On Wed, Apr 11, 2018 at 12:06:27PM +0200, Dmitry Vyukov wrote: > On Tue, Apr 10, 2018 at 7:02 PM, Paul E. McKenney > <paul...@linux.vnet.ibm.com> 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. > > > > 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? 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); > >> > > >> > > >