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BUG: KCSAN: data-race in kthread_stop / set_pf_worker

write to 0xffff8880ba9e40e4 of 4 bytes by task 16165 on cpu 0:
 set_pf_worker+0x58/0xb0 kernel/workqueue.c:2339
 rescuer_thread+0x7f/0x6a0 kernel/workqueue.c:2474
 kthread+0x1d4/0x200 drivers/block/aoe/aoecmd.c:1253
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352

read to 0xffff8880ba9e40e4 of 4 bytes by task 16142 on cpu 1:
 to_kthread kernel/kthread.c:75 [inline]
 kthread_stop+0x55/0x2d0 kernel/kthread.c:555
 destroy_workqueue+0x307/0x4a0 kernel/workqueue.c:4365
 ucma_close+0x192/0x1b0 drivers/infiniband/core/ucma.c:1771
 __fput+0x1e1/0x520 fs/file_table.c:280
 ____fput+0x1f/0x30 fs/file_table.c:313
 task_work_run+0xf6/0x130 kernel/task_work.c:113
 tracehook_notify_resume include/linux/tracehook.h:188 [inline]
 exit_to_usermode_loop+0x2b4/0x2c0 arch/x86/entry/common.c:163
 prepare_exit_to_usermode arch/x86/entry/common.c:194 [inline]
 syscall_return_slowpath arch/x86/entry/common.c:274 [inline]
 do_syscall_64+0x353/0x370 arch/x86/entry/common.c:300
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 16142 Comm: syz-executor.5 Not tainted 5.4.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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