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

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

[Don Zickus]

From: Dongdong Deng <dongdong.deng@windriver.com>

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:

      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
Signed-off-by: Don Zickus <dzickus@redhat.com>
---
 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;
+	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);
 
@@ -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);
 }
-- 
1.7.2.3

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

[Ingo Molnar]

* Don Zickus <dzickus@redhat.com> wrote:

> From: Dongdong Deng <dongdong.deng@windriver.com>
> 
> 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:
> 
>       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
> Signed-off-by: Don Zickus <dzickus@redhat.com>
> ---
>  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;

Please dont put statics into the middle of local variables - put them into file 
scope in a visible way.

> +	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);
>  
> @@ -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);

This spinlock is never actually used as a spinlock - it's a "in progress" flag. Why 
not use a flag and test_and_set_bit()?

Also, the irq disabling really needed, will this code ever be called with irqs on?

Thanks,

	Ingo

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

[DDD]

Ingo Molnar wrote:
> * Don Zickus <dzickus@redhat.com> wrote:
> 
>> From: Dongdong Deng <dongdong.deng@windriver.com>
>>
>> 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:
>>
>>       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
>> Signed-off-by: Don Zickus <dzickus@redhat.com>
>> ---
>>  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;
> 
> Please dont put statics into the middle of local variables - put them into file 
> scope in a visible way.

Got it. will change it.

> 
>> +	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);
>>  
>> @@ -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);
> 
> This spinlock is never actually used as a spinlock - it's a "in progress" flag. Why 
> not use a flag and test_and_set_bit()?

Yep, it's a "in progress" flag, I will change to use a flag to replace 
the spinlock.


> Also, the irq disabling really needed, will this code ever be called with irqs on?

This code could be called with irqs on.

If 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.


Thanks,

Dongdong

> 
> Thanks,
> 
> 	Ingo
> 

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

[Ingo Molnar]

* DDD <dongdong.deng@windriver.com> wrote:

> Ingo Molnar wrote:
> >* Don Zickus <dzickus@redhat.com> wrote:
> >
> >>From: Dongdong Deng <dongdong.deng@windriver.com>
> >>
> >>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:
> >>
> >>      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
> >>Signed-off-by: Don Zickus <dzickus@redhat.com>
> >>---
> >> 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;
> >
> >Please dont put statics into the middle of local variables - put
> >them into file scope in a visible way.
> 
> Got it. will change it.
> 
> >
> >>+	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);
> >>@@ -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);
> >
> >This spinlock is never actually used as a spinlock - it's a "in
> >progress" flag. Why not use a flag and test_and_set_bit()?
> 
> Yep, it's a "in progress" flag, I will change to use a flag to
> replace the spinlock.

Thanks.

> >Also, the irq disabling really needed, will this code ever be called with irqs on?
> 
> This code could be called with irqs on.
> 
> If 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.

Ok - please document that fact in the code.

Thanks,

	Ingo

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

[Dongdong Deng]

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:

      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
Signed-off-by: Don Zickus <dzickus@redhat.com>
---
 arch/x86/kernel/apic/hw_nmi.c |   23 +++++++++++++++++++++++
 arch/x86/kernel/apic/nmi.c    |   23 +++++++++++++++++++++++
 2 files changed, 46 insertions(+), 0 deletions(-)

diff --git a/arch/x86/kernel/apic/hw_nmi.c b/arch/x86/kernel/apic/hw_nmi.c
index cefd694..6084c3b 100644
--- a/arch/x86/kernel/apic/hw_nmi.c
+++ b/arch/x86/kernel/apic/hw_nmi.c
@@ -26,9 +26,27 @@ u64 hw_nmi_get_sample_period(void)
 }
 
 #ifdef ARCH_HAS_NMI_WATCHDOG
+/* "in progress" flag of arch_trigger_all_cpu_backtrace */
+static int backtrace_flag;
+
 void arch_trigger_all_cpu_backtrace(void)
 {
 	int i;
+	unsigned long flags;
+
+	/*
+	 * Have to disable irq here, as the
+	 * arch_trigger_all_cpu_backtrace() could be
+	 * triggered by "spin_lock()" with irqs on.
+	 */
+	local_irq_save(flags);
+
+	if (cmpxchg(&backtrace_flag, 0, 1) != 0)
+		/*
+		 * If there is already a trigger_all_cpu_backtrace() in progress
+		 * (backtrace_flag == 1), don't output double cpu dump infos.
+		 */
+		goto out_restore_irq;
 
 	cpumask_copy(to_cpumask(backtrace_mask), cpu_online_mask);
 
@@ -41,6 +59,11 @@ void arch_trigger_all_cpu_backtrace(void)
 			break;
 		mdelay(1);
 	}
+
+	cmpxchg(&backtrace_flag, 1, 0);
+
+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 c90041c..2d4b3a1 100644
--- a/arch/x86/kernel/apic/nmi.c
+++ b/arch/x86/kernel/apic/nmi.c
@@ -549,9 +549,27 @@ int do_nmi_callback(struct pt_regs *regs, int cpu)
 	return 0;
 }
 
+/* "in progress" flag of arch_trigger_all_cpu_backtrace */
+static int backtrace_flag;
+
 void arch_trigger_all_cpu_backtrace(void)
 {
 	int i;
+	unsigned long flags;
+
+	/*
+	 * Have to disable irq here, as the
+	 * arch_trigger_all_cpu_backtrace() could be
+	 * triggered by "spin_lock()" with irqs on.
+	 */
+	local_irq_save(flags);
+
+	if (cmpxchg(&backtrace_flag, 0, 1) != 0)
+		/*
+		 * If there is already a trigger_all_cpu_backtrace() in progress
+		 * (backtrace_flag == 1), don't output double cpu dump infos.
+		 */
+		goto out_restore_irq;
 
 	cpumask_copy(to_cpumask(backtrace_mask), cpu_online_mask);
 
@@ -564,4 +582,9 @@ void arch_trigger_all_cpu_backtrace(void)
 			break;
 		mdelay(1);
 	}
+
+	cmpxchg(&backtrace_flag, 1, 0);
+
+out_restore_irq:
+	local_irq_restore(flags);
 }
-- 
1.6.0.4

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

[Ingo Molnar]

* Dongdong Deng <dongdong.deng@windriver.com> wrote:

> +static int backtrace_flag;

> +	if (cmpxchg(&backtrace_flag, 0, 1) != 0)

Sorry to be a PITA, but i asked for test_and_set() because that's the simplest 
primitive. cmpxchg() semantics is not nearly as obvious and people regularly get it 
wrong :-/

Also, variables that cmpxchg or test_and_set operates on need to be long, not int.

	Ingo

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

[DDD]

Ingo Molnar wrote:
> * Dongdong Deng <dongdong.deng@windriver.com> wrote:
> 
>> +static int backtrace_flag;
> 
>> +	if (cmpxchg(&backtrace_flag, 0, 1) != 0)
> 
> Sorry to be a PITA, but i asked for test_and_set() because that's the simplest 
> primitive. cmpxchg() semantics is not nearly as obvious and people regularly get it 
> wrong :-/

As the 'backtrace_flag' could be accessed by multi-cpus on SMP at the 
same time, I use cmpxchg() for getting a atomic/memory barrier operation 
for 'backtrace_flag' variable.

If we use test_and_set, maybe we need smp_wmb() after test_and_set. (If 
I wrong, please correct me, thanks. :-) )

Should we still need to use test_and_set?
If need, I will use test_and_set at my next patch.

> 
> Also, variables that cmpxchg or test_and_set operates on need to be long, not int.

Got it, I will change it to 'unsigned long' type.

Thanks for your teaching.

Dongdong


> 
> 	Ingo
> 

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

[Ingo Molnar]

* DDD <dongdong.deng@windriver.com> wrote:

> Ingo Molnar wrote:
> >* Dongdong Deng <dongdong.deng@windriver.com> wrote:
> >
> >>+static int backtrace_flag;
> >
> >>+	if (cmpxchg(&backtrace_flag, 0, 1) != 0)
> >
> >Sorry to be a PITA, but i asked for test_and_set() because that's
> >the simplest primitive. cmpxchg() semantics is not nearly as
> >obvious and people regularly get it wrong :-/
> 
> As the 'backtrace_flag' could be accessed by multi-cpus on SMP at
> the same time, I use cmpxchg() for getting a atomic/memory barrier
> operation for 'backtrace_flag' variable.
>
> If we use test_and_set, maybe we need smp_wmb() after test_and_set.
> (If I wrong, please correct me, thanks. :-) )

No, test_and_set_bit() is SMP safe and is an implicit barrier as well - so no 
smp_wmb() or other barriers are needed.

Thanks,

	Ingo

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

[DDD]

Ingo Molnar wrote:
> * DDD <dongdong.deng@windriver.com> wrote:
> 
>> Ingo Molnar wrote:
>>> * Dongdong Deng <dongdong.deng@windriver.com> wrote:
>>>
>>>> +static int backtrace_flag;
>>>> +	if (cmpxchg(&backtrace_flag, 0, 1) != 0)
>>> Sorry to be a PITA, but i asked for test_and_set() because that's
>>> the simplest primitive. cmpxchg() semantics is not nearly as
>>> obvious and people regularly get it wrong :-/
>> As the 'backtrace_flag' could be accessed by multi-cpus on SMP at
>> the same time, I use cmpxchg() for getting a atomic/memory barrier
>> operation for 'backtrace_flag' variable.
>>
>> If we use test_and_set, maybe we need smp_wmb() after test_and_set.
>> (If I wrong, please correct me, thanks. :-) )
> 
> No, test_and_set_bit() is SMP safe and is an implicit barrier as well - so no 
> smp_wmb() or other barriers are needed.

Yep, the spin_lock of test_and_set_bit() could make sure that.

Thank you very much,
I will send out the new patch quickly. :-)

Dongdong

> 
> Thanks,
> 
> 	Ingo
> 

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

[Eric Dumazet]

Le jeudi 11 novembre 2010 à 10:23 +0100, Ingo Molnar a écrit :

> Also, variables that cmpxchg or test_and_set operates on need to be long, not int.

Hmm, ok for test_and_set(), it operates on a long.

cmpxchg() is ok on an int AFAIK. If not we have to make some changes :(

btrfs_orphan_cleanup() for example does this :

	if (cmpxchg(&root->orphan_cleanup_state, 0, ORPHAN_CLEANUP_STARTED))
		...


Same in build_ehash_secret() (net/ipv4/af_inet.c)

	cmpxchg(&inet_ehash_secret, 0, rnd);

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

[Ingo Molnar]

* Eric Dumazet <eric.dumazet@gmail.com> wrote:

> Le jeudi 11 novembre 2010 à 10:23 +0100, Ingo Molnar a écrit :
> 
> > Also, variables that cmpxchg or test_and_set operates on need to be long, not int.
> 
> Hmm, ok for test_and_set(), it operates on a long.
> 
> cmpxchg() is ok on an int AFAIK. If not we have to make some changes :(
> 
> btrfs_orphan_cleanup() for example does this :
> 
> 	if (cmpxchg(&root->orphan_cleanup_state, 0, ORPHAN_CLEANUP_STARTED))
> 		...
> 
> 
> Same in build_ehash_secret() (net/ipv4/af_inet.c)
> 
> 	cmpxchg(&inet_ehash_secret, 0, rnd);

You are right - cmpxchg() auto-detects the word size and thus should work on int 
too.

Thanks,

	Ingo

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

[Dongdong Deng]

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: 

      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;
+	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);
 
@@ -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);
 }
-- 
1.6.0.4

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

[DDD]

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: 
> 
>       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);
>  }

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

[Don Zickus]

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.

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

[DDD]

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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