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BUG: KCSAN: data-race in exit_signals / kthread_unpark

write to 0xffff888122958064 of 4 bytes by task 31835 on cpu 1:
 exit_signals+0x135/0x4c0 kernel/signal.c:2827
 do_exit+0x17b/0x1150 kernel/exit.c:744
 kthread+0x1aa/0x230 kernel/kthread.c:270
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352

read to 0xffff888122958064 of 4 bytes by task 31830 on cpu 0:
 to_kthread kernel/kthread.c:75 [inline]
 kthread_unpark+0x1f/0xd0 kernel/kthread.c:487
 kthread_stop+0xd5/0x370 kernel/kthread.c:570
 loop_unprepare_queue drivers/block/loop.c:917 [inline]
 __loop_clr_fd+0x39f/0x710 drivers/block/loop.c:1179
 loop_clr_fd drivers/block/loop.c:1259 [inline]
 lo_ioctl+0x24a/0xc50 drivers/block/loop.c:1628
 __blkdev_driver_ioctl block/ioctl.c:322 [inline]
 blkdev_ioctl+0x1a1/0x420 block/ioctl.c:718
 block_ioctl+0x91/0xb0 fs/block_dev.c:1996
 vfs_ioctl fs/ioctl.c:47 [inline]
 ksys_ioctl+0x101/0x150 fs/ioctl.c:763
 __do_sys_ioctl fs/ioctl.c:772 [inline]
 __se_sys_ioctl fs/ioctl.c:770 [inline]
 __x64_sys_ioctl+0x47/0x60 fs/ioctl.c:770
 do_syscall_64+0xc7/0x3b0 arch/x86/entry/common.c:295
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 31830 Comm: syz-executor.0 Not tainted 5.7.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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