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BUG: KCSAN: data-race in pcpu_balance_workfn / pcpu_free_area

write to 0xffffffff88f37c4c of 4 bytes by task 24596 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+0x578/0x620 mm/percpu.c:1253
 free_percpu+0x1cb/0x720 mm/percpu.c:2101
 htab_free_elems kernel/bpf/hashtab.c:243 [inline]
 prealloc_init kernel/bpf/hashtab.c:331 [inline]
 htab_map_alloc+0xa55/0xc70 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+0x420c/0x9990 kernel/bpf/syscall.c:4370
 __se_sys_bpf kernel/bpf/syscall.c:4346 [inline]
 __x64_sys_bpf+0x3d/0x50 kernel/bpf/syscall.c:4346
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffffffff88f37c4c of 4 bytes by task 7 on cpu 0:
 __pcpu_balance_workfn mm/percpu.c:2001 [inline]
 pcpu_balance_workfn+0x911/0xe40 mm/percpu.c:2068
 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:296

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 7 Comm: kworker/0:1 Not tainted 5.11.0-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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