syz-executor2 (8972) used greatest stack depth: 23624 bytes left
INFO: task syz-executor0:8355 blocked for more than 140 seconds.
      Not tainted 4.9.113-g47bbcd6 #10
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
syz-executor0   D27928  8355      1 0x00000004
 ffff8801d828e000 0000000000000000 ffff8801bf43d500 ffff8801d9aa3000
 ffff8801db321c18 ffff8801d472fb40 ffffffff839e909d 0000000000000002
 0000000041b58ab3 ffffffff843bbae3 00ffffff81221550 ffff8801db3224e8
Call Trace:
 [<ffffffff839ea69f>] schedule+0x7f/0x1b0 kernel/sched/core.c:3553
 [<ffffffff81602c89>] wb_wait_for_completion+0x149/0x1b0 fs/fs-writeback.c:221
 [<ffffffff81602ef7>] __writeback_inodes_sb_nr+0x207/0x300 fs/fs-writeback.c:2310
 [<ffffffff81603047>] writeback_inodes_sb_nr fs/fs-writeback.c:2327 [inline]
 [<ffffffff81603047>] writeback_inodes_sb+0x27/0x30 fs/fs-writeback.c:2342
 [<ffffffff8161ac03>] __sync_filesystem fs/sync.c:35 [inline]
 [<ffffffff8161ac03>] sync_filesystem+0x93/0x240 fs/sync.c:63
 [<ffffffff81579635>] generic_shutdown_super+0x75/0x300 fs/super.c:423
 [<ffffffff81579f6f>] kill_block_super+0x9f/0x120 fs/super.c:1127
 [<ffffffff8157a2e5>] deactivate_locked_super+0x75/0xd0 fs/super.c:310
 [<ffffffff8157acf1>] deactivate_super+0x91/0xd0 fs/super.c:341
 [<ffffffff815dd092>] cleanup_mnt+0xb2/0x160 fs/namespace.c:1134
 [<ffffffff815dd1c6>] __cleanup_mnt+0x16/0x20 fs/namespace.c:1141
 [<ffffffff8119838c>] 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
 [<ffffffff839f9f93>] 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: [<ffffffff8136675c>] check_hung_uninterruptible_tasks kernel/hung_task.c:168 [inline]
 #0:  (rcu_read_lock){......}, at: [<ffffffff8136675c>] watchdog+0x11c/0xa20 kernel/hung_task.c:239
 #1:  (tasklist_lock){.+.+..}, at: [<ffffffff81425de7>] debug_show_all_locks+0x79/0x218 kernel/locking/lockdep.c:4336
2 locks held by getty/3775:
 #0:  (&tty->ldisc_sem){++++++}, at: [<ffffffff839f8142>] ldsem_down_read+0x32/0x40 drivers/tty/tty_ldsem.c:367
 #1:  (&ldata->atomic_read_lock){+.+...}, at: [<ffffffff8211ee22>] n_tty_read+0x202/0x16e0 drivers/tty/n_tty.c:2142
1 lock held by syz-executor0/8355:
 #0:  (&type->s_umount_key#36){+.+...}, at: [<ffffffff8157ace9>] deactivate_super+0x89/0xd0 fs/super.c:340

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

NMI backtrace for cpu 1
CPU: 1 PID: 519 Comm: khungtaskd Not tainted 4.9.113-g47bbcd6 #10
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
 ffff8801d85ffd08 ffffffff81eb32a9 0000000000000000 0000000000000001
 0000000000000001 0000000000000001 ffffffff810b9bd0 ffff8801d85ffd40
 ffffffff81ebe5a7 0000000000000001 0000000000000000 0000000000000003
Call Trace:
 [<ffffffff81eb32a9>] __dump_stack lib/dump_stack.c:15 [inline]
 [<ffffffff81eb32a9>] dump_stack+0xc1/0x128 lib/dump_stack.c:51
 [<ffffffff81ebe5a7>] nmi_cpu_backtrace.cold.2+0x48/0x87 lib/nmi_backtrace.c:99
 [<ffffffff81ebe53a>] nmi_trigger_cpumask_backtrace+0x12a/0x14f lib/nmi_backtrace.c:60
 [<ffffffff810b9cd4>] arch_trigger_cpumask_backtrace+0x14/0x20 arch/x86/kernel/apic/hw_nmi.c:37
 [<ffffffff81366cf4>] trigger_all_cpu_backtrace include/linux/nmi.h:58 [inline]
 [<ffffffff81366cf4>] check_hung_task kernel/hung_task.c:125 [inline]
 [<ffffffff81366cf4>] check_hung_uninterruptible_tasks kernel/hung_task.c:182 [inline]
 [<ffffffff81366cf4>] watchdog+0x6b4/0xa20 kernel/hung_task.c:239
 [<ffffffff8119d0ad>] kthread+0x26d/0x300 kernel/kthread.c:211
 [<ffffffff839fa15c>] ret_from_fork+0x5c/0x70 arch/x86/entry/entry_64.S:373
Sending NMI from CPU 1 to CPUs 0:
NMI backtrace for cpu 0
CPU: 0 PID: 4131 Comm: kworker/u4:4 Not tainted 4.9.113-g47bbcd6 #10
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: writeback wb_workfn
c (flush-7:0)
c
task: ffff8801c2f8b000 task.stack: ffff8801d00a8000
RIP: 0010:[<ffffffff81236831>] 
c [<ffffffff81236831>] __lock_acquire+0x681/0x4070 kernel/locking/lockdep.c:3322
RSP: 0018:ffff8801d00af2a0  EFLAGS: 00000806
RAX: ffffffff85b2d6c0 RBX: ffff8801c2f8b918 RCX: 1ffff100385f1727
RDX: 0000000000000004 RSI: ffff8801c2f8b918 RDI: ffffffff8558c990
RBP: ffff8801d00af450 R08: ffff8801c2f8b938 R09: 0000000000000000
R10: ffff8801c2f8b000 R11: 1ffff100385f1722 R12: 0000000000000023
R13: 0000000000000002 R14: ffff8801c2f8b8b4 R15: 0000000000000000
FS:  0000000000000000(0000) GS:ffff8801db200000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fd6044c1140 CR3: 00000001ccb36000 CR4: 00000000001606f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Stack:
 ffff8801d00af2f0
c ffffffff81235647
c ffff8801ce27b270
c ffff8801c2f8b8c0
c
 0000000600000007
c ffff8801c2f8b000
c ffffffff839fa929
c ffff8801c2f8b918
c
 0000000000000003
c 0000000000000002
c ffff8801d00af310
c ffffffff81235a3b
c
Call Trace:
 [<ffffffff8123ac90>] lock_acquire+0x130/0x3e0 kernel/locking/lockdep.c:3756
 [<ffffffff81438c2a>] rcu_lock_acquire include/linux/rcupdate.h:493 [inline]
 [<ffffffff81438c2a>] rcu_read_lock include/linux/rcupdate.h:875 [inline]
 [<ffffffff81438c2a>] find_get_entry+0x4a/0x540 mm/filemap.c:1092
 [<ffffffff8143a05b>] pagecache_get_page+0x4b/0x720 mm/filemap.c:1203
 [<ffffffff81623fe2>] find_get_page_flags include/linux/pagemap.h:264 [inline]
 [<ffffffff81623fe2>] __find_get_block_slow+0xc2/0x330 fs/buffer.c:212
 [<ffffffff8162458e>] __find_get_block+0x23e/0x4c0 fs/buffer.c:1360
 [<ffffffff81625399>] __getblk_slow fs/buffer.c:1101 [inline]
 [<ffffffff81625399>] __getblk_gfp+0x189/0x710 fs/buffer.c:1386
 [<ffffffff81628d3d>] __bread_gfp+0x2d/0x270 fs/buffer.c:1420
 [<ffffffff818cc63b>] sb_bread include/linux/buffer_head.h:300 [inline]
 [<ffffffff818cc63b>] __fat_write_inode+0x2ab/0x980 fs/fat/inode.c:844
 [<ffffffff818d0849>] fat_write_inode+0x79/0x170 fs/fat/inode.c:893
 [<ffffffff8160238d>] write_inode fs/fs-writeback.c:1180 [inline]
 [<ffffffff8160238d>] __writeback_single_inode+0x86d/0x1020 fs/fs-writeback.c:1379
 [<ffffffff8160368c>] writeback_sb_inodes+0x4ac/0xe70 fs/fs-writeback.c:1584
 [<ffffffff8160468f>] wb_writeback+0x2cf/0xbd0 fs/fs-writeback.c:1760
 [<ffffffff8160dbbe>] wb_do_writeback fs/fs-writeback.c:1894 [inline]
 [<ffffffff8160dbbe>] wb_workfn+0x20e/0xdb0 fs/fs-writeback.c:1930
 [<ffffffff8118d181>] process_one_work+0x7e1/0x1500 kernel/workqueue.c:2092
 [<ffffffff8118df76>] worker_thread+0xd6/0x10a0 kernel/workqueue.c:2226
 [<ffffffff8119d0ad>] kthread+0x26d/0x300 kernel/kthread.c:211
 [<ffffffff839fa15c>] ret_from_fork+0x5c/0x70 arch/x86/entry/entry_64.S:373
Code: 
c54 
c24 
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c00 
c00 
c48 
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c00 
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c48 
cc1 
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c03 
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cb6 
c14 
c02 
c48 
cc7 
cc0 
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cd6 
cb2 
c85 
c<83> 
ce0 
c07 
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cc0 
c03 
c38 
cd0 
c7c 
c08 
c84 
cd2 
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c85 
c16 
c0f 
c00 
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c8b 
c0d 
c