INFO: task hung in _rcu_barrier

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: 4520f06b Linux 4.14.175
git tree: linux-4.14.y
console output: https://syzkaller.appspot.com/x/log.txt?x=1388e277e00000
kernel config: https://syzkaller.appspot.com/x/.config?x=93cf891381c0c347
dashboard link: https://syzkaller.appspot.com/bug?extid=697f5ad0fe2d523af622
compiler: gcc (GCC) 9.0.0 20181231 (experimental)

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+697f5a...@syzkaller.appspotmail.com

NOHZ: local_softirq_pending 08
NOHZ: local_softirq_pending 08
NOHZ: local_softirq_pending 08
NOHZ: local_softirq_pending 08
NOHZ: local_softirq_pending 08
INFO: task kworker/u4:0:5 blocked for more than 140 seconds.
Not tainted 4.14.175-syzkaller #0
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
kworker/u4:0 D25632 5 2 0x80000000
Workqueue: netns cleanup_net
Call Trace:
schedule+0x8d/0x1b0 kernel/sched/core.c:3428
schedule_timeout+0x946/0xe40 kernel/time/timer.c:1723
do_wait_for_common kernel/sched/completion.c:91 [inline]
__wait_for_common kernel/sched/completion.c:112 [inline]
wait_for_common kernel/sched/completion.c:123 [inline]
wait_for_completion+0x241/0x390 kernel/sched/completion.c:144
_rcu_barrier+0x285/0x400 kernel/rcu/tree.c:3611
ops_exit_list.isra.0+0x9d/0x140 net/core/net_namespace.c:142
cleanup_net+0x3bb/0x820 net/core/net_namespace.c:484
process_one_work+0x813/0x1540 kernel/workqueue.c:2116
worker_thread+0x5d1/0x1070 kernel/workqueue.c:2250
kthread+0x30d/0x420 kernel/kthread.c:232
ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:404

Showing all locks held in the system:
4 locks held by kworker/u4:0/5:
#0: ("%s""netns"){+.+.}, at: [<ffffffff813b67f7>] work_static include/linux/workqueue.h:199 [inline]
#0: ("%s""netns"){+.+.}, at: [<ffffffff813b67f7>] set_work_data kernel/workqueue.c:619 [inline]
#0: ("%s""netns"){+.+.}, at: [<ffffffff813b67f7>] set_work_pool_and_clear_pending kernel/workqueue.c:646 [inline]
#0: ("%s""netns"){+.+.}, at: [<ffffffff813b67f7>] process_one_work+0x727/0x1540 kernel/workqueue.c:2087
#1: (net_cleanup_work){+.+.}, at: [<ffffffff813b6831>] process_one_work+0x761/0x1540 kernel/workqueue.c:2091
#2: (net_mutex){+.+.}, at: [<ffffffff84fc8233>] cleanup_net+0x123/0x820 net/core/net_namespace.c:450
#3: (rcu_preempt_state.barrier_mutex){+.+.}, at: [<ffffffff814bbcc6>] _rcu_barrier+0x56/0x400 kernel/rcu/tree.c:3546
1 lock held by khungtaskd/1057:
#0: (tasklist_lock){.+.+}, at: [<ffffffff81465bb3>] debug_show_all_locks+0x7c/0x21a kernel/locking/lockdep.c:4544
1 lock held by syz-executor.4/30517:
#0: (rcu_preempt_state.barrier_mutex){+.+.}, at: [<ffffffff814bbcc6>] _rcu_barrier+0x56/0x400 kernel/rcu/tree.c:3546
1 lock held by syz-executor.2/30512:
#0: (rcu_preempt_state.barrier_mutex){+.+.}, at: [<ffffffff814bbcc6>] _rcu_barrier+0x56/0x400 kernel/rcu/tree.c:3546

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

NMI backtrace for cpu 0
CPU: 0 PID: 1057 Comm: khungtaskd Not tainted 4.14.175-syzkaller #0
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+0x13e/0x194 lib/dump_stack.c:58
nmi_cpu_backtrace.cold+0x57/0x93 lib/nmi_backtrace.c:101
nmi_trigger_cpumask_backtrace+0x139/0x17e lib/nmi_backtrace.c:62
trigger_all_cpu_backtrace include/linux/nmi.h:140 [inline]
check_hung_uninterruptible_tasks kernel/hung_task.c:195 [inline]
watchdog+0x5e2/0xb80 kernel/hung_task.c:274
kthread+0x30d/0x420 kernel/kthread.c:232
ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:404
Sending NMI from CPU 0 to CPUs 1:
NMI backtrace for cpu 1
CPU: 1 PID: 30058 Comm: syz-executor.0 Not tainted 4.14.175-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
task: ffff8880472a63c0 task.stack: ffff8881eaf98000
RIP: 0010:trace_hardirqs_off_caller+0x72/0x2b0 kernel/locking/lockdep.c:2946
RSP: 0000:ffff8881eaf9fc38 EFLAGS: 00000807
RAX: dffffc0000000000 RBX: ffff8880472a63c0 RCX: 1ffffffff10279a4
RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff8880472a6c3c
RBP: ffffffff8183674a R08: 0000000000000001 R09: 0000000000000001
R10: ffff8880472a6c68 R11: ffff8880472a63c0 R12: ffff8880472a63c0
R13: ffffffff81393ec1 R14: dffffc0000000000 R15: ffff8880472a6ae8
FS: 00007fa5b5765700(0000) GS:ffff8880aeb00000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000000003e979918 CR3: 000000020eb46000 CR4: 00000000001406e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
kmem_cache_free+0x14a/0x2b0 mm/slab.c:3759
__sigqueue_free kernel/signal.c:419 [inline]
dequeue_synchronous_signal kernel/signal.c:727 [inline]
get_signal+0xba1/0x1ca0 kernel/signal.c:2313
do_signal+0x7c/0x1690 arch/x86/kernel/signal.c:814
exit_to_usermode_loop+0x159/0x220 arch/x86/entry/common.c:160
prepare_exit_to_usermode+0x1af/0x210 arch/x86/entry/common.c:199
retint_user+0x8/0x18
RIP: 0033:0x405b89
RSP: 002b:00007fa5b5764a70 EFLAGS: 00010202
RAX: 000000003e979908 RBX: 00007fa5b57656d4 RCX: 0000000000416687
RDX: e0dec0719e25721d RSI: 0000000000000000 RDI: 000000000000000a
RBP: 000000000076c180 R08: 00007fa5b5764a70 R09: 000000000076c180
R10: 0000000000000000 R11: 0000000000000000 R12: 00000000ffffffff
R13: 0000000000000beb R14: 00000000004ce128 R15: 000000000076c18c
Code: 84 5e 01 00 00 65 4c 8b 24 25 40 ee 01 00 49 8d bc 24 7c 08 00 00 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 0f b6 14 02 <48> 89 f8 83 e0 07 83 c0 03 38 d0 7c 08 84 d2 0f 85 b8 01 00 00


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[syzbot]

Auto-closing this bug as obsolete.
Crashes did not happen for a while, no reproducer and no activity.