EXT4-fs error (device loop6): ext4_iget:4557: inode #2: comm syz-executor6: root inode unallocated
EXT4-fs (loop6): Remounting filesystem read-only
EXT4-fs (loop6): get root inode failed
EXT4-fs (loop6): mount failed
INFO: task syz-executor0:3864 blocked for more than 120 seconds.
      Not tainted 4.9.107-gdb2c520 #46
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
syz-executor0   D28024  3864      1 0x00000004
 ffff8801d74ab000 0000000000000000 ffff8801d56f4540 ffff8801c5508000
 ffff8801db221c18 ffff880190fbfb40 ffffffff839e8c1d 0000000000000002
 0000000041b58ab3 ffffffff843b9ea3 00ffffff81221500 ffff8801db2224e8
Call Trace:
 [<ffffffff839ea21f>] schedule+0x7f/0x1b0 kernel/sched/core.c:3557
 [<ffffffff81602a49>] wb_wait_for_completion+0x149/0x1b0 fs/fs-writeback.c:221
 [<ffffffff81602cb7>] __writeback_inodes_sb_nr+0x207/0x300 fs/fs-writeback.c:2310
 [<ffffffff81602e07>] writeback_inodes_sb_nr fs/fs-writeback.c:2327 [inline]
 [<ffffffff81602e07>] writeback_inodes_sb+0x27/0x30 fs/fs-writeback.c:2342
 [<ffffffff8161a9c3>] __sync_filesystem fs/sync.c:35 [inline]
 [<ffffffff8161a9c3>] sync_filesystem+0x93/0x240 fs/sync.c:63
 [<ffffffff81579455>] generic_shutdown_super+0x75/0x300 fs/super.c:423
 [<ffffffff81579d8f>] kill_block_super+0x9f/0x120 fs/super.c:1127
 [<ffffffff8157a105>] deactivate_locked_super+0x75/0xd0 fs/super.c:310
 [<ffffffff8157ab11>] deactivate_super+0x91/0xd0 fs/super.c:341
 [<ffffffff815dce52>] cleanup_mnt+0xb2/0x160 fs/namespace.c:1134
 [<ffffffff815dcf86>] __cleanup_mnt+0x16/0x20 fs/namespace.c:1141
 [<ffffffff8119832c>] task_work_run+0x10c/0x180 kernel/task_work.c:116
 [<ffffffff8100559c>] tracehook_notify_resume include/linux/tracehook.h:191 [inline]
 [<ffffffff8100559c>] exit_to_usermode_loop+0xfc/0x120 arch/x86/entry/common.c:161
 [<ffffffff810064d4>] prepare_exit_to_usermode arch/x86/entry/common.c:191 [inline]
 [<ffffffff810064d4>] syscall_return_slowpath arch/x86/entry/common.c:260 [inline]
 [<ffffffff810064d4>] do_syscall_64+0x364/0x490 arch/x86/entry/common.c:287
 [<ffffffff839f9b13>] entry_SYSCALL_64_after_swapgs+0x5d/0xdb

Showing all locks held in the system:
2 locks held by khungtaskd/519:
 #0:  (rcu_read_lock){......}, at: [<ffffffff8136662c>] check_hung_uninterruptible_tasks kernel/hung_task.c:168 [inline]
 #0:  (rcu_read_lock){......}, at: [<ffffffff8136662c>] watchdog+0x11c/0xa20 kernel/hung_task.c:239
 #1:  (tasklist_lock){.+.+..}, at: [<ffffffff81425cb7>] debug_show_all_locks+0x79/0x218 kernel/locking/lockdep.c:4336
2 locks held by getty/3743:
 #0:  (&tty->ldisc_sem){++++++}, at: [<ffffffff839f7cc2>] ldsem_down_read+0x32/0x40 drivers/tty/tty_ldsem.c:367
 #1:  (&ldata->atomic_read_lock){+.+...}, at: [<ffffffff8211fc62>] n_tty_read+0x202/0x16e0 drivers/tty/n_tty.c:2133
1 lock held by syz-executor0/3864:
 #0:  (&type->s_umount_key#36){++++..}, at: [<ffffffff8157ab09>] deactivate_super+0x89/0xd0 fs/super.c:340

=============================================

NMI backtrace for cpu 0
CPU: 0 PID: 519 Comm: khungtaskd Not tainted 4.9.107-gdb2c520 #46
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
 ffff8801d84bfd08 ffffffff81eb3da9 0000000000000000 0000000000000000
 0000000000000000 0000000000000001 ffffffff810b96a0 ffff8801d84bfd40
 ffffffff81ebf0a7 0000000000000000 0000000000000000 0000000000000003
Call Trace:
 [<ffffffff81eb3da9>] __dump_stack lib/dump_stack.c:15 [inline]
 [<ffffffff81eb3da9>] dump_stack+0xc1/0x128 lib/dump_stack.c:51
 [<ffffffff81ebf0a7>] nmi_cpu_backtrace.cold.2+0x48/0x87 lib/nmi_backtrace.c:99
 [<ffffffff81ebf03a>] nmi_trigger_cpumask_backtrace+0x12a/0x14f lib/nmi_backtrace.c:60
 [<ffffffff810b97a4>] arch_trigger_cpumask_backtrace+0x14/0x20 arch/x86/kernel/apic/hw_nmi.c:37
 [<ffffffff81366bc4>] trigger_all_cpu_backtrace include/linux/nmi.h:58 [inline]
 [<ffffffff81366bc4>] check_hung_task kernel/hung_task.c:125 [inline]
 [<ffffffff81366bc4>] check_hung_uninterruptible_tasks kernel/hung_task.c:182 [inline]
 [<ffffffff81366bc4>] watchdog+0x6b4/0xa20 kernel/hung_task.c:239
 [<ffffffff8119d04d>] kthread+0x26d/0x300 kernel/kthread.c:211
 [<ffffffff839f9cdc>] ret_from_fork+0x5c/0x70 arch/x86/entry/entry_64.S:373
Sending NMI from CPU 0 to CPUs 1:
NMI backtrace for cpu 1
CPU: 1 PID: 4807 Comm: kworker/u4:6 Not tainted 4.9.107-gdb2c520 #46
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: writeback wb_workfn
c (flush-7:0)
c
task: ffff8801c5508000 task.stack: ffff880184a10000
RIP: 0010:[<ffffffff8127be06>] 
c [<ffffffff8127be06>] debug_lockdep_rcu_enabled+0x26/0x90 kernel/rcu/update.c:264
RSP: 0018:ffff880184a17580  EFLAGS: 00000002
RAX: 0000000000000004 RBX: ffffffff846defa0 RCX: 1ffffffff0886481
RDX: 0000000000000000 RSI: 0000000000000001 RDI: ffffffff84c5e7c4
RBP: ffff880184a17580 R08: ffff8801c5508960 R09: 0000000000000001
R10: 0000000000000000 R11: 1ffff10038aa1127 R12: ffff8801d9c342e8
R13: ffff8801c5508000 R14: ffffea000699395c R15: ffffea0006993940
FS:  0000000000000000(0000) GS:ffff8801db300000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000000c42e7e9000 CR3: 000000000461e000 CR4: 00000000001606f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Stack:
 ffff880184a17638
c ffffffff8123b636
c ffff8801cbf61a90
c ffff8801c5508000
c
 ffffed0038aa1118
c ffff8801c55088c8
c ffffffff846defa0
c 0000000000000003
c
 ffff880184a17600
c ffff880184a17608
c ffffffff81f1b80b
c 0000000000000246
c
Call Trace:
 [<ffffffff8123b636>] trace_lock_release include/trace/events/lock.h:57 [inline]
 [<ffffffff8123b636>] lock_release+0x746/0xb80 kernel/locking/lockdep.c:3774
 [<ffffffff81438bf2>] rcu_lock_release include/linux/rcupdate.h:498 [inline]
 [<ffffffff81438bf2>] rcu_read_unlock include/linux/rcupdate.h:931 [inline]
 [<ffffffff81438bf2>] find_get_entry+0x142/0x540 mm/filemap.c:1132
 [<ffffffff81439f2b>] pagecache_get_page+0x4b/0x720 mm/filemap.c:1203
 [<ffffffff81625219>] find_or_create_page include/linux/pagemap.h:309 [inline]
 [<ffffffff81625219>] grow_dev_page fs/buffer.c:1005 [inline]
 [<ffffffff81625219>] grow_buffers fs/buffer.c:1078 [inline]
 [<ffffffff81625219>] __getblk_slow fs/buffer.c:1105 [inline]
 [<ffffffff81625219>] __getblk_gfp+0x249/0x710 fs/buffer.c:1386
 [<ffffffff81628afd>] __bread_gfp+0x2d/0x270 fs/buffer.c:1420
 [<ffffffff818d5d41>] sb_bread include/linux/buffer_head.h:300 [inline]
 [<ffffffff818d5d41>] fat_clusters_flush+0x111/0x380 fs/fat/misc.c:69
 [<ffffffff818cfcd2>] fat_write_inode+0xf2/0x170 fs/fat/inode.c:890
 [<ffffffff8160214d>] write_inode fs/fs-writeback.c:1180 [inline]
 [<ffffffff8160214d>] __writeback_single_inode+0x86d/0x1020 fs/fs-writeback.c:1379
 [<ffffffff8160344c>] writeback_sb_inodes+0x4ac/0xe70 fs/fs-writeback.c:1584
 [<ffffffff8160444f>] wb_writeback+0x2cf/0xbd0 fs/fs-writeback.c:1760
 [<ffffffff8160d97e>] wb_do_writeback fs/fs-writeback.c:1894 [inline]
 [<ffffffff8160d97e>] wb_workfn+0x20e/0xdb0 fs/fs-writeback.c:1930
 [<ffffffff8118d121>] process_one_work+0x7e1/0x1500 kernel/workqueue.c:2092
 [<ffffffff8118df16>] worker_thread+0xd6/0x10a0 kernel/workqueue.c:2226
 [<ffffffff8119d04d>] kthread+0x26d/0x300 kernel/kthread.c:211
 [<ffffffff839f9cdc>] ret_from_fork+0x5c/0x70 arch/x86/entry/entry_64.S:373
Code: 
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