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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 11/09/14 11:07, Jan Beulich wrote:
>>>> 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
>

Ah yes, and you haven't gotten around to changing the cpumask_*
functions wrt atomicity yet.

Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

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