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

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Date:	Mon, 21 Sep 2009 12:04:50 +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 Sun, 2009-09-20 at 19:55 -0700, Xiao Guangrong wrote:
>> 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.
>>
> 
> I am referring to the missing csd_lock_wait() here that you had in the
> first version of your patch. Let's say, if cpu X is going offline, we
> need to ensure that the smp_call_function() initiated by cpu X (i.e.,
> smp_call_function IPI sent to some other cpu's from cpu X) got serviced
> before cpu X goes offline. We can't do csd_lock_wait() here, as that
> might deadlock (as all the other cpu's are already in stop machine with
> interrupts disabled).
> 

It not happen because the preemption is disabled while send IPI request and
can't schedule to stop machine path, it also stop cpu down.

Thanks,
Xiao
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