_one_work+0x3e1/0x9a0 kernel/workqueue.c:2269
 worker_thread+0x665/0xbe0 kernel/workqueue.c:2415
 kthread+0x20d/0x230 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

read to 0xffffffff8952501c of 4 bytes by task 17958 on cpu 1:
 pcpu_alloc+0xb36/0x1370 mm/percpu.c:1835
 __alloc_percpu_gfp+0x22/0x30 mm/percpu.c:1894
 prealloc_init kernel/bpf/hashtab.c:269 [inline]
 htab_map_alloc+0x5ad/0xb70 kernel/bpf/hashtab.c:490
 find_and_alloc_map kernel/bpf/syscall.c:119 [inline]
 map_create kernel/bpf/syscall.c:820 [inline]
 __do_sys_bpf+0x4419/0x9a60 kernel/bpf/syscall.c:4160
 __se_sys_bpf kernel/bpf/syscall.c:4136 [inline]
 __x64_sys_bpf+0x3d/0x50 kernel/bpf/syscall.c:4136
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 17958 Comm: syz-executor.3 Not tainted 5.9.0-rc5-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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