Subject: Re: +
 generic-ipi-fix-the-race-between-generic_smp_call_function_-and-hotplug_cfd.patch
 added to -mm tree
From: Suresh Siddha <suresh.b.siddha@intel.com>
Reply-To: Suresh Siddha <suresh.b.siddha@intel.com>
To: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
       "akpm@linux-foundation.org" <akpm@linux-foundation.org>,
       "mm-commits@vger.kernel.org" <mm-commits@vger.kernel.org>,
       "jens.axboe@oracle.com" <jens.axboe@oracle.com>,
       "mingo@elte.hu" <mingo@elte.hu>,
       "nickpiggin@yahoo.com.au" <nickpiggin@yahoo.com.au>,
       "rusty@rustcorp.com.au" <rusty@rustcorp.com.au>,
       LKML <linux-kernel@vger.kernel.org>
In-Reply-To: <4AB6EB1C.5090106@cn.fujitsu.com>
References: <200907310030.n6V0Uqgw001644@imap1.linux-foundation.org>
	 <1252616988.7205.102.camel@laptop>  <4AAA0001.2060703@cn.fujitsu.com>
	 <1252696132.3756.21.camel@sbs-t61.sc.intel.com>
	 <4AADEF2F.5080504@cn.fujitsu.com>
	 <1252973802.2899.88.camel@sbs-t61.sc.intel.com>
	 <4AAEF5DC.4070308@cn.fujitsu.com>
	 <1253067602.2667.13.camel@sbs-t61.sc.intel.com>
	 <4AB1A652.1010000@cn.fujitsu.com>
	 <1253326599.3948.732.camel@sbs-t61.sc.intel.com>
	 <4AB6EB1C.5090106@cn.fujitsu.com>
Content-Type: text/plain
Organization: Intel Corp
Date: Sun, 20 Sep 2009 20:11:22 -0700
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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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