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

[Ingo Molnar <mingo@elte.hu>]

* 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