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Message-ID: <20150731153656.GB42272@redhat.com>
Date: Fri, 31 Jul 2015 11:36:56 -0400
From: Don Zickus <dzickus@...hat.com>
To: Jiri Olsa <jolsa@...nel.org>
Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>,
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: Re: [PATCH] watchdog: Fix race on single cpu boot
On Fri, Jul 31, 2015 at 01:56:45PM +0200, Jiri Olsa wrote:
> 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.
Hi Jiri,
Uli privately has been working on a patchset that cleans up a bunch of these
race conditions. We believe it should cover this case. It uses the
proc_mutex to synchronize everything.
I think he is reaching out to you. If you could try his patchset to see if
it fixes things, it might be a cleaner approach than what you are doing.
<small time goes by>
Ok, I was poked on IRC and you have already done this and it passed your
testcase. Let me get Uli to post his patches. :-)
Cheers,
Don
>
> 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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