On Mon, Apr 2, 2018 at 7:23 PM, Paul E. McKenney
<paul...@linux.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+2dbc55da20fa24637...@syzkaller.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?

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