Re: [PATCH] watchdog: Fix race on single cpu boot

[Don Zickus <dzickus@redhat.com>]

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@kernel.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
> 
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at  http://www.tux.org/lkml/