INFO: task syz-executor0:30773 blocked for more than 140 seconds.
      Not tainted 4.9.123-g8dd3fc2 #27
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
syz-executor0   D29360 30773   3879 0x00000004
 ffff88019d698000 0000000000000000 ffff8801b684a4c0 ffff88019cd18000
 ffff8801db221c18 ffff8801a16ffb58 ffffffff839f0dbd 0000000000000002
 0000000041b58ab3 ffffffff843bbc24 00ffffff81223870 ffff8801db2224e8
Call Trace:
 [<ffffffff839f23bf>] schedule+0x7f/0x1b0 kernel/sched/core.c:3553
 [<ffffffff81607479>] wb_wait_for_completion+0x149/0x1b0 fs/fs-writeback.c:221
 [<ffffffff8160cc09>] sync_inodes_sb+0x1a9/0x8d0 fs/fs-writeback.c:2412
 [<ffffffff8161ed33>] sync_inodes_one_sb+0x43/0x60 fs/sync.c:73
 [<ffffffff81581630>] iterate_supers+0x130/0x260 fs/super.c:593
 [<ffffffff8161f642>] sys_sync+0xa2/0x170 fs/sync.c:112
 [<ffffffff81006316>] do_syscall_64+0x1a6/0x490 arch/x86/entry/common.c:282
 [<ffffffff83a01c93>] 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: [<ffffffff81368a9c>] check_hung_uninterruptible_tasks kernel/hung_task.c:168 [inline]
 #0:  (rcu_read_lock){......}, at: [<ffffffff81368a9c>] watchdog+0x11c/0xa20 kernel/hung_task.c:239
 #1:  (tasklist_lock){.+.?..}, at: [<ffffffff81428308>] debug_show_all_locks+0x79/0x218 kernel/locking/lockdep.c:4336
2 locks held by getty/3783:
 #0:  (&tty->ldisc_sem){++++++}, at: [<ffffffff839ffe62>] ldsem_down_read+0x32/0x40 drivers/tty/tty_ldsem.c:367
 #1:  (&ldata->atomic_read_lock){+.+.+.}, at: [<ffffffff821253c2>] n_tty_read+0x202/0x16e0 drivers/tty/n_tty.c:2142
1 lock held by syz-executor0/30773:
 #0:  (&type->s_umount_key#49){++++..}, at: [<ffffffff815815e1>] iterate_supers+0xe1/0x260 fs/super.c:591
1 lock held by syz-executor0/30831:
 #0:  (&type->s_umount_key#49){++++..}, at: [<ffffffff815815e1>] iterate_supers+0xe1/0x260 fs/super.c:591

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

NMI backtrace for cpu 0
CPU: 0 PID: 519 Comm: khungtaskd Not tainted 4.9.123-g8dd3fc2 #27
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
 ffff8801d842fd08 ffffffff81eb9689 0000000000000000 0000000000000000
 0000000000000000 0000000000000001 ffffffff810b9fa0 ffff8801d842fd40
 ffffffff81ec49c7 0000000000000000 0000000000000000 0000000000000003
Call Trace:
 [<ffffffff81eb9689>] __dump_stack lib/dump_stack.c:15 [inline]
 [<ffffffff81eb9689>] dump_stack+0xc1/0x128 lib/dump_stack.c:51
 [<ffffffff81ec49c7>] nmi_cpu_backtrace.cold.2+0x48/0x87 lib/nmi_backtrace.c:99
 [<ffffffff81ec495a>] nmi_trigger_cpumask_backtrace+0x12a/0x14f lib/nmi_backtrace.c:60
 [<ffffffff810ba0a4>] arch_trigger_cpumask_backtrace+0x14/0x20 arch/x86/kernel/apic/hw_nmi.c:37
 [<ffffffff81369034>] trigger_all_cpu_backtrace include/linux/nmi.h:58 [inline]
 [<ffffffff81369034>] check_hung_task kernel/hung_task.c:125 [inline]
 [<ffffffff81369034>] check_hung_uninterruptible_tasks kernel/hung_task.c:182 [inline]
 [<ffffffff81369034>] watchdog+0x6b4/0xa20 kernel/hung_task.c:239
 [<ffffffff8119f3cd>] kthread+0x26d/0x300 kernel/kthread.c:211
 [<ffffffff83a01e5c>] 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: 6 Comm: kworker/u4:0 Not tainted 4.9.123-g8dd3fc2 #27
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: writeback wb_workfn
c (flush-7:0)
c
task: ffff8801d9a38000 task.stack: ffff8801d9a40000
RIP: 0010:[<ffffffff81360dd0>] 
c [<ffffffff81360dd0>] check_kcov_mode kernel/kcov.c:66 [inline]
RIP: 0010:[<ffffffff81360dd0>] 
c [<ffffffff81360dd0>] __sanitizer_cov_trace_pc+0x20/0x50 kernel/kcov.c:100
RSP: 0018:ffff8801d9a47638  EFLAGS: 00000246
RAX: ffff8801d9a38000 RBX: ffffea000719f040 RCX: 0000000000000006
RDX: 0000000000000000 RSI: ffffffff8143c6b2 RDI: ffffea000719f050
RBP: ffff8801d9a47638 R08: ffff8801d9a38988 R09: 0000000000000001
R10: 0000000000000000 R11: 1ffff1003b34712c R12: 0000000000000007
R13: dffffc0000000000 R14: 0000000002420848 R15: ffff8801b60142c8
FS:  0000000000000000(0000) GS:ffff8801db300000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fee885b6000 CR3: 00000001cd5e7000 CR4: 00000000001606f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Stack:
 ffff8801d9a47690
c ffffffff8143c6b2
c ffffea000719f060
c ffffea000719f040
c
 fffffbfff098c570
c 0000000000000000
c 0000000000000003
c ffffea000719f05c
c
 ffff8801b60142c8
c 0000000000000200
c dffffc0000000000
c ffff8801d9a47730
c
Call Trace:
 [<ffffffff8143c6b2>] pagecache_get_page+0x122/0x720 mm/filemap.c:1226
 [<ffffffff81629c49>] find_or_create_page include/linux/pagemap.h:309 [inline]
 [<ffffffff81629c49>] grow_dev_page fs/buffer.c:1005 [inline]
 [<ffffffff81629c49>] grow_buffers fs/buffer.c:1078 [inline]
 [<ffffffff81629c49>] __getblk_slow fs/buffer.c:1105 [inline]
 [<ffffffff81629c49>] __getblk_gfp+0x249/0x710 fs/buffer.c:1386
 [<ffffffff8162d52d>] __bread_gfp+0x2d/0x270 fs/buffer.c:1420
 [<ffffffff818db811>] sb_bread include/linux/buffer_head.h:300 [inline]
 [<ffffffff818db811>] fat_clusters_flush+0x111/0x380 fs/fat/misc.c:69
 [<ffffffff818d5772>] fat_write_inode+0xf2/0x170 fs/fat/inode.c:898
 [<ffffffff81606b7d>] write_inode fs/fs-writeback.c:1180 [inline]
 [<ffffffff81606b7d>] __writeback_single_inode+0x86d/0x1020 fs/fs-writeback.c:1379
 [<ffffffff81607e7c>] writeback_sb_inodes+0x4ac/0xe70 fs/fs-writeback.c:1584
 [<ffffffff8160893b>] __writeback_inodes_wb+0xfb/0x1e0 fs/fs-writeback.c:1653
 [<ffffffff816090c2>] wb_writeback+0x512/0xbd0 fs/fs-writeback.c:1762
 [<ffffffff816129f0>] wb_check_old_data_flush fs/fs-writeback.c:1877 [inline]
 [<ffffffff816129f0>] wb_do_writeback fs/fs-writeback.c:1901 [inline]
 [<ffffffff816129f0>] wb_workfn+0x850/0xdb0 fs/fs-writeback.c:1930
 [<ffffffff8118f461>] process_one_work+0x7e1/0x1500 kernel/workqueue.c:2092
 [<ffffffff81190256>] worker_thread+0xd6/0x10a0 kernel/workqueue.c:2226
 [<ffffffff8119f3cd>] kthread+0x26d/0x300 kernel/kthread.c:211
 [<ffffffff83a01e5c>] ret_from_fork+0x5c/0x70 arch/x86/entry/entry_64.S:373
Code: 
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c