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BUG: KCSAN: data-race in kthread_stop / set_pf_worker

write to 0xffff88809d5680e4 of 4 bytes by task 24145 on cpu 0:
 set_pf_worker+0x53/0xa0 kernel/workqueue.c:2334
 rescuer_thread+0x84/0x6a0 kernel/workqueue.c:2469
 kthread+0x1cb/0x1f0 kernel/kthread.c:255
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352

read to 0xffff88809d5680e4 of 4 bytes by task 24139 on cpu 1:
 to_kthread kernel/kthread.c:75 [inline]
 kthread_stop+0x97/0x370 kernel/kthread.c:555
 destroy_workqueue+0xd6/0x4c0 kernel/workqueue.c:4364
 ucma_close+0x184/0x1a0 drivers/infiniband/core/ucma.c:1771
 __fput+0x1c3/0x4f0 fs/file_table.c:280
 ____fput+0x1b/0x30 fs/file_table.c:313
 task_work_run+0xf5/0x130 kernel/task_work.c:113
 tracehook_notify_resume include/linux/tracehook.h:188 [inline]
 exit_to_usermode_loop+0x2ae/0x2c0 arch/x86/entry/common.c:164
 prepare_exit_to_usermode arch/x86/entry/common.c:195 [inline]
 syscall_return_slowpath arch/x86/entry/common.c:278 [inline]
 do_syscall_64+0x36e/0x390 arch/x86/entry/common.c:304
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 24139 Comm: syz-executor.1 Not tainted 5.6.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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