======================================================
[ INFO: possible circular locking dependency detected ]
4.9.80-g20c8a00 #38 Not tainted
-------------------------------------------------------
syzkaller034981/3798 is trying to acquire lock:
 (&bdev->bd_mutex){+.+.+.}, at: [<ffffffff81d36a3e>] blkdev_reread_part+0x1e/0x40 block/ioctl.c:189
but task is already holding lock:
 (&lo->lo_ctl_mutex#2){+.+.+.}, at: [<ffffffff824b84a9>] lo_compat_ioctl+0x109/0x140 drivers/block/loop.c:1515
which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

       lock_acquire+0x12e/0x410 kernel/locking/lockdep.c:3756
       __mutex_lock_common kernel/locking/mutex.c:521 [inline]
       mutex_lock_nested+0xbb/0x870 kernel/locking/mutex.c:621
       __lo_release drivers/block/loop.c:1568 [inline]
       lo_release+0x85/0x160 drivers/block/loop.c:1591
       __blkdev_put+0x5f7/0x7e0 fs/block_dev.c:1598
       blkdev_put+0x85/0x550 fs/block_dev.c:1663
       blkdev_close+0x8b/0xb0 fs/block_dev.c:1670
       __fput+0x28c/0x6e0 fs/file_table.c:208
       ____fput+0x15/0x20 fs/file_table.c:244
       task_work_run+0x115/0x190 kernel/task_work.c:116
       tracehook_notify_resume include/linux/tracehook.h:191 [inline]
       exit_to_usermode_loop+0xfc/0x120 arch/x86/entry/common.c:160
       prepare_exit_to_usermode arch/x86/entry/common.c:190 [inline]
       syscall_return_slowpath+0x1a0/0x1e0 arch/x86/entry/common.c:259
       entry_SYSCALL_64_fastpath+0xe6/0xe8

       lock_acquire+0x12e/0x410 kernel/locking/lockdep.c:3756
       __mutex_lock_common kernel/locking/mutex.c:521 [inline]
       mutex_lock_nested+0xbb/0x870 kernel/locking/mutex.c:621
       lo_open+0x1b/0xa0 drivers/block/loop.c:1548
       __blkdev_get+0x261/0xd50 fs/block_dev.c:1282
       blkdev_get+0x33b/0x960 fs/block_dev.c:1416
       blkdev_open+0x1a5/0x250 fs/block_dev.c:1571
       do_dentry_open+0x607/0xc60 fs/open.c:766
       vfs_open+0x105/0x220 fs/open.c:879
       do_last fs/namei.c:3408 [inline]
       path_openat+0x5ac/0x2910 fs/namei.c:3531
       do_filp_open+0x197/0x290 fs/namei.c:3566
       do_sys_open+0x366/0x620 fs/open.c:1072
       SYSC_open fs/open.c:1090 [inline]
       SyS_open+0x2d/0x40 fs/open.c:1085
       entry_SYSCALL_64_fastpath+0x29/0xe8

       check_prev_add kernel/locking/lockdep.c:1828 [inline]
       check_prevs_add kernel/locking/lockdep.c:1938 [inline]
       validate_chain kernel/locking/lockdep.c:2265 [inline]
       __lock_acquire+0x2bf9/0x3640 kernel/locking/lockdep.c:3345
       lock_acquire+0x12e/0x410 kernel/locking/lockdep.c:3756
       __mutex_lock_common kernel/locking/mutex.c:521 [inline]
       mutex_lock_nested+0xbb/0x870 kernel/locking/mutex.c:621
       blkdev_reread_part+0x1e/0x40 block/ioctl.c:189
       loop_reread_partitions+0x78/0xe0 drivers/block/loop.c:634
       loop_set_status+0x995/0xfc0 drivers/block/loop.c:1164
       loop_set_status_compat+0x9a/0x100 drivers/block/loop.c:1488
       lo_compat_ioctl+0x114/0x140 drivers/block/loop.c:1516
       compat_blkdev_ioctl+0x3e3/0x3bc0 block/compat_ioctl.c:751
       C_SYSC_ioctl fs/compat_ioctl.c:1602 [inline]
       compat_SyS_ioctl+0x15f/0x2050 fs/compat_ioctl.c:1549
       do_syscall_32_irqs_on arch/x86/entry/common.c:322 [inline]
       do_fast_syscall_32+0x2f7/0x890 arch/x86/entry/common.c:384
       entry_SYSENTER_compat+0x74/0x83 arch/x86/entry/entry_64_compat.S:127

other info that might help us debug this:

Chain exists of:
 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(&lo->lo_ctl_mutex#2);
                               lock(loop_index_mutex);
                               lock(&lo->lo_ctl_mutex#2);
  lock(&bdev->bd_mutex);

 *** DEADLOCK ***

1 lock held by syzkaller034981/3798:
 #0:  (&lo->lo_ctl_mutex#2){+.+.+.}, at: [<ffffffff824b84a9>] lo_compat_ioctl+0x109/0x140 drivers/block/loop.c:1515

stack backtrace:
CPU: 0 PID: 3798 Comm: syzkaller034981 Not tainted 4.9.80-g20c8a00 #38
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
 ffff8801d7477768 ffffffff81d94b69 ffffffff85386410 ffffffff85385840
 ffffffff853b32f0 ffff8801d95fe8d8 ffff8801d95fe000 ffff8801d74777b0
 ffffffff81238641 ffff8801d95fe8d8 00000000d95fe8b0 ffff8801d95fe8d8
Call Trace:
 [<ffffffff81d94b69>] __dump_stack lib/dump_stack.c:15 [inline]
 [<ffffffff81d94b69>] dump_stack+0xc1/0x128 lib/dump_stack.c:51
 [<ffffffff81238641>] print_circular_bug+0x271/0x310 kernel/locking/lockdep.c:1202
 [<ffffffff8123ea79>] check_prev_add kernel/locking/lockdep.c:1828 [inline]
 [<ffffffff8123ea79>] check_prevs_add kernel/locking/lockdep.c:1938 [inline]
 [<ffffffff8123ea79>] validate_chain kernel/locking/lockdep.c:2265 [inline]
 [<ffffffff8123ea79>] __lock_acquire+0x2bf9/0x3640 kernel/locking/lockdep.c:3345
 [<ffffffff8123fefe>] lock_acquire+0x12e/0x410 kernel/locking/lockdep.c:3756
 [<ffffffff838a971b>] __mutex_lock_common kernel/locking/mutex.c:521 [inline]
 [<ffffffff838a971b>] mutex_lock_nested+0xbb/0x870 kernel/locking/mutex.c:621
 [<ffffffff81d36a3e>] blkdev_reread_part+0x1e/0x40 block/ioctl.c:189
 [<ffffffff824b3038>] loop_reread_partitions+0x78/0xe0 drivers/block/loop.c:634
 [<ffffffff824b3a35>] loop_set_status+0x995/0xfc0 drivers/block/loop.c:1164
 [<ffffffff824b40fa>] loop_set_status_compat+0x9a/0x100 drivers/block/loop.c:1488
 [<ffffffff824b84b4>] lo_compat_ioctl+0x114/0x140 drivers/block/loop.c:1516
 [<ffffffff81d88933>] compat_blkdev_ioctl+0x3e3/0x3bc0 block/compat_ioctl.c:751
 [<ffffffff8167d0cf>] com