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BUG: KCSAN: data-race in exit_signals / kthread_unpark

write to 0xffff8881fb8fc064 of 4 bytes by task 28170 on cpu 0:
 exit_signals+0x124/0x580 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 0xffff8881fb8fc064 of 4 bytes by task 28168 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
 loop_clr_fd drivers/block/loop.c:1345 [inline]
 lo_ioctl+0x9d1/0x1190 drivers/block/loop.c:1703
 __blkdev_driver_ioctl block/ioctl.c:224 [inline]
 blkdev_ioctl+0x1cf/0x410 block/ioctl.c:620
 block_ioctl+0x6d/0x80 fs/block_dev.c:1871
 vfs_ioctl fs/ioctl.c:48 [inline]
 __do_sys_ioctl fs/ioctl.c:753 [inline]
 __se_sys_ioctl+0xcb/0x140 fs/ioctl.c:739
 __x64_sys_ioctl+0x3f/0x50 fs/ioctl.c:739
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 28168 Comm: syz-executor.3 Not tainted 5.9.0-rc4-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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