> 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..bfdab3b 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 unsigned long 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 (test_and_set_bit(0, &backtrace_flag))
> + /*
> + * 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);
> }
> +
> + clear_bit(0, &backtrace_flag);
