Message-ID: <CACT4Y+YdodVKL2h-Z4Svjyd6kug2ORmfiP4jripSC9PpYw4RiA@mail.gmail.com>
Date:   Fri, 20 Sep 2019 09:50:23 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     paulmck@...nel.org
Cc:     syzbot <syzbot+18379f2a19bc62c12565@...kaller.appspotmail.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>, avagin@...il.com,
        Christian Brauner <christian@...uner.io>, dbueso@...e.de,
        LKML <linux-kernel@...r.kernel.org>,
        Oleg Nesterov <oleg@...hat.com>, prsood@...eaurora.org,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tejun Heo <tj@...nel.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Subject: Re: INFO: rcu detected stall in sys_exit_group

 On Thu, Sep 19, 2019 at 10:12 PM Paul E. McKenney <paulmck@...nel.org> wrote:
>
> On Thu, Sep 19, 2019 at 07:39:03PM +0200, Dmitry Vyukov wrote:
> > On Thu, Sep 19, 2019 at 7:07 PM Paul E. McKenney <paulmck@...nel.org> wrote:
> > >
> > > On Wed, Sep 18, 2019 at 05:05:26PM +0200, Dmitry Vyukov wrote:
> > > > On Wed, Sep 18, 2019 at 1:19 PM syzbot
> > > > <syzbot+18379f2a19bc62c12565@...kaller.appspotmail.com> wrote:
> > > > >
> > > > > Hello,
> > > > >
> > > > > syzbot found the following crash on:
> > > > >
> > > > > HEAD commit:    a7f89616 Merge branch 'for-5.3-fixes' of git://git.kernel...
> > > > > git tree:       upstream
> > > > > console output: https://syzkaller.appspot.com/x/log.txt?x=15c33079600000
> > > > > kernel config:  https://syzkaller.appspot.com/x/.config?x=861a6f31647968de
> > > > > dashboard link: https://syzkaller.appspot.com/bug?extid=18379f2a19bc62c12565
> > > > > compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
> > > > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1066bb85600000
> > > > > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=13e9f75e600000
> > > > >
> > > > > Bisection is inconclusive: the bug happens on the oldest tested release.
> > > > >
> > > > > bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=154d4969600000
> > > > > final crash:    https://syzkaller.appspot.com/x/report.txt?x=174d4969600000
> > > > > console output: https://syzkaller.appspot.com/x/log.txt?x=134d4969600000
> > > > >
> > > > > IMPORTANT: if you fix the bug, please add the following tag to the commit:
> > > > > Reported-by: syzbot+18379f2a19bc62c12565@...kaller.appspotmail.com
> > > > >
> > > > > rcu: INFO: rcu_preempt self-detected stall on CPU
> > > > > rcu:    1-...!: (10499 ticks this GP) idle=63a/1/0x4000000000000002
> > > > > softirq=10978/10978 fqs=0
> > > > >         (t=10501 jiffies g=10601 q=227)
> > > > > rcu: rcu_preempt kthread starved for 10502 jiffies! g10601 f0x0
> > >
> > > The key point is the above line: RCU's grace-period kthread has not
> > > had a chance to run for 10,502 jiffies.
> > >
> > > > > RCU_GP_WAIT_FQS(5) ->state=0x402 ->cpu=0
> > >
> > > And it is sleeping normally.  The "RCU_GP_WAIT_FQS(5)" says that it was
> > > doing a fixed-time wait, which is normally for three jiffies, but never
> > > 10,000 of them.  Note that this kthread last ran on CPU 0.
> > >
> > > > > rcu: RCU grace-period kthread stack dump:
> > > > > rcu_preempt     I29040    10      2 0x80004000
> > > > > Call Trace:
> > > > >   context_switch kernel/sched/core.c:3254 [inline]
> > > > >   __schedule+0x755/0x1580 kernel/sched/core.c:3880
> > > > >   schedule+0xd9/0x260 kernel/sched/core.c:3947
> > > > >   schedule_timeout+0x486/0xc50 kernel/time/timer.c:1807
> > > > >   rcu_gp_fqs_loop kernel/rcu/tree.c:1611 [inline]
> > > > >   rcu_gp_kthread+0x9b2/0x18c0 kernel/rcu/tree.c:1768
> > > > >   kthread+0x361/0x430 kernel/kthread.c:255
> > > > >   ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352
> > >
> > > This stack trace is expected:  The RCU grace-period kthread is doing
> > > a fixed-time wait.  Note that this is a stack trace of this kthread,
> > > not necessarily of the CPU it was last running on.
> > >
> > > > > Sending NMI from CPU 1 to CPUs 0:
> > > > > INFO: NMI handler (nmi_cpu_backtrace_handler) took too long to run: 1.403
> > > > > msecs
> > >
> > > This is surprising.  Is this a guest OS?  If so, is the vCPU for CPU 0
> > > stuck somehow?  Did it get a SIGSTOP or some such?
> > >
> > > Clearly, if CPU 0 isn't running, RCU's grace-period kthread, which was
> > > last seen on CPU 0, might not be doing so well.
> > >
> > > OK, but we eventually did get a stack trace:
> > >
> > > > > NMI backtrace for cpu 0
> > > > > CPU: 0 PID: 10344 Comm: syz-executor933 Not tainted 5.3.0-rc8+ #0
> > > > > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> > > > > Google 01/01/2011
> > > > > RIP: 0010:hhf_dequeue+0x552/0xa20 net/sched/sch_hhf.c:436
> > > > > Code: ff ff 45 31 ff e9 b0 02 00 00 e8 49 05 ac fb 48 8d 43 f0 41 be 01 00
> > > > > 00 00 49 8d 95 c0 02 00 00 48 39 c2 74 34 e8 2e 05 ac fb <49> 8d bd ac 03
> > > > > 00 00 48 89 f8 48 c1 e8 03 42 0f b6 14 20 48 89 f8
> > > > > RSP: 0018:ffff8880ae809038 EFLAGS: 00000206
> > > > > RAX: ffff8880a3970100 RBX: ffff8880a8b1d538 RCX: ffffffff85c66b39
> > > > > RDX: 0000000000000100 RSI: ffffffff85c66fd2 RDI: 0000000000000005
> > > > > RBP: ffff8880ae809088 R08: ffff8880a3970100 R09: 0000000000000000
> > > > > R10: fffffbfff134afaf R11: ffff8880a3970100 R12: dffffc0000000000
> > > > > R13: ffff8880a8b1d240 R14: 0000000000000001 R15: 0000000000000000
> > > > > FS:  0000000000000000(0000) GS:ffff8880ae800000(0000) knlGS:0000000000000000
> > > > > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > > > CR2: 00000000006dab10 CR3: 0000000008c6d000 CR4: 00000000001406f0
> > > > > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > > > > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > > > > Call Trace:
> > > > >   <IRQ>
> > > > >   dequeue_skb net/sched/sch_generic.c:258 [inline]
> > > > >   qdisc_restart net/sched/sch_generic.c:361 [inline]
> > > > >   __qdisc_run+0x1e7/0x19d0 net/sched/sch_generic.c:379
> > > > >   __dev_xmit_skb net/core/dev.c:3533 [inline]
> > > > >   __dev_queue_xmit+0x16f1/0x3650 net/core/dev.c:3838
> > > > >   dev_queue_xmit+0x18/0x20 net/core/dev.c:3902
> > > > >   br_dev_queue_push_xmit+0x3f3/0x5c0 net/bridge/br_forward.c:52
> > > > >   NF_HOOK include/linux/netfilter.h:305 [inline]
> > > > >   NF_HOOK include/linux/netfilter.h:299 [inline]
> > > > >   br_forward_finish+0xfa/0x400 net/bridge/br_forward.c:65
> > > > >   NF_HOOK include/linux/netfilter.h:305 [inline]
> > > > >   NF_HOOK include/linux/netfilter.h:299 [inline]
> > > > >   __br_forward+0x641/0xb00 net/bridge/br_forward.c:109
> > > > >   deliver_clone+0x61/0xc0 net/bridge/br_forward.c:125
> > > > >   maybe_deliver+0x2c7/0x390 net/bridge/br_forward.c:181
> > > > >   br_flood+0x13a/0x3d0 net/bridge/br_forward.c:223
> > > > >   br_dev_xmit+0x98c/0x15a0 net/bridge/br_device.c:100
> > > > >   __netdev_start_xmit include/linux/netdevice.h:4406 [inline]
> > > > >   netdev_start_xmit include/linux/netdevice.h:4420 [inline]
> > > > >   xmit_one net/core/dev.c:3280 [inline]
> > > > >   dev_hard_start_xmit+0x1a3/0x9c0 net/core/dev.c:3296
> > > > >   __dev_queue_xmit+0x2b15/0x3650 net/core/dev.c:3869
> > > > >   dev_queue_xmit+0x18/0x20 net/core/dev.c:3902
> > > > >   neigh_hh_output include/net/neighbour.h:500 [inline]
> > > > >   neigh_output include/net/neighbour.h:509 [inline]
> > > > >   ip_finish_output2+0x1726/0x2570 net/ipv4/ip_output.c:228
> > > > >   __ip_finish_output net/ipv4/ip_output.c:308 [inline]
> > > > >   __ip_finish_output+0x5fc/0xb90 net/ipv4/ip_output.c:290
> > > > >   ip_finish_output+0x38/0x1f0 net/ipv4/ip_output.c:318
> > > > >   NF_HOOK_COND include/linux/netfilter.h:294 [inline]
> > > > >   ip_output+0x21f/0x640 net/ipv4/ip_output.c:432
> > > > >   dst_output include/net/dst.h:436 [inline]
> > > > >   ip_local_out+0xbb/0x190 net/ipv4/ip_output.c:125
> > > > >   igmpv3_sendpack+0x1b5/0x2c0 net/ipv4/igmp.c:426
> > > > >   igmpv3_send_cr net/ipv4/igmp.c:721 [inline]
> > > > >   igmp_ifc_timer_expire+0x687/0xa00 net/ipv4/igmp.c:809
> > >
> > > So this stack trace leads me to ask if networking has been hogging
> > > the CPU for the past 10,000 jiffies.  Perhaps there is a corner case
> > > that is not being addressed by the code that is supposed to move
> > > long-term processing from softirq to ksoftirqd?  Or perhaps more
> > > likely, the networking code isn't exiting its softirq handler, thus
> > > preventing the timer softirq handler from running, thus preventing
> > > RCU's grace-period kthread's sleep from ever ending.
> > >
> > > Is this consistent with what you are seeing?
> > >
> > > > This should have been parsed as "INFO: rcu detected stall in
> > > > igmp_ifc_timer_expire" which was already reported:
> > > > https://syzkaller.appspot.com/bug?id=330ce4f7626354cc6444c457c9a5e82d8a8c5055
> > > > So let's do:
> > > > #syz fix: sch_hhf: ensure quantum and hhf_non_hh_weight are non-zero
> > > >
> > > > +Paul, Tetsuo
> > > >
> > > > However, I cannot make sense of this kernel output (nor syzbot).
> > > > Here is full console output:
> > > > https://syzkaller.appspot.com/x/log.txt?x=15c33079600000
> > >
> > > I will bite...  What are all the "executing program" outputs?
> > >
> > > > This is "self-detected stall" which was detected in rcu_gp_kthread (?
> > > > usually these are detected in interrupts, no?)
> > >
> > > They are detected by the scheduling-clock interrupt handler, but
> > > stalls can be generated both at process and at interrupt levels.
> > >
> > > > and then the kthread runs on CPU 1 on top of the igmp_ifc_timer_expire
> > > > handler running in an interrupt (how can a kthread run on the
> > > > interrupt stack?)
> > > > and then it does NMI traceback for CPU 0, but that runs on CPU 1
> > > > (shouldn't NMI traceback run on CPU 0 too?)
> > > >
> > > > Any ideas what exactly happened here and how one can make sense of
> > > > such output to attribute it to some kernel activity that caused the
> > > > stall?
> > >
> > > My best guess based on what I am seeing is that a softirq handler
> > > is running for about ten seconds, which is too long.
> > >
> > > Do you have means for tracking softirq-handler durations?
> >
> > The "executing program" are produced by userspace. Kernel and
> > userspace outputs are multiplexed later to restore order of events.
> > Kernel output is prefixed with "[  351.648071][    C1]".
> >
> > Yes, the networking is stuck dead in an infinite loop. That's a known
> > bug, already fixed by:
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=d4d6ec6dac07f263f06d847d6f732d6855522845
> >
> > But I am interested why this report looks not like all other rcu
> > stalls and who should be the parsing logic to conclude that the stall
> > happened in igmp_ifc_timer_expire?
>
> Because the infinite loop happened to prevent the RCU grace-period
> kthread from making progress.  The odds of that are a bit low, so
> most of the stall warnings would look different.

But isn't it happens in all cases of infinite loops? Yet most other
reports look different and don't mention kthread. What's different
here?
Or is it another case of flaky reporting? We had that other case where
self-stall or remote-stall had the same timeout, so were reported
non-deterministically, or worse intermixed at the same time.


> I do expect to be revisiting the RCU CPU stall warning logic at some
> point.  Or are you asking how you could modify a script that figures
> this out?

Both. Fixing parsing may be easier and will fix all old kernels too.
You know nobody uses kernel HEAD, people use down to 3.18 and maybe
older. You are not going to backport these changes all the way back,
right? :)

> > Why it's detected by the kthread?
>
> It is not detected by the kthread.  The kthread has been interrupted
> indefinitely by the softirq processing, so is not in a position to
> detect much of anything.  It is instead detected as usual by the
> scheduling-clock interrupt.

I don't see any rcu-liveness-checking interrupt handler in any
tracebacks. Where is it? Is it useful to traceback the ktread in such
case at all?

What we do is the following, we find first apic_timer_interrupt on the
current CPU (presumably that the rcu-health-checking interrupt) and
then take the next "anchor" frame after that. This does not work in
this case, because the rcu-health-checking interrupt in missing.


> What is different is that the scheduling-clock interrupt detected
> that the grace-period kthread had not been running for an extended
> period of time.  It can detect this because the grace period kthread
> stores timestamps before each activity it undertakes.
>
> >                                   How it runs on top of an interrupt?
>
> It is not running on top of an interrupt.  Its stack was dumped
> separately.

I see. Usually the first stack is the traceback of the current stack.
So I was confused.

> > And why one cpu tracebacks another one?
>
> The usual reason is because neither CPU's quiescent state was reported
> to the RCU core, so the stall-warning code dumped both stacks.

But should the other CPU traceback _itself_? Rather than being traced
back by another CPU?
E.g. see this report:
https://github.com/google/syzkaller/blob/master/pkg/report/testdata/linux/report/350#L61-L83
Here the overall problem was detected by C2, but then C1 traces back itself.

... however even in that case C0 and C3 are traced by C2:
https://github.com/google/syzkaller/blob/master/pkg/report/testdata/linux/report/350#L84-L149
I can't understand this...
This makes understanding what happened harder because it's not easy to
exclude things on other CPUs.



>
> My turn.  Why dothere appear to be multiple levels of interrupt, as
> in one interrupt interrupting another interrupt?

I don't understand the question.
I see 1 apic_timer_interrupt on CPU 0 and 1 apic_timer_interrupt on CPU 1.

> > As of now syzkaller parsed it as "in sys_exit_group", which lead to
> > the creation of a new bug and another email, which is suboptimal.
>
> I suggest having syzkaller look for something like "rcu: rcu_[a-z]*
> kthread starved for".

And then what do we need to do to detect this as "in igmp_ifc_timer_expire"?

>  I don't have advice for handling the apparent
> multiple levels of interrupt.
>
>                                                         Thanx, Paul
>
> > > > >   call_timer_fn+0x1ac/0x780 kernel/time/timer.c:1322
> > > > >   expire_timers kernel/time/timer.c:1366 [inline]
> > > > >   __run_timers kernel/time/timer.c:1685 [inline]
> > > > >   __run_timers kernel/time/timer.c:1653 [inline]
> > > > >   run_timer_softirq+0x697/0x17a0 kernel/time/timer.c:1698
> > > > >   __do_softirq+0x262/0x98c kernel/softirq.c:292
> > > > >   invoke_softirq kernel/softirq.c:373 [inline]
> > > > >   irq_exit+0x19b/0x1e0 kernel/softirq.c:413
> > > > >   exiting_irq arch/x86/include/asm/apic.h:537 [inline]
> > > > >   smp_apic_timer_interrupt+0x1a3/0x610 arch/x86/kernel/apic/apic.c:1137
> > > > >   apic_timer_interrupt+0xf/0x20 arch/x86/entry/entry_64.S:830
> > > > >   </IRQ>
> > > > > RIP: 0010:__raw_write_unlock_irq include/linux/rwlock_api_smp.h:268 [inline]
> > > > > RIP: 0010:_raw_write_unlock_irq+0x54/0x90 kernel/locking/spinlock.c:343
> > > > > Code: c0 60 f4 d2 88 48 ba 00 00 00 00 00 fc ff df 48 c1 e8 03 80 3c 10 00
> > > > > 75 33 48 83 3d 05 bf 94 01 00 74 20 fb 66 0f 1f 44 00 00 <bf> 01 00 00 00
> > > > > e8 62 8c 10 fa 65 8b 05 93 c8 c3 78 85 c0 74 06 41
> > > > > RSP: 0018:ffff8880a406fd70 EFLAGS: 00000282 ORIG_RAX: ffffffffffffff13
> > > > > RAX: 1ffffffff11a5e8c RBX: ffff88809fbda740 RCX: 1ffffffff134b5ee
> > > > > RDX: dffffc0000000000 RSI: ffffffff8177f15e RDI: ffffffff873e3538
> > > > > RBP: ffff8880a406fd78 R08: ffff8880a3970100 R09: fffffbfff134afb0
> > > > > R10: fffffbfff134afaf R11: ffffffff89a57d7f R12: ffffffff88c090c0
> > > > > R13: 0000000000000011 R14: ffff8880a3970100 R15: 0000000000000000
> > > > >   exit_notify kernel/exit.c:745 [inline]
> > > > >   do_exit+0x13ab/0x2e50 kernel/exit.c:900
> > > > >   do_group_exit+0x135/0x360 kernel/exit.c:983
> > > > >   __do_sys_exit_group kernel/exit.c:994 [inline]
> > > > >   __se_sys_exit_group kernel/exit.c:992 [inline]
> > > > >   __x64_sys_exit_group+0x44/0x50 kernel/exit.c:992
> > > > >   do_syscall_64+0xfd/0x6a0 arch/x86/entry/common.c:296
> > > > >   entry_SYSCALL_64_after_hwframe+0x49/0xbe
> > > > > RIP: 0033:0x440f88
> > > > > Code: Bad RIP value.
> > > > > RSP: 002b:00007ffe302d02b8 EFLAGS: 00000246 ORIG_RAX: 00000000000000e7
> > > > > RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 0000000000440f88
> > > > > RDX: 0000000000000000 RSI: 000000000000003c RDI: 0000000000000000
> > > > > RBP: 00000000004c6eb0 R08: 00000000000000e7 R09: ffffffffffffffd0
> > > > > R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000001
> > > > > R13: 00000000006d95e0 R14: 0000000000000000 R15: 0000000000000000
> > > > > NMI backtrace for cpu 1
> > > > > CPU: 1 PID: 10345 Comm: syz-executor933 Not tainted 5.3.0-rc8+ #0
> > > > > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> > > > > Google 01/01/2011
> > > > > Call Trace:
> > > > >   <IRQ>
> > > > >   __dump_stack lib/dump_stack.c:77 [inline]
> > > > >   dump_stack+0x172/0x1f0 lib/dump_stack.c:113
> > > > >   nmi_cpu_backtrace.cold+0x70/0xb2 lib/nmi_backtrace.c:101
> > > > >   nmi_trigger_cpumask_backtrace+0x23b/0x28b lib/nmi_backtrace.c:62
> > > > >   arch_trigger_cpumask_backtrace+0x14/0x20 arch/x86/kernel/apic/hw_nmi.c:38
> > > > >   trigger_single_cpu_backtrace include/linux/nmi.h:164 [inline]
> > > > >   rcu_dump_cpu_stacks+0x183/0x1cf kernel/rcu/tree_stall.h:254
> > > > >   print_cpu_stall kernel/rcu/tree_stall.h:455 [inline]
> > > > >   check_cpu_stall kernel/rcu/tree_stall.h:529 [inline]
> > > > >   rcu_pending kernel/rcu/tree.c:2736 [inline]
> > > > >   rcu_sched_clock_irq.cold+0x4dd/0xc13 kernel/rcu/tree.c:2183
> > > > >   update_process_times+0x32/0x80 kernel/time/timer.c:1639
> > > > >   tick_sched_handle+0xa2/0x190 kernel/time/tick-sched.c:167
> > > > >   tick_sched_timer+0x53/0x140 kernel/time/tick-sched.c:1296
> > > > >   __run_hrtimer kernel/time/hrtimer.c:1389 [inline]
> > > > >   __hrtimer_run_queues+0x364/0xe40 kernel/time/hrtimer.c:1451
> > > > >   hrtimer_interrupt+0x314/0x770 kernel/time/hrtimer.c:1509
> > > > >   local_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1110 [inline]
> > > > >   smp_apic_timer_interrupt+0x160/0x610 arch/x86/kernel/apic/apic.c:1135
> > > > >   apic_timer_interrupt+0xf/0x20 arch/x86/entry/entry_64.S:830
> > > > >   </IRQ>
> > > > > RIP: 0010:cpu_relax arch/x86/include/asm/processor.h:656 [inline]
> > > > > RIP: 0010:virt_spin_lock arch/x86/include/asm/qspinlock.h:84 [inline]
> > > > > RIP: 0010:native_queued_spin_lock_slowpath+0x132/0x9f0
> > > > > kernel/locking/qspinlock.c:325
> > > > > Code: 00 00 00 48 8b 45 d0 65 48 33 04 25 28 00 00 00 0f 85 37 07 00 00 48
> > > > > 81 c4 98 00 00 00 5b 41 5c 41 5d 41 5e 41 5f 5d c3 f3 90 <e9> 73 ff ff ff
> > > > > 8b 45 98 4c 8d 65 d8 3d 00 01 00 00 0f 84 e5 00 00
> > > > > RSP: 0018:ffff8880946ef2f8 EFLAGS: 00000202 ORIG_RAX: ffffffffffffff13
> > > > > RAX: 0000000000000000 RBX: ffff8880a8b1d328 RCX: ffffffff81595c17
> > > > > RDX: 0000000000000000 RSI: 0000000000000004 RDI: ffff8880a8b1d328
> > > > > RBP: ffff8880946ef3b8 R08: 1ffff11015163a65 R09: ffffed1015163a66
> > > > > R10: ffffed1015163a65 R11: ffff8880a8b1d32b R12: 0000000000000001
> > > > > R13: 0000000000000003 R14: ffffed1015163a65 R15: 0000000000000001
> > > > >   pv_queued_spin_lock_slowpath arch/x86/include/asm/paravirt.h:654 [inline]
> > > > >   queued_spin_lock_slowpath arch/x86/include/asm/qspinlock.h:50 [inline]
> > > > >   queued_spin_lock include/asm-generic/qspinlock.h:81 [inline]
> > > > >   do_raw_spin_lock+0x20e/0x2e0 kernel/locking/spinlock_debug.c:113
> > > > >   __raw_spin_lock_bh include/linux/spinlock_api_smp.h:136 [inline]
> > > > >   _raw_spin_lock_bh+0x3b/0x50 kernel/locking/spinlock.c:175
> > > > >   spin_lock_bh include/linux/spinlock.h:343 [inline]
> > > > >   sch_tree_lock include/net/sch_generic.h:570 [inline]
> > > > >   hhf_change+0x2e3/0xad0 net/sched/sch_hhf.c:537
> > > > >   qdisc_change net/sched/sch_api.c:1321 [inline]
> > > > >   tc_modify_qdisc+0xfcf/0x1c50 net/sched/sch_api.c:1623
> > > > >   rtnetlink_rcv_msg+0x463/0xb00 net/core/rtnetlink.c:5223
> > > > >   netlink_rcv_skb+0x177/0x450 net/netlink/af_netlink.c:2477
> > > > >   rtnetlink_rcv+0x1d/0x30 net/core/rtnetlink.c:5241
> > > > >   netlink_unicast_kernel net/netlink/af_netlink.c:1302 [inline]
> > > > >   netlink_unicast+0x531/0x710 net/netlink/af_netlink.c:1328
> > > > >   netlink_sendmsg+0x8a5/0xd60 net/netlink/af_netlink.c:1917
> > > > >   sock_sendmsg_nosec net/socket.c:637 [inline]
> > > > >   sock_sendmsg+0xd7/0x130 net/socket.c:657
> > > > >   ___sys_sendmsg+0x803/0x920 net/socket.c:2311
> > > > >   __sys_sendmsg+0x105/0x1d0 net/socket.c:2356
> > > > >   __do_sys_sendmsg net/socket.c:2365 [inline]
> > > > >   __se_sys_sendmsg net/socket.c:2363 [inline]
> > > > >   __x64_sys_sendmsg+0x78/0xb0 net/socket.c:2363
> > > > >   do_syscall_64+0xfd/0x6a0 arch/x86/entry/common.c:296
> > > > >   entry_SYSCALL_64_after_hwframe+0x49/0xbe
> > > > > RIP: 0033:0x442399
> > > > > Code: e8 9c 07 03 00 48 83 c4 18 c3 0f 1f 80 00 00 00 00 48 89 f8 48 89 f7
> > > > > 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff
> > > > > ff 0f 83 3b 0a fc ff c3 66 2e 0f 1f 84 00 00 00 00
> > > > > RSP: 002b:00007ffe302d02f8 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
> > > > > RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 0000000000442399
> > > > > RDX: 0000000000000000 RSI: 0000000020000240 RDI: 0000000000000003
> > > > > RBP: 000000000003c361 R08: 0000000000000000 R09: 0000000000000000
> > > > > R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
> > > > > R13: 00000000004032f0 R14: 0000000000000000 R15: 0000000000000000
> > > > >
> > > > >
> > > > > ---
> > > > > This bug is generated by a bot. It may contain errors.
> > > > > See https://goo.gl/tpsmEJ for more information about syzbot.
> > > > > syzbot engineers can be reached at syzkaller@...glegroups.com.
> > > > >
> > > > > syzbot will keep track of this bug report. See:
> > > > > https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
> > > > > For information about bisection process see: https://goo.gl/tpsmEJ#bisection
> > > > > syzbot can test patches for this bug, for details see:
> > > > > https://goo.gl/tpsmEJ#testing-patches
> > > > >
> > > > > --
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> > > > > To view this discussion on the web visit https://groups.google.com/d/msgid/syzkaller-bugs/000000000000674b3d0592d2015b%40google.com.
> > >
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>
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