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BUG: KCSAN: data-race in pcpu_balance_workfn / pcpu_free_area

write to 0xffffffff88f3dccc of 4 bytes by task 18529 on cpu 0:
 pcpu_update_empty_pages mm/percpu.c:559 [inline]
 pcpu_block_update_hint_free mm/percpu.c:994 [inline]
 pcpu_free_area+0x597/0x640 mm/percpu.c:1254
 free_percpu+0x1cb/0x740 mm/percpu.c:2102
 htab_free_elems kernel/bpf/hashtab.c:243 [inline]
 prealloc_destroy+0xee/0x1a0 kernel/bpf/hashtab.c:337
 htab_map_free+0x45/0x340 kernel/bpf/hashtab.c:1373
 map_create kernel/bpf/syscall.c:905 [inline]
 __do_sys_bpf+0x92cb/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

read to 0xffffffff88f3dccc of 4 bytes by task 9115 on cpu 1:
 __pcpu_balance_workfn mm/percpu.c:2002 [inline]
 pcpu_balance_workfn+0x9a2/0xf10 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

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 9115 Comm: kworker/1:4 Not tainted 5.12.0-rc7-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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