Re: [Xen-devel] [PATCH 1/2] x86: suppress event check IPI to MWAITing CPUs

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 11.09.14 at 12:02, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 11/09/14 10:40, Jan Beulich wrote:
>> @@ -349,7 +359,7 @@ void mwait_idle_with_hints(unsigned int 
>>       * Timer deadline passing is the event on which we will be woken via
>>       * cpuidle_mwait_wakeup. So check it now that the location is armed.
>>       */
>> -    if ( expires > NOW() || expires == 0 )
>> +    if ( (expires > NOW() || expires == 0) && !softirq_pending(cpu) )
>>      {
>>          cpumask_set_cpu(cpu, &cpuidle_mwait_flags);
> 
> Don't you need a smp_wmb() or better here for the results of
> cpumask_set_cpu() to be guaranteed to be externally visible?  mwait does
> not appear to be a serialising instruction, and doesn't appear to have
> any ordering guarantees in the manual.

I think cpumask_set_cpu() using a LOCKed r-m-w instruction should
provide all the needed ordering.

Jan


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