Re: Crashes in perf_event_ctx_lock_nested

From: Don Zickus
Date: Tue Oct 31 2017 - 14:48:58 EST

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On Mon, Oct 30, 2017 at 03:45:12PM -0700, Guenter Roeck wrote:
> Hi Thomas,
>
> we are seeing the following crash in v4.14-rc5/rc7 if CONFIG_HARDLOCKUP_DETECTOR
> is enabled.
>
> [ 5.908021] NMI watchdog: Enabled. Permanently consumes one hw-PMU counter.
> [ 5.915836]
> ==================================================================
> [ 5.917325] Unsafe core_pattern used with fs.suid_dumpable=2.
> [ 5.917325] Pipe handler or fully qualified core dump path required.
> [ 5.917325] Set kernel.core_pattern before fs.suid_dumpable.
> [ 5.924046] udevd[147]: starting version 225
> [ 5.948520] BUG: KASAN: null-ptr-deref in perf_event_ctx_lock_nested.isra.71+0x22/0x89
> [ 5.957380] Read of size 8 at addr 0000000000000208 by task watchdog/2/21
> [ 5.964973]
> [ 5.966646] CPU: 2 PID: 21 Comm: watchdog/2 Not tainted 4.14.0-rc7 #30
> [ 5.973947] Hardware name: Google Eve/Eve, BIOS Google_Eve.9584.95.0 09/27/2017
> [ 5.982128] Call Trace:
> [ 5.984874] dump_stack+0x4d/0x63
> [ 5.988585] kasan_report+0x24b/0x295
> [ 5.992691] ? watchdog_nmi_enable+0x12/0x12
> [ 5.997477] __asan_load8+0x81/0x83
> [ 6.001388] perf_event_ctx_lock_nested.isra.71+0x22/0x89
> [ 6.007437] perf_event_enable+0xf/0x27
> [ 6.011737] hardlockup_detector_perf_enable+0x3e/0x40
> [ 6.017493] watchdog_nmi_enable+0xe/0x12
> [ 6.021990] watchdog_enable+0x8c/0xc5
> [ 6.026195] smpboot_thread_fn+0x27a/0x3c7
> [ 6.030788] ? sort_range+0x22/0x22
> [ 6.034701] kthread+0x221/0x231
> [ 6.038321] ? kthread_flush_work+0x120/0x120
> [ 6.043204] ret_from_fork+0x22/0x30
> [ 6.047207]
> ==================================================================
> ...
> [ 6.134561] BUG: unable to handle kernel NULL pointer dereference at 0000000000000208
> [ 6.143316] IP: perf_event_ctx_lock_nested.isra.71+0x22/0x89
> [ 6.149645] PGD 0 P4D 0
> [ 6.152478] Oops: 0000 [#1] PREEMPT SMP KASAN
> [ 6.157350] Modules linked in:
> [ 6.160766] CPU: 2 PID: 21 Comm: watchdog/2 Tainted: G B 4.14.0-rc7 #30
> [ 6.169422] Hardware name: Google Eve/Eve, BIOS Google_Eve.9584.95.0 09/27/2017
> [ 6.177583] task: ffff8803eacd1100 task.stack: ffff8803eacf8000
> [ 6.184206] RIP: 0010:perf_event_ctx_lock_nested.isra.71+0x22/0x89
> [ 6.191118] RSP: 0018:ffff8803eacffe10 EFLAGS: 00010246
> [ 6.196962] RAX: 0000000000000296 RBX: 0000000000000000 RCX: ffffffffa52ee8a5
> [ 6.204941] RDX: d8ecf37b519af800 RSI: 0000000000000003 RDI: ffffffffa6274610
> [ 6.212911] RBP: ffff8803eacffe30 R08: dffffc0000000000 R09: 0000000000000000
> [ 6.220888] R10: ffffed007d59ffa9 R11: ffffc9000044c1a1 R12: 0000000000000000
> [ 6.228867] R13: ffff8803eacd1100 R14: 0000000000000208 R15: ffffffffa535ce54
> [ 6.231476] EXT4-fs (mmcblk0p1): mounted filesystem with ordered data mode. Opts: commit=600
> [ 6.237449] EXT4-fs (mmcblk0p8): mounted filesystem with ordered data mode. Opts: (null)
> [ 6.255332] FS: 0000000000000000(0000) GS:ffff8803ed500000(0000) knlGS:0000000000000000
> [ 6.264384] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ 6.270812] CR2: 0000000000000208 CR3: 0000000430615001 CR4: 00000000003606e0
> [ 6.278789] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [ 6.286761] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> [ 6.294741] Call Trace:
> [ 6.297480] perf_event_enable+0xf/0x27
> [ 6.301771] hardlockup_detector_perf_enable+0x3e/0x40
> [ 6.307515] watchdog_nmi_enable+0xe/0x12
> [ 6.311990] watchdog_enable+0x8c/0xc5
> [ 6.316176] smpboot_thread_fn+0x27a/0x3c7
> [ 6.320757] ? sort_range+0x22/0x22
> [ 6.324650] kthread+0x221/0x231
> [ 6.328251] ? kthread_flush_work+0x120/0x120
> [ 6.333114] ret_from_fork+0x22/0x30
> [ 6.337107] Code: a5 e8 70 58 f6 ff 5b 5d c3 55 48 89 e5 41 56 4c 8d b7 08 02
> 00 00 41 55 41 54 49 89 fc 53 e8 1a e9 f5 ff 4c 89 f7 e8 8d d3 07 00 <49> 8b 9c
> 24 08 02 00 00 31 d2 be 01 00 00 00 48 8d bb b0 00 00
> [ 6.358230] RIP: perf_event_ctx_lock_nested.isra.71+0x22/0x89 RSP: ffff8803eacffe10
> [ 6.366779] CR2: 0000000000000208
> [ 6.370477] ---[ end trace ff68e1917f0a2044 ]---
> [ 6.383531] Kernel panic - not syncing: Fatal exception
> [ 6.389640] Kernel Offset: 0x24200000 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffffbfffffff)
>
> The problem is a heisenbug - slight changes in the code, such as added logging,
> can make it disappear.
>
> I added some logging and a long msleep() in hardlockup_detector_perf_cleanup().
> Here is the result:
>
> [ 0.274361] NMI watchdog: ############ hardlockup_detector_perf_init
> [ 0.274915] NMI watchdog: ############ hardlockup_detector_event_create(0)
> [ 0.277049] NMI watchdog: ############ hardlockup_detector_perf_cleanup
> [ 0.277593] NMI watchdog: ############ hardlockup_detector_perf_enable(0)
> [ 0.278027] NMI watchdog: ############ hardlockup_detector_event_create(0)
> [ 1.312044] NMI watchdog: ############ hardlockup_detector_perf_cleanup done
> [ 1.385122] NMI watchdog: ############ hardlockup_detector_perf_enable(1)
> [ 1.386028] NMI watchdog: ############ hardlockup_detector_event_create(1)
> [ 1.466102] NMI watchdog: ############ hardlockup_detector_perf_enable(2)
> [ 1.475536] NMI watchdog: ############ hardlockup_detector_event_create(2)
> [ 1.535099] NMI watchdog: ############ hardlockup_detector_perf_enable(3)
> [ 1.535101] NMI watchdog: ############ hardlockup_detector_event_create(3)
> [ 7.222816] NMI watchdog: ############ hardlockup_detector_perf_disable(0)
> [ 7.230567] NMI watchdog: ############ hardlockup_detector_perf_disable(1)
> [ 7.243138] NMI watchdog: ############ hardlockup_detector_perf_disable(2)
> [ 7.250966] NMI watchdog: ############ hardlockup_detector_perf_disable(3)
> [ 7.258826] NMI watchdog: ############ hardlockup_detector_perf_enable(1)
> [ 7.258827] NMI watchdog: ############ hardlockup_detector_perf_cleanup
> [ 7.258831] NMI watchdog: ############ hardlockup_detector_perf_enable(2)
> [ 7.258833] NMI watchdog: ############ hardlockup_detector_perf_enable(0)
> [ 7.258834] NMI watchdog: ############ hardlockup_detector_event_create(2)
> [ 7.258835] NMI watchdog: ############ hardlockup_detector_event_create(0)
> [ 7.260169] NMI watchdog: ############ hardlockup_detector_perf_enable(3)
> [ 7.260170] NMI watchdog: ############ hardlockup_detector_event_create(3)
> [ 7.494251] NMI watchdog: ############ hardlockup_detector_event_create(1)
> [ 8.287135] NMI watchdog: ############ hardlockup_detector_perf_cleanup done
>
> Looks like there are a number of problems: hardlockup_detector_event_create()
> creates the event data structure even if it is already created, and
> hardlockup_detector_perf_cleanup() runs unprotected and in parallel to
> the enable/create functions.

Hi,

Thomas created a deferred cleanup mechanism to help with hotplugging, so yes
an event tries to be re-created if the deferred cleanup wasn't run yet. There
is probably a bug there to remove the re-created event off of the deferred
'dead_events_mask'. And maybe some locking there too, though I don't know
if adding locking around that queue re-introduces the deadlock to the
hotplug code. See commit 941154bd6937a710ae9193a3c733c0029e5ae7b8 for
details.

>
> ALso, the following message is seen twice.
>
> [ 0.278758] NMI watchdog: Enabled. Permanently consumes one hw-PMU counter.
> [ 7.258838] NMI watchdog: Enabled. Permanently consumes one hw-PMU counter.
>
> I don't offer a proposed patch since I have no idea how to best solve the
> problem.
>
> Also, is the repeated enable/disable/cleanup as part of the normal boot
> really necessary ?

Yes, part of it. The first event_create(0) is a hardware check. Does the
hardware support perf counters. The event is removed after the check. If
the check passes, allow all cpus to create the event. Hence the second
event_create(0). Otherwise, disable the counters from being enabled on
other cpus.

What doesn't make sense is the perf_disable(0-3) at the 7 second mark. Not
sure why that happens. But it seems to cause the problem by exposing a race
of disabling the event and then re-creating it.

What is odd, is the panic.

hardlockup_detector_perf_enable ->
hardlockup_detector_event_create
perf_event_enable ->
panic()

due to the delay of hardlockup_detector_perf_cleanup(), I would have assumed
hardlockup_detector_event_create() would have warned and returned
had the cleanup not been invoked yet.

The only race I can see here:

CPU A CPU B
hardlockup_detector_perf_cleanup
hardlockup_detector_perf_enable perf_event_release_kernel
hardlockup_detector_event_create
per_cpu(watchdog_ev, cpu) = NULL;
perf_event_enable(this_cpu_read(watchdog_ev)
^^^NULL

I am guessing adding a check in hardlockup_detector_perf_enable() would
prove that, but as you said, it might may the problem go into hiding.

The below patch probably doesn't solve the problem, just makes the race
condition a lot smaller? Though I don't know if this causes more problems
internally to perf with two attached events temporarily.

Cheers,
Don

diff --git a/kernel/watchdog_hld.c b/kernel/watchdog_hld.c
index 71a62ceacdc8..0b9bd1e0bf57 100644
--- a/kernel/watchdog_hld.c
+++ b/kernel/watchdog_hld.c
@@ -221,12 +221,16 @@ void hardlockup_detector_perf_cleanup(void)
struct perf_event *event = per_cpu(watchdog_ev, cpu);

/*
+ * Clear immediately to avoid delay of releasing event.
+ */
+ per_cpu(watchdog_ev, cpu) = NULL;
+
+ /*
* Required because for_each_cpu() reports unconditionally
* CPU0 as set on UP kernels. Sigh.
*/
if (event)
perf_event_release_kernel(event);
- per_cpu(watchdog_ev, cpu) = NULL;
}
cpumask_clear(&dead_events_mask);
}

Next message: Michal Hocko: "Re: [PATCH] fs, mm: account filp and names caches to kmemcg"
Previous message: Sumit Saxena: "RE: [PATCH] scsi: megaraid: Convert timers to use timer_setup()"
In reply to: Guenter Roeck: "Re: Crashes in perf_event_ctx_lock_nested"
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