======================================================
[ INFO: possible circular locking dependency detected ]
4.4.166+ #17 Not tainted
-------------------------------------------------------
syz-executor769/2073 is trying to acquire lock:
 (&bdev->bd_mutex){+.+.+.}, at: [<ffffffff81a721ee>] blkdev_reread_part+0x1e/0x40 block/ioctl.c:189

but task is already holding lock:
 (&lo->lo_ctl_mutex#2){+.+...}, at: [<ffffffff81d410f3>] lo_compat_ioctl+0x103/0x140 drivers/block/loop.c:1566

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

       [<ffffffff812037de>] lock_acquire+0x15e/0x450 kernel/locking/lockdep.c:3592
       [<ffffffff827095a2>] __mutex_lock_common kernel/locking/mutex.c:521 [inline]
       [<ffffffff827095a2>] mutex_lock_nested+0xc2/0xb60 kernel/locking/mutex.c:621
       [<ffffffff81d411b5>] __lo_release drivers/block/loop.c:1620 [inline]
       [<ffffffff81d411b5>] lo_release+0x85/0x1a0 drivers/block/loop.c:1643
       [<ffffffff81548806>] __blkdev_put+0x636/0x840 fs/block_dev.c:1535
       [<ffffffff81549885>] blkdev_put+0x85/0x560 fs/block_dev.c:1600
       [<ffffffff81549deb>] blkdev_close+0x8b/0xb0 fs/block_dev.c:1607
       [<ffffffff81497685>] __fput+0x235/0x6f0 fs/file_table.c:208
       [<ffffffff81497bc5>] ____fput+0x15/0x20 fs/file_table.c:244
       [<ffffffff8112f8bc>] task_work_run+0x21c/0x2d0 kernel/task_work.c:115
       [<ffffffff81003edd>] tracehook_notify_resume include/linux/tracehook.h:191 [inline]
       [<ffffffff81003edd>] exit_to_usermode_loop+0x13d/0x160 arch/x86/entry/common.c:188
       [<ffffffff81005784>] prepare_exit_to_usermode arch/x86/entry/common.c:221 [inline]
       [<ffffffff81005784>] syscall_return_slowpath+0x254/0x2d0 arch/x86/entry/common.c:286
       [<ffffffff82715ee1>] int_ret_from_sys_call+0x25/0xa3

       [<ffffffff812037de>] lock_acquire+0x15e/0x450 kernel/locking/lockdep.c:3592
       [<ffffffff827095a2>] __mutex_lock_common kernel/locking/mutex.c:521 [inline]
       [<ffffffff827095a2>] mutex_lock_nested+0xc2/0xb60 kernel/locking/mutex.c:621
       [<ffffffff81d36d3b>] lo_open+0x1b/0xa0 drivers/block/loop.c:1600
       [<ffffffff81548cbe>] __blkdev_get+0x2ae/0xdf0 fs/block_dev.c:1213
       [<ffffffff8154ba8a>] blkdev_get+0x2da/0x920 fs/block_dev.c:1353
       [<ffffffff8154c2e5>] blkdev_open+0x1a5/0x250 fs/block_dev.c:1508
       [<ffffffff8148c4cd>] do_dentry_open+0x38d/0xbd0 fs/open.c:749
       [<ffffffff8148fc3a>] vfs_open+0x12a/0x210 fs/open.c:862
       [<ffffffff814bff20>] do_last fs/namei.c:3269 [inline]
       [<ffffffff814bff20>] path_openat+0xc10/0x3f10 fs/namei.c:3406
       [<ffffffff814c6eb7>] do_filp_open+0x197/0x270 fs/namei.c:3440
       [<ffffffff8149055c>] do_sys_open+0x31c/0x610 fs/open.c:1038
       [<ffffffff8149087d>] SYSC_open fs/open.c:1056 [inline]
       [<ffffffff8149087d>] SyS_open+0x2d/0x40 fs/open.c:1051
       [<ffffffff82715da1>] entry_SYSCALL_64_fastpath+0x1e/0x9a

       [<ffffffff81200314>] check_prev_add kernel/locking/lockdep.c:1853 [inline]
       [<ffffffff81200314>] check_prevs_add kernel/locking/lockdep.c:1958 [inline]
       [<ffffffff81200314>] validate_chain kernel/locking/lockdep.c:2144 [inline]
       [<ffffffff81200314>] __lock_acquire+0x3cd4/0x5530 kernel/locking/lockdep.c:3213
       [<ffffffff812037de>] lock_acquire+0x15e/0x450 kernel/locking/lockdep.c:3592
       [<ffffffff827095a2>] __mutex_lock_common kernel/locking/mutex.c:521 [inline]
       [<ffffffff827095a2>] mutex_lock_nested+0xc2/0xb60 kernel/locking/mutex.c:621
       [<ffffffff81a721ee>] blkdev_reread_part+0x1e/0x40 block/ioctl.c:189
       [<ffffffff81d3836c>] loop_reread_partitions+0x7c/0x90 drivers/block/loop.c:648
       [<ffffffff81d38dfe>] loop_set_status+0xa7e/0xfe0 drivers/block/loop.c:1192
       [<ffffffff81d393fa>] loop_set_status_compat+0x9a/0xf0 drivers/block/loop.c:1539
       [<ffffffff81d410fe>] lo_compat_ioctl+0x10e/0x140 drivers/block/loop.c:1567
       [<ffffffff81a9fa34>] compat_blkdev_ioctl+0x3a4/0x3546 block/compat_ioctl.c:751
       [<ffffffff8159463f>] C_SYSC_ioctl fs/compat_ioctl.c:1592 [inline]
       [<ffffffff8159463f>] compat_SyS_ioctl+0x4af/0x2220 fs/compat_ioctl.c:1544
       [<ffffffff810060fe>] do_syscall_32_irqs_on arch/x86/entry/common.c:330 [inline]
       [<ffffffff810060fe>] do_fast_syscall_32+0x31e/0xa80 arch/x86/entry/common.c:397
       [<ffffffff82717550>] sysenter_flags_fixed+0xd/0x1a

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 syz-executor769/2073:
 #0:  (&lo->lo_ctl_mutex#2){+.+...}, at: [<ffffffff81d410f3>] lo_compat_ioctl+0x103/0x140 drivers/block/loop.c:1566

stack backtrace:
CPU: 0 PID: 2073 Comm: syz-executor769 Not tainted 4.4.166+ #17
 0000000000000000 d966f09b8dcbfade ffff8801d41ff5d0 ffffffff81aa62ad
 ffffffff83a9b890 ffffffff83a9acc0 ffff8800b7ad4740 ffffffff83ac0580
 ffff8800b7ad5028 ffff8801d41ff620 ffffffff813a9549 ffff8800b7ad4740
Call Trace:
 [<ffffffff81aa62ad>] __dump_stack lib/dump_stack.c:15 [inline]
 [<ffffffff81aa62ad>] dump_stack+0xc1/0x124 lib/dump_stack.c:51
 [<ffffffff813a9549>] print_circular_bug.cold.31+0x2f6/0x435 kernel/locking/lockdep.c:1226
 [<fffffff