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BUG: KCSAN: data-race in pcpu_alloc / pcpu_balance_workfn

write to 0xffffffff88f3cccc of 4 bytes by task 9691 on cpu 1:
 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 0xffffffff88f3cccc of 4 bytes by task 5950 on cpu 0:
 pcpu_alloc+0xaed/0x1090 mm/percpu.c:1835
 __alloc_percpu_gfp+0x22/0x30 mm/percpu.c:1894
 bpf_map_alloc_percpu+0xab/0x120 kernel/bpf/syscall.c:435
 prealloc_init kernel/bpf/hashtab.c:296 [inline]
 htab_map_alloc+0x814/0xc80 kernel/bpf/hashtab.c:508
 find_and_alloc_map kernel/bpf/syscall.c:122 [inline]
 map_create kernel/bpf/syscall.c:828 [inline]
 __do_sys_bpf+0x420c/0x99c0 kernel/bpf/syscall.c:4375
 __se_sys_bpf kernel/bpf/syscall.c:4351 [inline]
 __x64_sys_bpf+0x3d/0x50 kernel/bpf/syscall.c:4351
 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: 0 PID: 5950 Comm: syz-executor.0 Tainted: G        W         5.12.0-rc6-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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