INFO: task hung in __sb_start_write

[syzbot]

Hello,

syzbot hit the following crash on upstream commit
06dd3dfeea60e2a6457a6aedf97afc8e6d2ba497 (Thu Apr 5 03:07:20 2018 +0000)
Merge tag 'char-misc-4.17-rc1' of
git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/char-misc
syzbot dashboard link:
https://syzkaller.appspot.com/bug?extid=3f5d21408ea1b850c50c

So far this crash happened 2 times on upstream.
Unfortunately, I don't have any reproducer for this crash yet.
Raw console output:
https://syzkaller.appspot.com/x/log.txt?id=5226049759608832
Kernel config: https://syzkaller.appspot.com/x/.config?id=216543573824217049
compiler: gcc (GCC) 8.0.1 20180301 (experimental)
CC: [linux-...@vger.kernel.org linux-...@vger.kernel.org
vi...@zeniv.linux.org.uk]

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+3f5d21...@syzkaller.appspotmail.com
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details.
If you forward the report, please keep this part and the footer.

kworker/dying (22) used greatest stack depth: 8440 bytes left
INFO: task syz-executor1:4457 blocked for more than 120 seconds.
Not tainted 4.16.0+ #1
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
syz-executor1 D21448 4457 4449 0x00000000
Call Trace:
context_switch kernel/sched/core.c:2848 [inline]
__schedule+0x807/0x1e40 kernel/sched/core.c:3490
schedule+0xef/0x430 kernel/sched/core.c:3549
__rwsem_down_read_failed_common kernel/locking/rwsem-xadd.c:269 [inline]
rwsem_down_read_failed+0x400/0x6e0 kernel/locking/rwsem-xadd.c:286
call_rwsem_down_read_failed+0x18/0x30 arch/x86/lib/rwsem.S:94
__down_read arch/x86/include/asm/rwsem.h:83 [inline]
__percpu_down_read+0x15d/0x200 kernel/locking/percpu-rwsem.c:85
percpu_down_read_preempt_disable include/linux/percpu-rwsem.h:49 [inline]
percpu_down_read include/linux/percpu-rwsem.h:59 [inline]
__sb_start_write+0x2d7/0x300 fs/super.c:1340
sb_start_write include/linux/fs.h:1548 [inline]
mnt_want_write+0x3f/0xc0 fs/namespace.c:386
do_unlinkat+0x2a3/0xa10 fs/namei.c:4027
SYSC_unlink fs/namei.c:4092 [inline]
SyS_unlink+0x26/0x30 fs/namei.c:4090
do_syscall_64+0x29e/0x9d0 arch/x86/entry/common.c:287
entry_SYSCALL_64_after_hwframe+0x42/0xb7
RIP: 0033:0x455027
RSP: 002b:00007ffe9645abc8 EFLAGS: 00000202 ORIG_RAX: 0000000000000057
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 0000000000455027
RDX: 0000000001272983 RSI: 00007ffe9645ac60 RDI: 00007ffe9645ac60
RBP: 00007ffe9645c970 R08: 0000000000000000 R09: 0000000000000010
R10: 000000000000000a R11: 0000000000000202 R12: 0000000001272940
R13: 0000000000000000 R14: 000000000000041b R15: 000000000003297e

Showing all locks held in the system:
2 locks held by khungtaskd/878:
#0: 00000000e417309b (rcu_read_lock){....}, at:
check_hung_uninterruptible_tasks kernel/hung_task.c:175 [inline]
#0: 00000000e417309b (rcu_read_lock){....}, at: watchdog+0x1ff/0xf60
kernel/hung_task.c:249
#1: 00000000a42a84b8 (tasklist_lock){.+.+}, at:
debug_show_all_locks+0xde/0x34a kernel/locking/lockdep.c:4470
2 locks held by rs:main Q:Reg/4288:
#0: 00000000658bb541 (&f->f_pos_lock){+.+.}, at: __fdget_pos+0x1a9/0x1e0
fs/file.c:766
#1: 0000000052aff78e (sb_writers#4){++++}, at: file_start_write
include/linux/fs.h:2709 [inline]
#1: 0000000052aff78e (sb_writers#4){++++}, at: vfs_write+0x452/0x560
fs/read_write.c:548
2 locks held by getty/4381:
#0: 00000000990269ba (&tty->ldisc_sem){++++}, at:
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: 000000009f1d2051 (&ldata->atomic_read_lock){+.+.}, at:
n_tty_read+0x321/0x1cc0 drivers/tty/n_tty.c:2131
2 locks held by getty/4382:
#0: 00000000ecd434e6 (&tty->ldisc_sem){++++}, at:
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: 0000000034d736c1 (&ldata->atomic_read_lock){+.+.}, at:
n_tty_read+0x321/0x1cc0 drivers/tty/n_tty.c:2131
2 locks held by getty/4383:
#0: 0000000013764982 (&tty->ldisc_sem){++++}, at:
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: 00000000202a6e1f (&ldata->atomic_read_lock){+.+.}, at:
n_tty_read+0x321/0x1cc0 drivers/tty/n_tty.c:2131
2 locks held by getty/4384:
#0: 00000000ba3d2e6d (&tty->ldisc_sem){++++}, at:
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: 00000000e3323037 (&ldata->atomic_read_lock){+.+.}, at:
n_tty_read+0x321/0x1cc0 drivers/tty/n_tty.c:2131
2 locks held by getty/4385:
#0: 00000000a3add0e7 (&tty->ldisc_sem){++++}, at:
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: 0000000081f7b524 (&ldata->atomic_read_lock){+.+.}, at:
n_tty_read+0x321/0x1cc0 drivers/tty/n_tty.c:2131
2 locks held by getty/4386:
#0: 00000000a73f06ea (&tty->ldisc_sem){++++}, at:
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: 000000002de50dc3 (&ldata->atomic_read_lock){+.+.}, at:
n_tty_read+0x321/0x1cc0 drivers/tty/n_tty.c:2131
2 locks held by getty/4387:
#0: 0000000087cccc64 (&tty->ldisc_sem){++++}, at:
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: 000000000a3822a6 (&ldata->atomic_read_lock){+.+.}, at:
n_tty_read+0x321/0x1cc0 drivers/tty/n_tty.c:2131
1 lock held by syz-executor1/4457:
#0: 0000000052aff78e (sb_writers#4){++++}, at: sb_start_write
include/linux/fs.h:1548 [inline]
#0: 0000000052aff78e (sb_writers#4){++++}, at: mnt_want_write+0x3f/0xc0
fs/namespace.c:386
1 lock held by syz-executor7/4459:
#0: 0000000052aff78e (sb_writers#4){++++}, at: sb_start_write
include/linux/fs.h:1548 [inline]
#0: 0000000052aff78e (sb_writers#4){++++}, at: mnt_want_write+0x3f/0xc0
fs/namespace.c:386
1 lock held by syz-executor5/4460:
#0: 0000000052aff78e (sb_writers#4){++++}, at: sb_start_write
include/linux/fs.h:1548 [inline]
#0: 0000000052aff78e (sb_writers#4){++++}, at: mnt_want_write+0x3f/0xc0
fs/namespace.c:386
1 lock held by syz-executor3/4462:
#0: 0000000052aff78e (sb_writers#4){++++}, at: sb_start_write
include/linux/fs.h:1548 [inline]
#0: 0000000052aff78e (sb_writers#4){++++}, at: mnt_want_write+0x3f/0xc0
fs/namespace.c:386
2 locks held by syz-executor6/14836:
#0: 00000000f86d1ee3 (&mm->mmap_sem){++++}, at:
__do_page_fault+0x381/0xe40 arch/x86/mm/fault.c:1328
#1: 0000000002ee4fbd (sb_pagefaults){++++}, at: sb_start_pagefault
include/linux/fs.h:1577 [inline]
#1: 0000000002ee4fbd (sb_pagefaults){++++}, at:
ext4_page_mkwrite+0x1c8/0x1420 fs/ext4/inode.c:6074
1 lock held by syz-executor0/14842:
#0: 0000000052aff78e (sb_writers#4){++++}, at: sb_start_write
include/linux/fs.h:1548 [inline]
#0: 0000000052aff78e (sb_writers#4){++++}, at: mnt_want_write+0x3f/0xc0
fs/namespace.c:386
1 lock held by syz-executor4/14843:
#0: 0000000052aff78e (sb_writers#4){++++}, at: sb_start_write
include/linux/fs.h:1548 [inline]
#0: 0000000052aff78e (sb_writers#4){++++}, at: mnt_want_write+0x3f/0xc0
fs/namespace.c:386
1 lock held by syz-executor2/14849:
#0: 0000000052aff78e (sb_writers#4){++++}, at: sb_start_write
include/linux/fs.h:1548 [inline]
#0: 0000000052aff78e (sb_writers#4){++++}, at: mnt_want_write+0x3f/0xc0
fs/namespace.c:386

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

NMI backtrace for cpu 1
CPU: 1 PID: 878 Comm: khungtaskd Not tainted 4.16.0+ #1
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
Call Trace:
__dump_stack lib/dump_stack.c:17 [inline]
dump_stack+0x1b9/0x29f lib/dump_stack.c:53
nmi_cpu_backtrace.cold.4+0x19/0xce lib/nmi_backtrace.c:103
nmi_trigger_cpumask_backtrace+0x151/0x192 lib/nmi_backtrace.c:62
arch_trigger_cpumask_backtrace+0x14/0x20 arch/x86/kernel/apic/hw_nmi.c:38
trigger_all_cpu_backtrace include/linux/nmi.h:138 [inline]
check_hung_task kernel/hung_task.c:132 [inline]
check_hung_uninterruptible_tasks kernel/hung_task.c:190 [inline]
watchdog+0xc10/0xf60 kernel/hung_task.c:249
kthread+0x345/0x410 kernel/kthread.c:238
ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:411
Sending NMI from CPU 1 to CPUs 0:
NMI backtrace for cpu 0 skipped: idling at native_safe_halt+0x6/0x10
arch/x86/include/asm/irqflags.h:54


---
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[Dmitry Vyukov]

#syz upstream

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