Re: INFO: rcu detected stall in sys_exit_group

From: Paul E. McKenney
Date: Thu Sep 19 2019 - 16:12:07 EST

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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@xxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxxxxxx
> > > >
> > > > 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.

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?

> 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.

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.

> 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.

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

> 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". 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@xxxxxxxxxxxxxxxxx
> > > >
> > > > 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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