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BUG: KCSAN: data-race in exit_signals / kthread_unpark

write to 0xffff8880918de024 of 4 bytes by task 4973 on cpu 0:
 exit_signals+0x130/0x460 kernel/signal.c:2822
 do_exit+0x17b/0x1880 kernel/exit.c:750
 kthread+0x17b/0x1f0 kernel/kthread.c:257
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352

read to 0xffff8880918de024 of 4 bytes by task 5092 on cpu 1:
 to_kthread kernel/kthread.c:75 [inline]
 kthread_unpark+0x1f/0xd0 kernel/kthread.c:474
 kthread_stop+0xd5/0x370 kernel/kthread.c:557
 loop_unprepare_queue drivers/block/loop.c:895 [inline]
 __loop_clr_fd+0x39f/0x710 drivers/block/loop.c:1157
 lo_release+0x140/0x150 drivers/block/loop.c:1830
 __blkdev_put+0x29c/0x3e0 fs/block_dev.c:1896
 blkdev_put+0x84/0x2b0 fs/block_dev.c:1958
 blkdev_close+0x4e/0x60 fs/block_dev.c:1965
 __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: 5092 Comm: blkid 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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