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BUG: KCSAN: data-race in pcpu_balance_workfn / pcpu_free_area

read-write to 0xffffffff8830cd4c of 4 bytes by task 8812 on cpu 1:
 pcpu_update_empty_pages mm/percpu.c:558 [inline]
 pcpu_block_update_hint_free mm/percpu.c:993 [inline]
 pcpu_free_area+0x531/0x5d0 mm/percpu.c:1253
 free_percpu+0x1ca/0x710 mm/percpu.c:2101
 htab_free_elems kernel/bpf/hashtab.c:217 [inline]
 prealloc_destroy kernel/bpf/hashtab.c:310 [inline]
 htab_map_free+0x303/0x3f0 kernel/bpf/hashtab.c:1311
 bpf_map_free_deferred+0x6a/0xa0 kernel/bpf/syscall.c:468
 process_one_work+0x3e1/0x950 kernel/workqueue.c:2272
 worker_thread+0x635/0xb90 kernel/workqueue.c:2418
 kthread+0x1fa/0x220 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:296

read to 0xffffffff8830cd4c of 4 bytes by task 14374 on cpu 0:
 __pcpu_balance_workfn mm/percpu.c:2001 [inline]
 pcpu_balance_workfn+0x8f1/0xdd0 mm/percpu.c:2068
 process_one_work+0x3e1/0x950 kernel/workqueue.c:2272
 worker_thread+0x635/0xb90 kernel/workqueue.c:2418
 kthread+0x1fa/0x220 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:296

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 14374 Comm: kworker/0:10 Not tainted 5.10.0-rc2-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: events pcpu_balance_workfn
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