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Date: Fri, 31 Jul 2015 13:56:45 +0200
From: Jiri Olsa <jolsa@...nel.org>
To: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc: lkml <linux-kernel@...r.kernel.org>,
Ingo Molnar <mingo@...nel.org>, Michal Hocko <mhocko@...e.cz>,
Ulrich Obergfell <uobergfe@...hat.com>,
Stephane Eranian <eranian@...gle.com>,
Andi Kleen <ak@...ux.intel.com>,
Paul Bunyan <pbunyan@...hat.com>
Subject: [PATCH] watchdog: Fix race on single cpu boot
We are experiencing following crashes under kdump
kernel execution:
[ 1.930428] BUG: unable to handle kernel NULL pointer dereference at 0000000000000080
SNIP
[ 2.478604] Call Trace:
[ 2.479364] [<ffffffff81158f06>] perf_event_enable+0x16/0x40
[ 2.481035] [<ffffffff81119a2c>] watchdog_nmi_enable+0x10c/0x170
[ 2.482906] [<ffffffff81119c14>] watchdog_enable+0x54/0xc0
[ 2.484583] [<ffffffff810ad209>] smpboot_thread_fn+0xb9/0x1a0
[ 2.486302] [<ffffffff81634bb9>] ? schedule+0x29/0x70
[ 2.487811] [<ffffffff810ad150>] ? lg_double_unlock+0x90/0x90
[ 2.4810a484f>] kthread+0xcf/0xe0
[ 2.991478] [<ffffffff810a4780>] ? kthread_create_on_node+0x140/0x140
[ 2.993519] [<ffffffff8163fad8>] ret_from_fork+0x58/0x90
[ 2.995192] [<ffffffff810a4780>] ? kthread_create_on_node+0x140/0x140
[ 3.005787] RIP [<ffffffff8115744a>] perf_event_ctx_lock_nested.isra.50+0x5a/0xa0
[ 3.008350] RSP <ffff88003199fde0>
[ 3.009417] CR2: 0000000000000080
[ 3.010432] ---[ end trace 892dd7be017c5cde ]---
[ 3.011792] Kernel panic - not syncing: Fatal exception
[ 3.013379] Rebooting in 10 seconds..
It's caused by watchdog_nmi_disable_all/watchdog_nmi_enable_all calls
executed by fixup_ht_bug (as fallback for HT bug) and watchdog_enable
function executed by watchdog's smp_hotplug_thread setup callback.
The watchdog_enable function calls watchdog_nmi_enable which is not serialized
with watchdog_nmi_disable_all/watchdog_nmi_enable_all. We see following race
during the boot.
(booting with 'maxcpus=1' to emulate kdump mode and execute HT bug fallback).
---
kernel_init
kernel_init_freeable
lockup_detector_init
smpboot_register_percpu_thread
-> schedules out to the new kthread
---
smpboot_thread_fn
watchdog_enable
watchdog_nmi_enable
perf_event_create_kernel_counter
per_cpu(watchdog_ev, cpu) = event
perf_event_enable
perf_event_ctx_lock_nested
-> schedules out back to the kernel_init
---
kernel_init
...
do_basic_setup
do_initcalls
fixup_ht_bug
watchdog_nmi_disable_all
-> release per_cpu(watchdog_ev, cpu) initialized by watchdog_enable
watchdog_nmi_enable_all
...
-> schedules out to the smpboot_thread_fn
---
smpboot_thread_fn
...
-> crash in perf_event_ctx_lock_nested because the event
was released
Protecting watchdog_nmi_(enable|disable) calls with new per-cpu mutex.
I can easily reproduce this on SNB server with 'maxcpus=1' command line
option, and I guess it could be triggered on any server that deals with
the HT bug. I dig out following:
case 42: /* 32nm SandyBridge */
case 45: /* 32nm SandyBridge-E/EN/EP */
case 58: /* 22nm IvyBridge */
case 62: /* 22nm IvyBridge-EP/EX */
case 60: /* 22nm Haswell Core */
case 63: /* 22nm Haswell Server */
case 69: /* 22nm Haswell ULT */
case 70: /* 22nm Haswell + GT3e (Intel Iris Pro graphics) */
Signed-off-by: Jiri Olsa <jolsa@...nel.org>
---
kernel/watchdog.c | 22 ++++++++++++++++++++--
1 file changed, 20 insertions(+), 2 deletions(-)
diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index a6ffa43f2993..8fcc75116277 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -82,6 +82,7 @@ static DEFINE_PER_CPU(bool, hard_watchdog_warn);
static DEFINE_PER_CPU(bool, watchdog_nmi_touch);
static DEFINE_PER_CPU(unsigned long, hrtimer_interrupts_saved);
static DEFINE_PER_CPU(struct perf_event *, watchdog_ev);
+static DEFINE_PER_CPU(struct mutex, watchdog_ev_lock);
#endif
static unsigned long soft_lockup_nmi_warn;
@@ -526,7 +527,10 @@ static unsigned long cpu0_err;
static int watchdog_nmi_enable(unsigned int cpu)
{
struct perf_event_attr *wd_attr;
- struct perf_event *event = per_cpu(watchdog_ev, cpu);
+ struct perf_event *event;
+
+ mutex_lock(&per_cpu(watchdog_ev_lock, cpu));
+ event = per_cpu(watchdog_ev, cpu);
/* nothing to do if the hard lockup detector is disabled */
if (!(watchdog_enabled & NMI_WATCHDOG_ENABLED))
@@ -557,6 +561,8 @@ static int watchdog_nmi_enable(unsigned int cpu)
goto out_save;
}
+ mutex_unlock(&per_cpu(watchdog_ev_lock, cpu));
+
/*
* Disable the hard lockup detector if _any_ CPU fails to set up
* set up the hardware perf event. The watchdog() function checks
@@ -593,12 +599,16 @@ out_save:
out_enable:
perf_event_enable(per_cpu(watchdog_ev, cpu));
out:
+ mutex_unlock(&per_cpu(watchdog_ev_lock, cpu));
return 0;
}
static void watchdog_nmi_disable(unsigned int cpu)
{
- struct perf_event *event = per_cpu(watchdog_ev, cpu);
+ struct perf_event *event;
+
+ mutex_lock(&per_cpu(watchdog_ev_lock, cpu));
+ event = per_cpu(watchdog_ev, cpu);
if (event) {
perf_event_disable(event);
@@ -611,6 +621,7 @@ static void watchdog_nmi_disable(unsigned int cpu)
/* watchdog_nmi_enable() expects this to be zero initially. */
cpu0_err = 0;
}
+ mutex_unlock(&per_cpu(watchdog_ev_lock, cpu));
}
void watchdog_nmi_enable_all(void)
@@ -928,6 +939,8 @@ int proc_watchdog_cpumask(struct ctl_table *table, int write,
void __init lockup_detector_init(void)
{
+ int cpu __maybe_unused;
+
set_sample_period();
#ifdef CONFIG_NO_HZ_FULL
@@ -942,6 +955,11 @@ void __init lockup_detector_init(void)
cpumask_copy(&watchdog_cpumask, cpu_possible_mask);
#endif
+#ifdef CONFIG_HARDLOCKUP_DETECTOR
+ for_each_possible_cpu(cpu)
+ mutex_init(&per_cpu(watchdog_ev_lock, cpu));
+#endif
+
if (watchdog_enabled)
watchdog_enable_all_cpus();
}
--
2.4.3
--
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