Re: [PATCH 1/3] smp/ipi: Remove redundant cfd->cpumask_ipi mask

[Preeti U Murthy <preeti@linux.vnet.ibm.com>]

Hi Wang,

On 07/06/2013 11:33 AM, Wang YanQing wrote:
> On Sat, Jul 06, 2013 at 10:59:39AM +0530, Preeti U Murthy wrote:
>> Hi Wang,
>>
>> On 07/06/2013 08:43 AM, Wang YanQing wrote:
>>> On Fri, Jul 05, 2013 at 09:57:01PM +0530, Preeti U Murthy wrote:
>>>> cfd->cpumask_ipi is used only in smp_call_function_many().The existing
>>>> comment around it says that this additional mask is used because
>>>> cfd->cpumask can get overwritten.
>>>>
>>>> There is no reason why the cfd->cpumask can be overwritten, since this
>>>> is a per_cpu mask; nobody can change it but us and we are
>>>> called with preemption disabled.
>>>
>>> The ChangeLog for f44310b98ddb7f0d06550d73ed67df5865e3eda5
>>> which import cfd->cpumask_ipi saied the reason why we need
>>> it:
>>>
>>> "    As explained by Linus as well:
>>>     
>>>      |
>>>      | Once we've done the "list_add_rcu()" to add it to the
>>>      | queue, we can have (another) IPI to the target CPU that can
>>>      | now see it and clear the mask.
>>>      |
>>>      | So by the time we get to actually send the IPI, the mask might
>>>      | have been cleared by another IPI.
>>
>> I am unable to understand where the cfd->cpumask of the source cpu is
>> getting cleared. Surely not by itself, since it is preempt disabled.
>> Also why should it get cleared?
> 
> Assume we have three CPUs: A,B,C
> 
> A call smp_call_function_many to notify C do something,
> and current it execute on finished below codes:
> 
> "for_each_cpu(cpu, cfd->cpumask) {
>                 struct call_single_data *csd = per_cpu_ptr(cfd->csd, cpu);
>                 struct call_single_queue *dst =
>                                         &per_cpu(call_single_queue, cpu);
>                 unsigned long flags;
> 
>                 csd_lock(csd);
>                 csd->func = func;
>                 csd->info = info;
> 
>                 raw_spin_lock_irqsave(&dst->lock, flags);
>                 list_add_tail(&csd->list, &dst->list);
>                 raw_spin_unlock_irqrestore(&dst->lock, flags);
>         }
> "
> You see "list_add_tail(&csd->list, &dst->list);", it pass the address of csd,
> and A stop before call arch_send_call_function_ipi_mask due interrupt.
> 
> At this time B send ipi to C also, then C will see the csd passed by A,
> then C will clear itself in the cfd->cpumask.

Ah ok! Thank you very much for this clarification :)

Regards
Preeti U Murthy