On Mon, Apr 9, 2018 at 8:11 PM, Paul E. McKenney
<paul...@linux.vnet.ibm.com> wrote:
> On Mon, Apr 09, 2018 at 06:28:16PM +0200, Dmitry Vyukov wrote:
>> On Mon, Apr 9, 2018 at 6:20 PM, Paul E. McKenney
>> <paul...@linux.vnet.ibm.com> wrote:
>> > On Mon, Apr 09, 2018 at 02:54:20PM +0200, Dmitry Vyukov wrote:
>> >> 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?
>> >
>> > 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.


>                                                         Thanx, Paul
>
> ------------------------------------------------------------------------
>
> commit 6a5ab1e68f8636d8823bb5a9aee35fc44c2be866
> Author: Paul E. McKenney <paul...@linux.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 <dvyu...@google.com>
>     Signed-off-by: Paul E. McKenney <paul...@linux.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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