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    Subject[PATCH 7/9] x86: Avoid calling arch_trigger_all_cpu_backtrace() at the same time
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    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 taken as
    unknown reason NMI interrupts.

    This patch uses a test_and_set to avoid the problem, and stop the
    arch_trigger_all_cpu_backtrace() from calling to avoid dumping a
    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 | 13 +++++++++++++
    1 files changed, 13 insertions(+), 0 deletions(-)

    diff --git a/arch/x86/kernel/apic/hw_nmi.c b/arch/x86/kernel/apic/hw_nmi.c
    index ddb29bf..b36dd01 100644
    --- a/arch/x86/kernel/apic/hw_nmi.c
    +++ b/arch/x86/kernel/apic/hw_nmi.c
    @@ -36,10 +36,20 @@ EXPORT_SYMBOL(touch_nmi_watchdog);
    /* For reliability, we're prepared to waste bits here. */
    static DECLARE_BITMAP(backtrace_mask, NR_CPUS) __read_mostly;

    +/* "in progress" flag of arch_trigger_all_cpu_backtrace */
    +static unsigned long backtrace_flag;
    +
    void arch_trigger_all_cpu_backtrace(void)
    {
    int i;

    + 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.
    + */
    + return;
    +
    cpumask_copy(to_cpumask(backtrace_mask), cpu_online_mask);

    printk(KERN_INFO "sending NMI to all CPUs:\n");
    @@ -51,6 +61,9 @@ void arch_trigger_all_cpu_backtrace(void)
    break;
    mdelay(1);
    }
    +
    + clear_bit(0, &backtrace_flag);
    + smp_mb__after_clear_bit();
    }

    static int __kprobes
    --
    1.7.3.2


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