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BUG: KCSAN: data-race in exit_signals / kthread_unpark

write to 0xffff88808d60d024 of 4 bytes by task 16577 on cpu 0:
 exit_signals+0x118/0x550 kernel/signal.c:2845
 do_exit+0x19a/0x16c0 kernel/exit.c:762
 kthread+0x215/0x230 kernel/kthread.c:294
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

read to 0xffff88808d60d024 of 4 bytes by task 13070 on cpu 1:
 to_kthread kernel/kthread.c:83 [inline]
 kthread_unpark+0x1f/0x150 kernel/kthread.c:511
 kthread_stop+0xa3/0x310 kernel/kthread.c:594
 loop_unprepare_queue drivers/block/loop.c:931 [inline]
 __loop_clr_fd+0x42e/0x5b0 drivers/block/loop.c:1265
 lo_release+0x101/0x130 drivers/block/loop.c:1926
 __blkdev_put+0x36e/0x4a0 fs/block_dev.c:1784
 blkdev_put+0x1e6/0x210 fs/block_dev.c:1846
 blkdev_close+0x47/0x50 fs/block_dev.c:1853
 __fput+0x246/0x4d0 fs/file_table.c:281
 ____fput+0x11/0x20 fs/file_table.c:314
 task_work_run+0x8e/0x110 kernel/task_work.c:141
 tracehook_notify_resume include/linux/tracehook.h:188 [inline]
 exit_to_user_mode_loop kernel/entry/common.c:165 [inline]
 exit_to_user_mode_prepare+0x1b2/0x210 kernel/entry/common.c:192
 syscall_exit_to_user_mode+0x16/0x30 kernel/entry/common.c:267
 do_syscall_64+0x45/0x80 arch/x86/entry/common.c:56
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 13070 Comm: systemd-udevd Not tainted 5.9.0-rc8-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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