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Date: Sun, 07 Jul 2013 22:15:06 +0530
From: Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To: Wang YanQing <udknight@...il.com>, xiaoguangrong@...fujitsu.com,
mingo@...e.hu, paulmck@...ux.vnet.ibm.com,
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tj@...nel.org, akpm@...ux-foundation.org,
svaidy@...ux.vnet.ibm.com, shli@...nel.org, tglx@...utronix.de,
lig.fnst@...fujitsu.com, anton@...ba.org,
torvalds@...ux-foundation.org, jbeulich@...e.com
Subject: Re: [PATCH 1/3] smp/ipi: Remove redundant cfd->cpumask_ipi mask
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
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