Message-ID: <CACT4Y+YLZeMSz1F_YEmbNVF=cxBJcOMOC6KNDXCddei49JMs4Q@mail.gmail.com>
Date:   Fri, 20 Sep 2019 09:54:52 +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 Fri, Sep 20, 2019 at 9:50 AM Dmitry Vyukov <dvyukov@...gle.com> wrote:
>
>  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"?

Another thing in this report that confuses me is this:
The machine has 2 CPUs. The report contains 2 NMI tracebacks, both for
CPU 0 and CPU 1. A CPU generally does not do NMI traceback for itself
(right? it could do just traceback). The question is: what CPU the
code that prints this runs on then?

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