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BUG: KCSAN: data-race in exit_signals / kthread_unpark

write to 0xffff888010d4e02c of 4 bytes by task 14087 on cpu 0:
 exit_signals+0x10d/0x540 kernel/signal.c:2865
 do_exit+0x1b4/0x1690 kernel/exit.c:781
 kthread+0x213/0x230 kernel/kthread.c:294
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:296

read to 0xffff888010d4e02c of 4 bytes by task 8435 on cpu 1:
 to_kthread kernel/kthread.c:83 [inline]
 kthread_unpark+0x1f/0x150 kernel/kthread.c:536
 kthread_stop+0xa6/0x2c0 kernel/kthread.c:619
 loop_unprepare_queue drivers/block/loop.c:926 [inline]
 __loop_clr_fd+0x432/0x5b0 drivers/block/loop.c:1254
 lo_release+0xfb/0x120 drivers/block/loop.c:1915
 __blkdev_put+0x40c/0x460 fs/block_dev.c:1579
 blkdev_put+0x222/0x2a0 fs/block_dev.c:1632
 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+0x17c/0x1b0 kernel/entry/common.c:201
 __syscall_exit_to_user_mode_work kernel/entry/common.c:283 [inline]
 syscall_exit_to_user_mode+0x20/0x40 kernel/entry/common.c:294
 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: 8435 Comm: syz-executor.1 Not tainted 5.11.0-rc7-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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