linux-kernel - Re: [PATCH 2/2] x86, mce: Add persistent MCE event

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Date:	Thu, 22 Mar 2012 17:27:48 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Borislav Petkov <bp@...64.org>
CC:	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] x86, mce: Add persistent MCE event

On 03/22/2012 05:10 PM, Borislav Petkov wrote:

> On Thu, Mar 22, 2012 at 02:06:29PM +0530, Srivatsa S. Bhat wrote:
>>> +err_unwind:
>>> +	err = -EINVAL;
>>> +	for (--cpu; cpu >= 0; cpu--)
>>> +		perf_rm_persistent_on_cpu(cpu, &per_cpu(mce_ev, cpu));
>>> +
>>
>>
>> *Totally* theoretical question: How do you know that the cpu_online_mask isn't
>> sparse? In other words, what if some CPUs weren't booted? Then this for-loop
>> wouldn't be very good..
>>
>> Oh, now I see that perf_rm_persistent_on_cpu() probably handles that case well..
>> So no issues I guess.. ?
> 
> Right, this could theoretically come around to bite us in some obscure
> cases, so we probably fix it from the get-go.
> 
>> (Moreover, we will probably have bigger issues at hand if some CPU didn't
>> boot..)
>>
>> (The code looked funny, so I thought of pointing it out, whether or not it
>> actually is worrisome. Sorry for the noise, if any).
> 
> Right, no, thanks for pointing it out.
> 
> I'll probably do something like the following:
> 
> 	for (--cpu; cpu >= 0; cpu--)
> 		if (cpu_online(cpu))
> 			perf_rm_persistent_on_cpu(cpu, &per_cpu(mce_ev, cpu));
> 
> to be on the safe side from that perspective.
> 


You can do that or something like the following, to make it more readable:

int cpunum;

for_each_online_cpu(cpunum) {
	if (cpunum == cpu)
		break;
	perf_rm_persistent_on_cpu(cpunum, &per_cpu(mce_ev, cpunum));
}

It is of course, up to you.. whichever form you prefer..

Regards,
Srivatsa S. Bhat

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