linux-kernel - Re: [PATCH 1/3] smp/ipi: Remove redundant cfd->cpumask

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Date:	Sat, 6 Jul 2013 14:03:17 +0800
From:	Wang YanQing <udknight@...il.com>
To:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
Cc:	xiaoguangrong@...fujitsu.com, mingo@...e.hu,
	paulmck@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	a.p.zijlstra@...llo.nl, npiggin@...e.de,
	deepthi@...ux.vnet.ibm.com, peterz@...radead.org,
	rusty@...tcorp.com.au, heiko.carstens@...ibm.com,
	rostedt@...dmis.org, miltonm@....com,
	srivatsa.bhat@...ux.vnet.ibm.com, jens.axboe@...cle.com,
	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

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.

Thanks.

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