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

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Date:	Sun, 20 Sep 2009 20:11:22 -0700
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	Xiao Guangrong <xiaoguangrong@...fujitsu.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

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).

> > 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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