linux-kernel - Re: + generic-ipi-fix-the-race-between-generic_smp_call_function_-and-hotplug

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Date:	Mon, 21 Sep 2009 10:55:24 +0800
From:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"mm-commits@...r.kernel.org" <mm-commits@...r.kernel.org>,
	"jens.axboe@...cle.com" <jens.axboe@...cle.com>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"nickpiggin@...oo.com.au" <nickpiggin@...oo.com.au>,
	"rusty@...tcorp.com.au" <rusty@...tcorp.com.au>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: + generic-ipi-fix-the-race-between-generic_smp_call_function_-and-hotplug_cfd.patch
 added to -mm tree



Suresh Siddha wrote:
> On Wed, 2009-09-16 at 20:00 -0700, Xiao Guangrong wrote: 
>> How about manual check/handle pending IPI interruption in the CPU context?
>> like this:
>> --- a/kernel/cpu.c
>> +++ b/kernel/cpu.c
>> @@ -173,6 +173,9 @@ static int __ref take_cpu_down(void *_param)
>>  	struct take_cpu_down_param *param = _param;
>>  	int err;
>>  
>> +	generic_smp_call_function_interrupt();
>> +	generic_smp_call_function_single_interrupt();
>> +
> 
> At this place, how will you ensure that the smp_call_function initiated
> by this dying cpu has reached and got serviced at its destination?
> 

Suresh, sorry for my poor English, Do you mean that how we ensure it has
pending IPI request in the dying cpu?

generic_smp_call_function_*() will check it, if the cpu has pending request,
then handle it, else directly return.

> All the other cpu's have disabled interrupts in the stop machine state
> by the time we come here and we can't wait.
> 

Why we can't wait? It manual check/handle the pending IPI request not wait
interruption happen.

It not has race here because all cpu's interruption is disabled, and it not
make stop machine slow because only the dying cpu can enter take_cpu_down(),
we just wait the dying cpu handle it's pending request. 

Am I misunderstand something?

Thanks,
Xiao
 
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