Re: [PATCH] x86: avoid calling arch_trigger_all_cpu_backtrace() at the same time on SMP

[DDD <dongdong.deng@windriver.com>]

Don Zickus wrote:
> On Mon, Oct 18, 2010 at 07:00:15PM +0800, DDD wrote:
>> CC to Ingo's mingo@elte.hu and add some code explains for this patch.
>>
>> Dongdong
>>
>>
>> Dongdong Deng wrote:
>>> The spin_lock_debug/rcu_cpu_stall detector uses
>>> trigger_all_cpu_backtrace() to dump cpu backtrace.
>>> Therefore it is possible that trigger_all_cpu_backtrace()
>>> could be called at the same time on different CPUs, which
>>> triggers and 'unknown reason NMI' warning. The following case
>>> illustrates the problem:
> 
> The patch seems reasonable.  I can queue it up.  Ingo wrote the original
> code and knows which spin locks to use better than me, perhaps he can find
> a moment to comment.

Hello Don,

Thanks for taking care and queuing this patch. :-)

Dongdong


> 
> Cheers,
> Don
> 
>>>      CPU1                    CPU2                     ...   CPU N
>>>                       trigger_all_cpu_backtrace()
>>>                       set "backtrace_mask" to cpu mask
>>>                               |
>>> generate NMI interrupts  generate NMI interrupts       ...
>>>    \                          |                               /
>>>     \                         |                              /
>>>   The "backtrace_mask" will be cleaned by the first NMI interrupt
>>>  at nmi_watchdog_tick(), then the following NMI interrupts generated
>>> by other cpus's arch_trigger_all_cpu_backtrace() will be took as
>>> unknown reason NMI interrupts.
>>>
>>> This patch uses a lock to avoid the problem, and stop the
>>> arch_trigger_all_cpu_backtrace() calling to avoid dumping double cpu
>>> backtrace info when there is already a trigger_all_cpu_backtrace()
>>> in progress.
>>>
>>> Signed-off-by: Dongdong Deng <dongdong.deng@windriver.com>
>>> Reviewed-by: Bruce Ashfield <bruce.ashfield@windriver.com>
>>> CC: Thomas Gleixner <tglx@linutronix.de>
>>> CC: Ingo Molnar <mingo@redhat.com>
>>> CC: "H. Peter Anvin" <hpa@zytor.com>
>>> CC: x86@kernel.org
>>> CC: linux-kernel@vger.kernel.org
>>> ---
>>> arch/x86/kernel/apic/hw_nmi.c |   14 ++++++++++++++
>>> arch/x86/kernel/apic/nmi.c    |   14 ++++++++++++++
>>> 2 files changed, 28 insertions(+), 0 deletions(-)
>>>
>>> diff --git a/arch/x86/kernel/apic/hw_nmi.c b/arch/x86/kernel/apic/hw_nmi.c
>>> index cefd694..3aea0a5 100644
>>> --- a/arch/x86/kernel/apic/hw_nmi.c
>>> +++ b/arch/x86/kernel/apic/hw_nmi.c
>>> @@ -29,6 +29,16 @@ u64 hw_nmi_get_sample_period(void)
>>> void arch_trigger_all_cpu_backtrace(void)
>>> {
>>> 	int i;
>>> +	static arch_spinlock_t lock = __ARCH_SPIN_LOCK_UNLOCKED;
>> Why an arch spin lock vs just using a raw spin lock?
>> for example. static DEFINE_RAW_SPINLOCK(lock);
>>
>> The spin_lock_debug detector was used in raw_spinlock too.
>>
>> arch_trigger_all_cpu_backtrace() -->
>>   raw_spin_lock(lock)      -->
>>   _raw_spin_lock(lock)     -->
>>   __raw_spin_lock(lock)    -->
>>   do_raw_spin_lock(lock)   -->
>>   __spin_lock_debug(lock)  -->
>>   trigger_all_cpu_backtrace()
>>
>> Therefor, we have to use arch spin lock here.
>>
>>
>>
>>> +	unsigned long flags;
>>> +
>>> +	local_irq_save(flags);
>> Why have to save the irq's here?
>>
>> When the arch_trigger_all_cpu_backtrace() was triggered by
>> "spin_lock()"'s spin_lock_debug detector, it is possible that
>> the irq is enabled, thus we have to save and disable it here.
>>
>>
>>
>>> +	if (!arch_spin_trylock(&lock))
>>> +		/*
>>> +		 * If there is already a trigger_all_cpu_backtrace()
>>> +		 * in progress, don't output double cpu dump infos.
>>> +		 */
>>> +		goto out_restore_irq;
>>> 	cpumask_copy(to_cpumask(backtrace_mask), cpu_online_mask);
>>> @@ -41,6 +51,10 @@ void arch_trigger_all_cpu_backtrace(void)
>>> 			break;
>>> 		mdelay(1);
>>> 	}
>>> +
>>> +	arch_spin_unlock(&lock);
>>> +out_restore_irq:
>>> +	local_irq_restore(flags);
>>> }
>>> static int __kprobes
>>> diff --git a/arch/x86/kernel/apic/nmi.c b/arch/x86/kernel/apic/nmi.c
>>> index a43f71c..5fa8a13 100644
>>> --- a/arch/x86/kernel/apic/nmi.c
>>> +++ b/arch/x86/kernel/apic/nmi.c
>>> @@ -552,6 +552,16 @@ int do_nmi_callback(struct pt_regs *regs, int cpu)
>>> void arch_trigger_all_cpu_backtrace(void)
>>> {
>>> 	int i;
>>> +	static arch_spinlock_t lock = __ARCH_SPIN_LOCK_UNLOCKED;
>>> +	unsigned long flags;
>>> +
>>> +	local_irq_save(flags);
>>> +	if (!arch_spin_trylock(&lock))
>>> +		/*
>>> +		 * If there is already a trigger_all_cpu_backtrace()
>>> +		 * in progress, don't output double cpu dump infos.
>>> +		 */
>>> +		goto out_restore_irq;
>>> 	cpumask_copy(to_cpumask(backtrace_mask), cpu_online_mask);
>>> @@ -564,4 +574,8 @@ void arch_trigger_all_cpu_backtrace(void)
>>> 			break;
>>> 		mdelay(1);
>>> 	}
>>> +
>>> +	arch_spin_unlock(&lock);
>>> +out_restore_irq:
>>> +	local_irq_restore(flags);
>>> }
>>
>>
>>
>>
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>