Message-ID: <CACT4Y+a-6ONFDNdwG4UPdyWdbnyAk=PmLnBHNR0rMUa1ksB--w@mail.gmail.com>
Date:   Thu, 12 Apr 2018 11:39:42 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Paul McKenney <paulmck@...ux.vnet.ibm.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        syzbot <syzbot+2dbc55da20fa246378fd@...kaller.appspotmail.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Peter Zijlstra <peterz@...radead.org>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: INFO: task hung in perf_trace_event_unreg

On Wed, Apr 11, 2018 at 9:36 PM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
>> >> >> >> <paulmck@...ux.vnet.ibm.com> wrote:
>> >> >> >> >> >> >> >>
>> >> >> >> >> >> >> >> > Hello,
>> >> >> >> >> >> >> >> >
>> >> >> >> >> >> >> >> > syzbot hit the following crash on upstream commit
>> >> >> >> >> >> >> >> > 0adb32858b0bddf4ada5f364a84ed60b196dbcda (Sun Apr 1 21:20:27 2018 +0000)
>> >> >> >> >> >> >> >> > Linux 4.16
>> >> >> >> >> >> >> >> > syzbot dashboard link:
>> >> >> >> >> >> >> >> > https://syzkaller.appspot.com/bug?extid=2dbc55da20fa246378fd
>> >> >> >> >> >> >> >> >
>> >> >> >> >> >> >> >> > Unfortunately, I don't have any reproducer for this crash yet.
>> >> >> >> >> >> >> >> > Raw console output:
>> >> >> >> >> >> >> >> > https://syzkaller.appspot.com/x/log.txt?id=5487937873510400
>> >> >> >> >> >> >> >> > Kernel config:
>> >> >> >> >> >> >> >> > https://syzkaller.appspot.com/x/.config?id=-2374466361298166459
>> >> >> >> >> >> >> >> > compiler: gcc (GCC) 7.1.1 20170620
>> >> >> >> >> >> >> >> >
>> >> >> >> >> >> >> >> > IMPORTANT: if you fix the bug, please add the following tag to the commit:
>> >> >> >> >> >> >> >> > Reported-by: syzbot+2dbc55da20fa246378fd@...kaller.appspotmail.com
>> >> >> >> >> >> >> >> > It will help syzbot understand when the bug is fixed. See footer for
>> >> >> >> >> >> >> >> > details.
>> >> >> >> >> >> >> >> > If you forward the report, please keep this part and the footer.
>> >> >> >> >> >> >> >> >
>> >> >> >> >> >> >> >> > REISERFS warning (device loop4): super-6502 reiserfs_getopt: unknown mount
>> >> >> >> >> >> >> >> > option "g �;e�K�׫>pquota"
>> >> >> >> >> >> >> >
>> >> >> >> >> >> >> > Might not hurt to look into the above, though perhaps this is just syzkaller
>> >> >> >> >> >> >> > playing around with mount options.
>> >> >> >> >> >> >> >
>> >> >> >> >> >> >> >> > INFO: task syz-executor3:10803 blocked for more than 120 seconds.
>> >> >> >> >> >> >> >> >        Not tainted 4.16.0+ #10
>> >> >> >> >> >> >> >> > "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>> >> >> >> >> >> >> >> > syz-executor3   D20944 10803   4492 0x80000002
>> >> >> >> >> >> >> >> > Call Trace:
>> >> >> >> >> >> >> >> >   context_switch kernel/sched/core.c:2862 [inline]
>> >> >> >> >> >> >> >> >   __schedule+0x8fb/0x1ec0 kernel/sched/core.c:3440
>> >> >> >> >> >> >> >> >   schedule+0xf5/0x430 kernel/sched/core.c:3499
>> >> >> >> >> >> >> >> >   schedule_timeout+0x1a3/0x230 kernel/time/timer.c:1777
>> >> >> >> >> >> >> >> >   do_wait_for_common kernel/sched/completion.c:86 [inline]
>> >> >> >> >> >> >> >> >   __wait_for_common kernel/sched/completion.c:107 [inline]
>> >> >> >> >> >> >> >> >   wait_for_common kernel/sched/completion.c:118 [inline]
>> >> >> >> >> >> >> >> >   wait_for_completion+0x415/0x770 kernel/sched/completion.c:139
>> >> >> >> >> >> >> >> >   __wait_rcu_gp+0x221/0x340 kernel/rcu/update.c:414
>> >> >> >> >> >> >> >> >   synchronize_sched.part.64+0xac/0x100 kernel/rcu/tree.c:3212
>> >> >> >> >> >> >> >> >   synchronize_sched+0x76/0xf0 kernel/rcu/tree.c:3213
>> >> >> >> >> >> >> >>
>> >> >> >> >> >> >> >> I don't think this is a perf issue. Looks like something is preventing
>> >> >> >> >> >> >> >> rcu_sched from completing. If there's a CPU that is running in kernel
>> >> >> >> >> >> >> >> space and never scheduling, that can cause this issue. Or if RCU
>> >> >> >> >> >> >> >> somehow missed a transition into idle or user space.
>> >> >> >> >> >> >> >
>> >> >> >> >> >> >> > The RCU CPU stall warning below strongly supports this position ...
>> >> >> >> >> >> >>
>> >> >> >> >> >> >> I think this is this guy then:
>> >> >> >> >> >> >>
>> >> >> >> >> >> >> https://syzkaller.appspot.com/bug?id=17f23b094cd80df750e5b0f8982c521ee6bcbf40
>> >> >> >> >> >> >>
>> >> >> >> >> >> >> #syz dup: INFO: rcu detected stall in __process_echoes
>> >> >> >> >> >> >
>> >> >> >> >> >> > Seems likely to me!
>> >> >> >> >> >> >
>> >> >> >> >> >> >> Looking retrospectively at the various hang/stall bugs that we have, I
>> >> >> >> >> >> >> think we need some kind of priority between them. I.e. we have rcu
>> >> >> >> >> >> >> stalls, spinlock stalls, workqueue hangs, task hangs, silent machine
>> >> >> >> >> >> >> hang and maybe something else. It would be useful if they fire
>> >> >> >> >> >> >> deterministically according to priorities. If there is an rcu stall,
>> >> >> >> >> >> >> that's always detected as CPU stall. Then if there is no RCU stall,
>> >> >> >> >> >> >> but a workqueue stall, then that's always detected as workqueue stall,
>> >> >> >> >> >> >> etc.
>> >> >> >> >> >> >> Currently if we have an RCU stall (effectively CPU stall), that can be
>> >> >> >> >> >> >> detected either RCU stall or a task hung, producing 2 different bug
>> >> >> >> >> >> >> reports (which is bad).
>> >> >> >> >> >> >> One can say that it's only a matter of tuning timeouts, but at least
>> >> >> >> >> >> >> task hung detector has a problem that if you set timeout to X, it can
>> >> >> >> >> >> >> detect hung anywhere between X and 2*X. And on one hand we need quite
>> >> >> >> >> >> >> large timeout (a minute may not be enough), and on the other hand we
>> >> >> >> >> >> >> can't wait for an hour just to make sure that the machine is indeed
>> >> >> >> >> >> >> dead (these things happen every few minutes).
>> >> >> >> >> >> >
>> >> >> >> >> >> > I suppose that we could have a global variable that was set to the
>> >> >> >> >> >> > priority of the complaint in question, which would suppress all
>> >> >> >> >> >> > lower-priority complaints.  Might need to be opt-in, though -- I would
>> >> >> >> >> >> > guess that not everyone is going to be happy with one complaint suppressing
>> >> >> >> >> >> > others, especially given the possibility that the two complaints might
>> >> >> >> >> >> > be about different things.
>> >> >> >> >> >> >
>> >> >> >> >> >> > Or did you have something more deft in mind?
>> >> >> >> >> >>
>> >> >> >> >> >>
>> >> >> >> >> >> syzkaller generally looks only at the first report. One does not know
>> >> >> >> >> >> if/when there will be a second one, or the second one can be induced
>> >> >> >> >> >> by the first one, and we generally want clean reports on a non-tainted
>> >> >> >> >> >> kernel. So we don't just need to suppress lower priority ones, we need
>> >> >> >> >> >> to produce the right report first.
>> >> >> >> >> >> I am thinking maybe setting:
>> >> >> >> >> >>  - rcu stalls at 1.5 minutes
>> >> >> >> >> >>  - workqueue stalls at 2 minutes
>> >> >> >> >> >>  - task hungs at 2.5 minutes
>> >> >> >> >> >>  - and no output whatsoever at 3 minutes
>> >> >> >> >> >> Do I miss anything? I think at least spinlocks. Should they go before
>> >> >> >> >> >> or after rcu?
>> >> >> >> >> >
>> >> >> >> >> > That is what I know of, but the Linux kernel being what it is, there is
>> >> >> >> >> > probably something more out there.  If not now, in a few months.  The
>> >> >> >> >> > RCU CPU stall timeout can be set on the kernel-boot command line, but
>> >> >> >> >> > you probably already knew that.
>> >> >> >> >>
>> >> >> >> >> Well, it's all based solely on a large number of patches and stopgaps.
>> >> >> >> >> If we fix main problems for today, it's already good.
>> >> >> >> >
>> >> >> >> > Fair enough!
>> >> >> >> >
>> >> >> >> >> > Just for comparison, back in DYNIX/ptx days the RCU CPU stall timeout
>> >> >> >> >> > was 1.5 -seconds-.  ;-)
>> >> >> >> >>
>> >> >> >> >> Have you tried to instrument every basic block with a function call to
>> >> >> >> >> collect coverage, check every damn memory access for validity, enable
>> >> >> >> >> all thinkable and unthinkable debug configs and put the insanest load
>> >> >> >> >> one can imagine from a swarm of parallel threads? It makes things a
>> >> >> >> >> bit slower ;)
>> >> >> >> >
>> >> >> >> > Given that we wouldn't have had enough CPU or memory to accommodate
>> >> >> >> > all of that back in DYNIX/ptx days, I am forced to answer "no".  ;-)
>> >> >> >> >
>> >> >> >> >> >> This will require fixing task hung. Have not yet looked at workqueue detector.
>> >> >> >> >> >> Does at least RCU respect the given timeout more or less precisely?
>> >> >> >> >> >
>> >> >> >> >> > Assuming that there is at least one CPU capable of taking scheduling-clock
>> >> >> >> >> > interrupts, it should respect the timeout to within a few jiffies.
>> >> >> >>
>> >> >> >>
>> >> >> >> Hi Paul,
>> >> >> >>
>> >> >> >> Speaking of stalls and rcu, we are seeing lots of crashes that go like this:
>> >> >> >>
>> >> >> >> INFO: rcu_sched self-detected stall on CPU[  404.992530] INFO:
>> >> >> >> rcu_sched detected stalls on CPUs/tasks:
>> >> >> >> INFO: rcu_sched self-detected stall on CPU[  454.347448] INFO:
>> >> >> >> rcu_sched detected stalls on CPUs/tasks:
>> >> >> >> INFO: rcu_sched self-detected stall on CPU[  396.073634] INFO:
>> >> >> >> rcu_sched detected stalls on CPUs/tasks:
>> >> >> >>
>> >> >> >> or like this:
>> >> >> >>
>> >> >> >> INFO: rcu_sched self-detected stall on CPU
>> >> >> >> INFO: rcu_sched detected stalls on CPUs/tasks:
>> >> >> >> 0-....: (125000 ticks this GP) idle=0ba/1/4611686018427387906
>> >> >> >> softirq=57641/57641 fqs=31151
>> >> >> >> 0-....: (125000 ticks this GP) idle=0ba/1/4611686018427387906
>> >> >> >> softirq=57641/57641 fqs=31151
>> >> >> >>  (t=125002 jiffies g=31656 c=31655 q=910)
>> >> >> >>
>> >> >> >>  INFO: rcu_sched self-detected stall on CPU
>> >> >> >> INFO: rcu_sched detected stalls on CPUs/tasks:
>> >> >> >> 0-....: (125000 ticks this GP) idle=49a/1/4611686018427387906
>> >> >> >> softirq=65194/65194 fqs=31231
>> >> >> >> 0-....: (125000 ticks this GP) idle=49a/1/4611686018427387906
>> >> >> >> softirq=65194/65194 fqs=31231
>> >> >> >>  (t=125002 jiffies g=34421 c=34420 q=1119)
>> >> >> >> (detected by 1, t=125002 jiffies, g=34421, c=34420, q=1119)
>> >> >> >>
>> >> >> >>
>> >> >> >> and then there is an unintelligible mess of 2 reports. Such crashes go
>> >> >> >> to trash bin, because we can't even say which function hanged. It
>> >> >> >> seems that in all cases 2 different rcu stall detection facilities
>> >> >> >> race with each other. Is it possible to make them not race?
>> >> >> >
>> >> >> > How about the following (untested, not for mainline) patch?  It suppresses
>> >> >> > all but the "main" RCU flavor, which is rcu_sched for !PREEMPT builds and
>> >> >> > rcu_preempt otherwise.  Either way, this is the RCU flavor corresponding
>> >> >> > to synchronize_rcu().  This works well in the common case where there
>> >> >> > is almost always an RCU grace period in flight.
>> >> >> >
>> >> >> > One reason that this patch is not for mainline is that I am working on
>> >> >> > merging the RCU-bh, RCU-preempt, and RCU-sched flavors into one thing,
>> >> >> > at which point there won't be any races.  But that might be a couple
>> >> >> > merge windows away from now.
>> >> >> >
>> >> >> >                                                         Thanx, Paul
>> >> >> >
>> >> >> > ------------------------------------------------------------------------
>> >> >> >
>> >> >> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
>> >> >> > index 381b47a68ac6..31f7818f2d63 100644
>> >> >> > --- a/kernel/rcu/tree.c
>> >> >> > +++ b/kernel/rcu/tree.c
>> >> >> > @@ -1552,7 +1552,7 @@ static void check_cpu_stall(struct rcu_state *rsp, struct rcu_data *rdp)
>> >> >> >         struct rcu_node *rnp;
>> >> >> >
>> >> >> >         if ((rcu_cpu_stall_suppress && !rcu_kick_kthreads) ||
>> >> >> > -           !rcu_gp_in_progress(rsp))
>> >> >> > +           !rcu_gp_in_progress(rsp) || rsp != rcu_state_p)
>> >> >> >                 return;
>> >> >> >         rcu_stall_kick_kthreads(rsp);
>> >> >> >         j = jiffies;
>> >> >>
>> >> >> But doesn't they both relate to the same rcu flavor? They both say
>> >> >> rcu_sched. I assumed that the difference is "self-detected" vs "on
>> >> >> CPUs/tasks", i.e. on the current CPU vs on other CPUs.
>> >> >
>> >> > Right you are!
>> >> >
>> >> > One approach would be to increase the value of RCU_STALL_RAT_DELAY,
>> >> > which is currently two jiffies to (say) 20 jiffies.  This is in
>> >> > kernel/rcu/tree.h.  But this would fail on a sufficiently overloaded
>> >> > system -- and the failure of the two-jiffy delay is a bit of a surprise,
>> >> > given interrupts disabled and all that.  Are you by any chance loaded
>> >> > heavily enough to see vCPU preemption?
>> >> >
>> >> > I could avoid at least some of these timing issues instead using cmpxchg()
>> >> > on ->jiffies_stall to allow only one CPU in, but leave the non-atomic
>> >> > update to discourage overly long stall prints from running into the
>> >> > next one.  This is not perfect, either, and is roughly equivalent to
>> >> > setting RCU_STALL_RAT_DELAY to many second's worth of jiffies, but
>> >> > avoiding that minute's delay.  But it should get rid of the duplication
>> >> > in almost all cases, though it could allow a stall warning to overlap
>> >> > with a later stall warning for that same grace period.  Which can
>> >> > already happen anyway.  Also, a tens-of-seconds vCPU preemption can
>> >> > still cause concurrent stall warnings, but if that is happening to you,
>> >> > the concurrent stall warnings are probably the least of your problems.
>> >> > Besides, we do need at least one CPU to actually report the stall, which
>> >> > won't happen if that CPU's vCPU is indefinitely preempted.  So there is
>> >> > only so much I can do about that particular corner case.
>> >> >
>> >> > So how does the following (untested) patch work for you?
>> >>
>> >> Looks good to me.
>> >>
>> >> We run on VMs, so we can well have vCPU preemption.
>> >
>> > Very good!  Please do get me a Tested-by when you get to that point.
>>
>> Unfortunately I don't have a good way to test it until it's submitted
>> upstream. While we are seeing thousands of such instances, they happen
>> episodically on a farm of test machines. But they are still harmful,
>> especially when the system tries to reproduce a bug, because it's
>> mid-way through and thinks it got a hook, but then suddenly boom! it
>> gets some mess that it can't parse and now it does not know if it's
>> still the same bug, or maybe a different bug triggered by the same
>> program, so it does not know how to properly attribute the reproducer.
>> You can see these cases as they happen here (under report/log links in
>> the table):
>> https://syzkaller.appspot.com/bug?id=d5bc3e0c66d200d72216ab343a67c4327e4a3452
>> When the patch is submitted, the rate should go down.
>
> OK, I will bite...  How do you test fixes to problems that syzkaller finds?

I don't. I can't. No one can test that many fixes.

Normally syzbot provides reproducers for bugs. Then you have 2
choices: (1) test it yourself (if you debugged it, you probably
already have everything setup for this), or (2) ask syzbot to test the
patch on this particular reproducer.
Some bugs don't have reproducers. Then you either localize the bug and
write a test, or go with the old good "it must be correct, right?".
Even for the second case, syzbot will notify if the bug happens again
after the fix is landed, or it's silent, then presumably the fix
indeed fixed the bug.

Now, this is not a syzbot bug (syzbot reports bugs itself from own
email address). This is more like you looked at somebody else dmsg and
like "oh, this looks bad, let me copy-paste and report it".
So can also go with the old good "it must be correct, right?" and
assess how well it goes after few weeks when it reaches syzbot, or
someone needs to write a test for rcu.

This could have been handled with some kind of "cluster-wide" test,
but I don't see how it is feasible. See this for details:
https://groups.google.com/d/msg/syzkaller-bugs/7ucgCkAJKSk/skZjgavRAQAJ
Especially the part that someone will need to go through and triage
hundreds of crashes and assess that they are not related to the new
patch, and do something with then afterwards.




>> >> > ------------------------------------------------------------------------
>> >> >
>> >> > commit 6a5ab1e68f8636d8823bb5a9aee35fc44c2be866
>> >> > Author: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
>> >> > Date:   Mon Apr 9 11:04:46 2018 -0700
>> >> >
>> >> >     rcu: Exclude near-simultaneous RCU CPU stall warnings
>> >> >
>> >> >     There is a two-jiffy delay between the time that a CPU will self-report
>> >> >     an RCU CPU stall warning and the time that some other CPU will report a
>> >> >     warning on behalf of the first CPU.  This has worked well in the past,
>> >> >     but on busy systems, it is possible for the two warnings to overlap,
>> >> >     which makes interpreting them extremely difficult.
>> >> >
>> >> >     This commit therefore uses a cmpxchg-based timing decision that
>> >> >     allows only one report in a given one-minute period (assuming default
>> >> >     stall-warning Kconfig parameters).  This approach will of course fail
>> >> >     if you are seeing minute-long vCPU preemption, but in that case the
>> >> >     overlapping RCU CPU stall warnings are the least of your worries.
>> >> >
>> >> >     Reported-by: Dmitry Vyukov <dvyukov@...gle.com>
>> >> >     Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
>> >> >
>> >> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
>> >> > index 381b47a68ac6..b7246bcbf633 100644
>> >> > --- a/kernel/rcu/tree.c
>> >> > +++ b/kernel/rcu/tree.c
>> >> > @@ -1429,8 +1429,6 @@ static void print_other_cpu_stall(struct rcu_state *rsp, unsigned long gpnum)
>> >> >                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
>> >> >                 return;
>> >> >         }
>> >> > -       WRITE_ONCE(rsp->jiffies_stall,
>> >> > -                  jiffies + 3 * rcu_jiffies_till_stall_check() + 3);
>> >> >         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
>> >> >
>> >> >         /*
>> >> > @@ -1481,6 +1479,10 @@ static void print_other_cpu_stall(struct rcu_state *rsp, unsigned long gpnum)
>> >> >                         sched_show_task(current);
>> >> >                 }
>> >> >         }
>> >> > +       /* Rewrite if needed in case of slow consoles. */
>> >> > +       if (ULONG_CMP_GE(jiffies, READ_ONCE(rsp->jiffies_stall)))
>> >> > +               WRITE_ONCE(rsp->jiffies_stall,
>> >> > +                          jiffies + 3 * rcu_jiffies_till_stall_check() + 3);
>> >> >
>> >> >         rcu_check_gp_kthread_starvation(rsp);
>> >> >
>> >> > @@ -1525,6 +1527,7 @@ static void print_cpu_stall(struct rcu_state *rsp)
>> >> >         rcu_dump_cpu_stacks(rsp);
>> >> >
>> >> >         raw_spin_lock_irqsave_rcu_node(rnp, flags);
>> >> > +       /* Rewrite if needed in case of slow consoles. */
>> >> >         if (ULONG_CMP_GE(jiffies, READ_ONCE(rsp->jiffies_stall)))
>> >> >                 WRITE_ONCE(rsp->jiffies_stall,
>> >> >                            jiffies + 3 * rcu_jiffies_till_stall_check() + 3);
>> >> > @@ -1548,6 +1551,7 @@ static void check_cpu_stall(struct rcu_state *rsp, struct rcu_data *rdp)
>> >> >         unsigned long gpnum;
>> >> >         unsigned long gps;
>> >> >         unsigned long j;
>> >> > +       unsigned long jn;
>> >> >         unsigned long js;
>> >> >         struct rcu_node *rnp;
>> >> >
>> >> > @@ -1586,14 +1590,17 @@ static void check_cpu_stall(struct rcu_state *rsp, struct rcu_data *rdp)
>> >> >             ULONG_CMP_GE(gps, js))
>> >> >                 return; /* No stall or GP completed since entering function. */
>> >> >         rnp = rdp->mynode;
>> >> > +       jn = jiffies + 3 * rcu_jiffies_till_stall_check() + 3;
>> >> >         if (rcu_gp_in_progress(rsp) &&
>> >> > -           (READ_ONCE(rnp->qsmask) & rdp->grpmask)) {
>> >> > +           (READ_ONCE(rnp->qsmask) & rdp->grpmask) &&
>> >> > +           cmpxchg(&rsp->jiffies_stall, js, jn) == js) {
>> >> >
>> >> >                 /* We haven't checked in, so go dump stack. */
>> >> >                 print_cpu_stall(rsp);
>> >> >
>> >> >         } else if (rcu_gp_in_progress(rsp) &&
>> >> > -                  ULONG_CMP_GE(j, js + RCU_STALL_RAT_DELAY)) {
>> >> > +                  ULONG_CMP_GE(j, js + RCU_STALL_RAT_DELAY) &&
>> >> > +                  cmpxchg(&rsp->jiffies_stall, js, jn) == js) {
>> >> >
>> >> >                 /* They had a few time units to dump stack, so complain. */
>> >> >                 print_other_cpu_stall(rsp, gpnum);
>> >> >
>> >>
>> >
>>
>

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