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BUG: KCSAN: data-race in exit_signals / kthread_unpark

read-write to 0xffff8881352ff02c of 4 bytes by task 21206 on cpu 0:
 exit_signals+0x102/0x530 kernel/signal.c:2862
 do_exit+0x15d/0x1630 kernel/exit.c:779
 kthread+0x205/0x220 kernel/kthread.c:294
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:296

read to 0xffff8881352ff02c of 4 bytes by task 8475 on cpu 1:
 to_kthread kernel/kthread.c:83 [inline]
 kthread_unpark+0x1f/0x140 kernel/kthread.c:511
 kthread_stop+0xa6/0x2c0 kernel/kthread.c:594
 loop_unprepare_queue drivers/block/loop.c:926 [inline]
 __loop_clr_fd+0x42d/0x5a0 drivers/block/loop.c:1254
 lo_release+0xfb/0x120 drivers/block/loop.c:1915
 __blkdev_put+0x3f9/0x450 fs/block_dev.c:1570
 blkdev_put+0x212/0x290 fs/block_dev.c:1623
 kill_block_super+0x7b/0x90 fs/super.c:1396
 deactivate_locked_super+0x6a/0xb0 fs/super.c:335
 deactivate_super+0x85/0x90 fs/super.c:366
 cleanup_mnt+0x277/0x2e0 fs/namespace.c:1118
 __cleanup_mnt+0x15/0x20 fs/namespace.c:1125
 task_work_run+0x8e/0x110 kernel/task_work.c:140
 tracehook_notify_resume include/linux/tracehook.h:189 [inline]
 exit_to_user_mode_loop kernel/entry/common.c:174 [inline]
 exit_to_user_mode_prepare+0x16b/0x1a0 kernel/entry/common.c:201
 __syscall_exit_to_user_mode_work kernel/entry/common.c:291 [inline]
 syscall_exit_to_user_mode+0x20/0x40 kernel/entry/common.c:302
 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: 8475 Comm: syz-executor.5 Not tainted 5.10.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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