INFO: task hung in blkdev_issue_flush

[syzbot]

Hello,

syzbot hit the following crash on upstream commit
18b7fd1c93e5204355ddbf2608a097d64df81b88 (Sat Apr 14 15:50:50 2018 +0000)
Merge branch 'akpm' (patches from Andrew)
syzbot dashboard link:
https://syzkaller.appspot.com/bug?extid=e711500e2ec1ca3755a1

Unfortunately, I don't have any reproducer for this crash yet.
Raw console output:
https://syzkaller.appspot.com/x/log.txt?id=4523021393985536
Kernel config:
https://syzkaller.appspot.com/x/.config?id=-8852471259444315113
compiler: gcc (GCC) 8.0.1 20180413 (experimental)
CC: [ax...@kernel.dk linux...@vger.kernel.org
linux-...@vger.kernel.org]

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Reported-by: syzbot+e71150...@syzkaller.appspotmail.com
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R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000014
R13: 0000000000000280 R14: 00000000006f6ca0 R15: 0000000000000000
INFO: task loop0:7043 blocked for more than 120 seconds.
Not tainted 4.16.0+ #3
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
loop0 D25224 7043 2 0x80000000
Call Trace:
context_switch kernel/sched/core.c:2848 [inline]
__schedule+0x801/0x1e30 kernel/sched/core.c:3490
schedule+0xef/0x430 kernel/sched/core.c:3549
schedule_timeout+0x1b5/0x240 kernel/time/timer.c:1777
io_schedule_timeout+0x26/0x80 kernel/sched/core.c:5157
do_wait_for_common kernel/sched/completion.c:83 [inline]
__wait_for_common kernel/sched/completion.c:104 [inline]
wait_for_common_io kernel/sched/completion.c:121 [inline]
wait_for_completion_io+0x3e7/0x870 kernel/sched/completion.c:169
submit_bio_wait+0x13c/0x1e0 block/bio.c:947
blkdev_issue_flush+0x226/0x320 block/blk-flush.c:548
blkdev_fsync+0x93/0xd0 fs/block_dev.c:632
vfs_fsync_range+0x140/0x220 fs/sync.c:197
vfs_fsync+0x29/0x30 fs/sync.c:211
lo_req_flush drivers/block/loop.c:444 [inline]
do_req_filebacked drivers/block/loop.c:572 [inline]
loop_handle_cmd drivers/block/loop.c:1748 [inline]
loop_queue_work+0x14fe/0x3c6d drivers/block/loop.c:1762
kthread_worker_fn+0x357/0xa30 kernel/kthread.c:642
loop_kthread_worker_fn+0x51/0x60 drivers/block/loop.c:841
kthread+0x345/0x410 kernel/kthread.c:238
ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:411

Showing all locks held in the system:
2 locks held by khungtaskd/890:
#0: 000000002599374e (rcu_read_lock){....}, at:
check_hung_uninterruptible_tasks kernel/hung_task.c:175 [inline]
#0: 000000002599374e (rcu_read_lock){....}, at: watchdog+0x1ff/0xf60
kernel/hung_task.c:249
#1: 000000003fb6ce90 (tasklist_lock){.+.+}, at:
debug_show_all_locks+0xde/0x34a kernel/locking/lockdep.c:4470
1 lock held by rsyslogd/4349:
#0: 00000000f84c3033 (&f->f_pos_lock){+.+.}, at: __fdget_pos+0x1a9/0x1e0
fs/file.c:766
2 locks held by getty/4440:
#0: 000000008e360c27 (&tty->ldisc_sem){++++}, at:
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: 00000000c0193b78 (&ldata->atomic_read_lock){+.+.}, at:
n_tty_read+0x321/0x1cc0 drivers/tty/n_tty.c:2131
2 locks held by getty/4441:
#0: 00000000cec16b98 (&tty->ldisc_sem){++++}, at:
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: 0000000070384c74 (&ldata->atomic_read_lock){+.+.}, at:
n_tty_read+0x321/0x1cc0 drivers/tty/n_tty.c:2131
2 locks held by getty/4442:
#0: 00000000f71980c8 (&tty->ldisc_sem){++++}, at:
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: 000000000d1d8a50 (&ldata->atomic_read_lock){+.+.}, at:
n_tty_read+0x321/0x1cc0 drivers/tty/n_tty.c:2131
2 locks held by getty/4443:
#0: 000000008a8bfdfb (&tty->ldisc_sem){++++}, at:
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: 000000001d59f19e (&ldata->atomic_read_lock){+.+.}, at:
n_tty_read+0x321/0x1cc0 drivers/tty/n_tty.c:2131
2 locks held by getty/4444:
#0: 00000000b66f2749 (&tty->ldisc_sem){++++}, at:
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: 00000000ff483da9 (&ldata->atomic_read_lock){+.+.}, at:
n_tty_read+0x321/0x1cc0 drivers/tty/n_tty.c:2131
2 locks held by getty/4445:
#0: 000000003f352a77 (&tty->ldisc_sem){++++}, at:
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: 00000000b214f03f (&ldata->atomic_read_lock){+.+.}, at:
n_tty_read+0x321/0x1cc0 drivers/tty/n_tty.c:2131
2 locks held by getty/4446:
#0: 00000000e75a15cd (&tty->ldisc_sem){++++}, at:
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: 0000000084c9fd1f (&ldata->atomic_read_lock){+.+.}, at:
n_tty_read+0x321/0x1cc0 drivers/tty/n_tty.c:2131

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

NMI backtrace for cpu 1
CPU: 1 PID: 890 Comm: khungtaskd Not tainted 4.16.0+ #3
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
Call Trace:
__dump_stack lib/dump_stack.c:77 [inline]
dump_stack+0x1b9/0x294 lib/dump_stack.c:113
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]

This bug accumulates reports with different root causes, the older
ones seems to be due to the /dev/loop related bug. Let's untangle the
old reports from newer ones:

#syz dup: INFO: task hung in lo_ioctl

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