INFO: task hung in netdev_run_todo

[syzbot]

Hello,

syzbot hit the following crash on upstream commit
581e400ff935d34d95811258586128bf11baef15 (Wed Feb 7 22:29:34 2018 +0000)
Merge tag 'modules-for-v4.16' of
git://git.kernel.org/pub/scm/linux/kernel/git/jeyu/linux

Unfortunately, I don't have any reproducer for this crash yet.
Raw console output is attached.
compiler: gcc (GCC) 7.1.1 20170620
.config is attached.
user-space arch: i386

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Reported-by: syzbot+8cd13a...@syzkaller.appspotmail.com
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details.
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INFO: task kworker/u4:1:21 blocked for more than 120 seconds.
Not tainted 4.15.0+ #213
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
kworker/u4:1 D12208 21 2 0x80000000
Workqueue: netns cleanup_net
Call Trace:
context_switch kernel/sched/core.c:2857 [inline]
__schedule+0x8e2/0x2040 kernel/sched/core.c:3435
schedule+0xf5/0x430 kernel/sched/core.c:3494
schedule_preempt_disabled+0x10/0x20 kernel/sched/core.c:3552
__mutex_lock_common kernel/locking/mutex.c:833 [inline]
__mutex_lock+0xaad/0x1a80 kernel/locking/mutex.c:893
mutex_lock_nested+0x16/0x20 kernel/locking/mutex.c:908
rtnl_lock+0x17/0x20 net/core/rtnetlink.c:74
netdev_wait_allrefs+0x1cb/0x410 net/core/dev.c:8035
netdev_run_todo+0x478/0xae0 net/core/dev.c:8129
rtnl_unlock+0xe/0x10 net/core/rtnetlink.c:108
default_device_exit_batch+0x4b4/0x5d0 net/core/dev.c:8930
ops_exit_list.isra.6+0x100/0x150 net/core/net_namespace.c:145
cleanup_net+0x6a3/0xcc0 net/core/net_namespace.c:517
process_one_work+0xbbf/0x1af0 kernel/workqueue.c:2113
worker_thread+0x223/0x1990 kernel/workqueue.c:2247
kthread+0x33c/0x400 kernel/kthread.c:238
ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:429

Showing all locks held in the system:
4 locks held by kworker/u4:1/21:
#0: ((wq_completion)"%s""netns"){+.+.}, at: [<000000008671663e>]
process_one_work+0xaaf/0x1af0 kernel/workqueue.c:2084
#1: (net_cleanup_work){+.+.}, at: [<0000000098e1951b>]
process_one_work+0xb01/0x1af0 kernel/workqueue.c:2088
#2: (net_mutex){+.+.}, at: [<000000009da3dcc8>] cleanup_net+0x242/0xcc0
net/core/net_namespace.c:484
#3: (rtnl_mutex){+.+.}, at: [<000000000b60ddd2>] rtnl_lock+0x17/0x20
net/core/rtnetlink.c:74
2 locks held by khungtaskd/758:
#0: (rcu_read_lock){....}, at: [<0000000032c7899a>]
check_hung_uninterruptible_tasks kernel/hung_task.c:175 [inline]
#0: (rcu_read_lock){....}, at: [<0000000032c7899a>] watchdog+0x1c5/0xd60
kernel/hung_task.c:249
#1: (tasklist_lock){.+.+}, at: [<000000008744ddaf>]
debug_show_all_locks+0xd3/0x3d0 kernel/locking/lockdep.c:4470
1 lock held by rsyslogd/3947:
#0: (&f->f_pos_lock){+.+.}, at: [<00000000f50ca942>]
__fdget_pos+0x12b/0x190 fs/file.c:765
2 locks held by getty/4070:
#0: (&tty->ldisc_sem){++++}, at: [<000000004fdd9acb>]
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: (&ldata->atomic_read_lock){+.+.}, at: [<0000000033740686>]
n_tty_read+0x2ef/0x1a00 drivers/tty/n_tty.c:2131
2 locks held by getty/4071:
#0: (&tty->ldisc_sem){++++}, at: [<000000004fdd9acb>]
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: (&ldata->atomic_read_lock){+.+.}, at: [<0000000033740686>]
n_tty_read+0x2ef/0x1a00 drivers/tty/n_tty.c:2131
2 locks held by getty/4072:
#0: (&tty->ldisc_sem){++++}, at: [<000000004fdd9acb>]
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: (&ldata->atomic_read_lock){+.+.}, at: [<0000000033740686>]
n_tty_read+0x2ef/0x1a00 drivers/tty/n_tty.c:2131
2 locks held by getty/4073:
#0: (&tty->ldisc_sem){++++}, at: [<000000004fdd9acb>]
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: (&ldata->atomic_read_lock){+.+.}, at: [<0000000033740686>]
n_tty_read+0x2ef/0x1a00 drivers/tty/n_tty.c:2131
2 locks held by getty/4074:
#0: (&tty->ldisc_sem){++++}, at: [<000000004fdd9acb>]
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: (&ldata->atomic_read_lock){+.+.}, at: [<0000000033740686>]
n_tty_read+0x2ef/0x1a00 drivers/tty/n_tty.c:2131
2 locks held by getty/4075:
#0: (&tty->ldisc_sem){++++}, at: [<000000004fdd9acb>]
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: (&ldata->atomic_read_lock){+.+.}, at: [<0000000033740686>]
n_tty_read+0x2ef/0x1a00 drivers/tty/n_tty.c:2131
2 locks held by getty/4076:
#0: (&tty->ldisc_sem){++++}, at: [<000000004fdd9acb>]
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: (&ldata->atomic_read_lock){+.+.}, at: [<0000000033740686>]
n_tty_read+0x2ef/0x1a00 drivers/tty/n_tty.c:2131
1 lock held by syz-executor2/4154:
#0: (rtnl_mutex){+.+.}, at: [<000000000b60ddd2>] rtnl_lock+0x17/0x20
net/core/rtnetlink.c:74
3 locks held by kworker/1:3/7524:
#0: ((wq_completion)"%s"("ipv6_addrconf")){+.+.}, at:
[<000000008671663e>] process_one_work+0xaaf/0x1af0 kernel/workqueue.c:2084
#1: ((addr_chk_work).work){+.+.}, at: [<0000000098e1951b>]
process_one_work+0xb01/0x1af0 kernel/workqueue.c:2088
#2: (rtnl_mutex){+.+.}, at: [<000000000b60ddd2>] rtnl_lock+0x17/0x20
net/core/rtnetlink.c:74
1 lock held by syz-executor0/10595:
#0: (rtnl_mutex){+.+.}, at: [<000000000b60ddd2>] rtnl_lock+0x17/0x20
net/core/rtnetlink.c:74
1 lock held by syz-executor3/13841:
#0: (sk_lock-AF_INET6){+.+.}, at: [<0000000063f9b07b>] lock_sock
include/net/sock.h:1463 [inline]
#0: (sk_lock-AF_INET6){+.+.}, at: [<0000000063f9b07b>]
compat_ipv6_getsockopt+0x1f1/0x390 net/ipv6/ipv6_sockglue.c:1412
1 lock held by syz-executor3/13845:
#0: (rtnl_mutex){+.+.}, at: [<000000000b60ddd2>] rtnl_lock+0x17/0x20
net/core/rtnetlink.c:74
1 lock held by syz-executor3/13851:
#0: (rtnl_mutex){+.+.}, at: [<000000000b60ddd2>] rtnl_lock+0x17/0x20
net/core/rtnetlink.c:74
1 lock held by syz-executor3/13862:
#0: (rtnl_mutex){+.+.}, at: [<000000000b60ddd2>] rtnl_lock+0x17/0x20
net/core/rtnetlink.c:74
1 lock held by syz-executor0/13866:
#0: (rtnl_mutex){+.+.}, at: [<000000000b60ddd2>] rtnl_lock+0x17/0x20
net/core/rtnetlink.c:74
1 lock held by syz-executor2/13868:
#0: (rtnl_mutex){+.+.}, at: [<000000000b60ddd2>] rtnl_lock+0x17/0x20
net/core/rtnetlink.c:74

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

NMI backtrace for cpu 1
CPU: 1 PID: 758 Comm: khungtaskd Not tainted 4.15.0+ #213
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+0x194/0x257 lib/dump_stack.c:53
nmi_cpu_backtrace+0x1d2/0x210 lib/nmi_backtrace.c:103
nmi_trigger_cpumask_backtrace+0x122/0x180 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+0x90c/0xd60 kernel/hung_task.c:249
kthread+0x33c/0x400 kernel/kthread.c:238
ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:429
Sending NMI from CPU 1 to CPUs 0:
NMI backtrace for cpu 0
CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.15.0+ #213
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
RIP: 0010:rcu_lockdep_current_cpu_online+0x52/0x190 kernel/rcu/tree.c:1123
RSP: 0018:ffff8801db407780 EFLAGS: 00000282
RAX: ffffed003b680ef0 RBX: 1ffff1003b680ef0 RCX: 1ffff1003b680ef6
RDX: dffffc0000000000 RSI: ffffffff86b290c0 RDI: ffff8801db423750
RBP: ffff8801db407808 R08: 1ffff1003b680e47 R09: 0000000000000002
R10: ffff8801db4076f8 R11: 0000000000000020 R12: 1ffff1003b680f20
R13: ffff8801d9e66a00 R14: ffff8801db4079c0 R15: 1ffff1003b680f14
FS: 0000000000000000(0000) GS:ffff8801db400000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000000000240f000 CR3: 0000000006a22003 CR4: 00000000001606f0
DR0: 0000000020000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000600
Call Trace:
<IRQ>
rcu_read_lock_held+0x94/0xc0 kernel/rcu/update.c:331
rebalance_domains+0x945/0xcc0 kernel/sched/fair.c:9147
run_rebalance_domains+0x378/0x770 kernel/sched/fair.c:9405
__do_softirq+0x2d7/0xb85 kernel/softirq.c:285
invoke_softirq kernel/softirq.c:365 [inline]
irq_exit+0x1cc/0x200 kernel/softirq.c:405
scheduler_ipi+0x318/0x820 kernel/sched/core.c:1804
smp_reschedule_interrupt+0xe6/0x650 arch/x86/kernel/smp.c:277
reschedule_interrupt+0xa9/0xb0 arch/x86/entry/entry_64.S:853
</IRQ>
RIP: 0010:native_safe_halt+0x6/0x10 arch/x86/include/asm/irqflags.h:54
RSP: 0018:ffffffff86a07c38 EFLAGS: 00000286 ORIG_RAX: ffffffffffffff02
RAX: dffffc0000000000 RBX: 1ffffffff0d40f8a RCX: 0000000000000000
RDX: 1ffffffff0d59278 RSI: 0000000000000001 RDI: ffffffff86ac93c0
RBP: ffffffff86a07c38 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
R13: ffffffff86a07cf0 R14: ffffffff87269560 R15: 0000000000000000
arch_safe_halt arch/x86/include/asm/paravirt.h:93 [inline]
default_idle+0xbf/0x430 arch/x86/kernel/process.c:354
arch_cpu_idle+0xa/0x10 arch/x86/kernel/process.c:345
default_idle_call+0x36/0x90 kernel/sched/idle.c:98
cpuidle_idle_call kernel/sched/idle.c:156 [inline]
do_idle+0x24a/0x3b0 kernel/sched/idle.c:246
cpu_startup_entry+0x104/0x120 kernel/sched/idle.c:351
rest_init+0xed/0xf0 init/main.c:436
start_kernel+0x7f1/0x819 init/main.c:716
x86_64_start_reservations+0x2a/0x2c arch/x86/kernel/head64.c:378
x86_64_start_kernel+0x77/0x7a arch/x86/kernel/head64.c:359
secondary_startup_64+0xa5/0xb0 arch/x86/kernel/head_64.S:237
Code: c7 85 78 ff ff ff b3 8a b5 41 48 8d 04 13 48 c7 45 80 b5 14 7d 86 48
c7 45 88 50 e2 5d 81 c7 00 f1 f1 f1 f1 c7 40 04 00 f2 f2 f2 <c7> 40 08 f3
f3 f3 f3 b8 01 00 00 00 65 8b 0d 9b 0a a4 7e 81 e1


---
This bug is generated by a dumb bot. It may contain errors.
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[Dmitry Vyukov]

Presumably this is just a flake due to too slow execution. A thread
can't actually take the mutex for 2 minutes, but nothing has hanged
entirely. See detailed discussion here:
https://groups.google.com/d/msg/syzkaller-bugs/JwoXRjOYNnY/pIZWcXAoBQAJ

[Dmitry Vyukov]

After https://github.com/google/syzkaller/commit/37dc03de04826cc0d5d1e3699832b0a3113d40af
this should be re-detected as "task hung in rtnl_lock"

#syz invalid