Re: [Xen-devel] [PATCH 2/2] x86/crash: Disable the watchdog NMIs on the crashing cpu.

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 18/11/13 09:26, Jan Beulich wrote:
>>>> On 15.11.13 at 21:32, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
>> --- a/xen/arch/x86/crash.c
>> +++ b/xen/arch/x86/crash.c
>> @@ -118,6 +118,7 @@ static void nmi_shootdown_cpus(void)
>>      unsigned long msecs;
>>      int i, cpu = smp_processor_id();
>>  
>> +    disable_lapic_nmi_watchdog();
>>      local_irq_disable();
>>  
>>      crashing_cpu = cpu;
> _If_ you do this here, I wonder why it's being done before
> disabling interrupts.
>
> But then again I wonder whether it wouldn't be better to do
> this even earlier (i.e. by passing a flag to watchdog_disable()),
> as the NMI watchdog becomes useless with that call being done
> from kexec_common_shutdown().

Disabling interrupts here is more defensive coding than anything else. 
It is not expected to be able to get here with interrupts enabled, but
in a crash

Putting this in watchdog_disable() would result in a race condition. 
disable_lapic_nmi_watchdog() mutates global state, meaning that it can
only possibly run correctly on a single cpu. In an ideal world with
plenty of time, the lapic watchdog code could be improved.  However,
restricting its use until after one_cpu_only() is the easiest fix.

~Andrew

>
>> --- a/xen/arch/x86/nmi.c
>> +++ b/xen/arch/x86/nmi.c
>> @@ -165,7 +165,7 @@ static void nmi_timer_fn(void *unused)
>>      set_timer(&this_cpu(nmi_timer), NOW() + MILLISECS(1000));
>>  }
>>  
>> -static void disable_lapic_nmi_watchdog(void)
>> +void disable_lapic_nmi_watchdog(void)
> The suggested alternative would also make it unnecessary to
> make this function non-static...
>
> Jan
>


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