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BUG: KCSAN: data-race in pcpu_balance_workfn / pcpu_free_area

read-write to 0xffffffff88315a4c of 4 bytes by task 13666 on cpu 0:
 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:243 [inline]
 prealloc_init kernel/bpf/hashtab.c:331 [inline]
 htab_map_alloc+0xa56/0xc70 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+0x3fea/0x99f0 kernel/bpf/syscall.c:4364
 __se_sys_bpf kernel/bpf/syscall.c:4340 [inline]
 __x64_sys_bpf+0x3d/0x50 kernel/bpf/syscall.c:4340
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffffffff88315a4c of 4 bytes by task 11111 on cpu 1:
 __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:2275
 worker_thread+0x635/0xb90 kernel/workqueue.c:2421
 kthread+0x1fd/0x220 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:296

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 11111 Comm: kworker/1:5 Not tainted 5.11.0-rc4-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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