linux-kernel - Re: [PATCH v2 5/7] x86, acpi, cpu-hotplug: Introduce apicid_to_cpuid[] array to store persistent cpuid <-> apicid mapping.

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Date:	Mon, 28 Sep 2015 09:57:14 +0800
From:	Tang Chen <tangchen@...fujitsu.com>
To:	Tejun Heo <tj@...nel.org>
CC:	<jiang.liu@...ux.intel.com>, <mika.j.penttila@...il.com>,
	<mingo@...hat.com>, <akpm@...ux-foundation.org>,
	<rjw@...ysocki.net>, <hpa@...or.com>, <yasu.isimatu@...il.com>,
	<isimatu.yasuaki@...fujitsu.com>, <kamezawa.hiroyu@...fujitsu.com>,
	<izumi.taku@...fujitsu.com>, <gongzhaogang@...pur.com>,
	<qiaonuohan@...fujitsu.com>, <x86@...nel.org>,
	<linux-acpi@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux-mm@...ck.org>, <tangchen@...fujitsu.com>
Subject: Re: [PATCH v2 5/7] x86, acpi, cpu-hotplug: Introduce apicid_to_cpuid[]
 array to store persistent cpuid <-> apicid mapping.


On 09/27/2015 01:56 AM, Tejun Heo wrote:
> On Sat, Sep 26, 2015 at 05:52:09PM +0800, Tang Chen wrote:
>>>> +static int allocate_logical_cpuid(int apicid)
>>>> +{
>>>> +	int i;
>>>> +
>>>> +	/*
>>>> +	 * cpuid <-> apicid mapping is persistent, so when a cpu is up,
>>>> +	 * check if the kernel has allocated a cpuid for it.
>>>> +	 */
>>>> +	for (i = 0; i < max_logical_cpuid; i++) {
>>>> +		if (cpuid_to_apicid[i] == apicid)
>>>> +			return i;
>>>> +	}
>>>> +
>>>> +	/* Allocate a new cpuid. */
>>>> +	if (max_logical_cpuid >= nr_cpu_ids) {
>>>> +		WARN_ONCE(1, "Only %d processors supported."
>>>> +			     "Processor %d/0x%x and the rest are ignored.\n",
>>>> +			     nr_cpu_ids - 1, max_logical_cpuid, apicid);
>>>> +		return -1;
>>>> +	}
>>> So, the original code didn't have this failure mode, why is this
>>> different for the new code?
>> It is not different. Since max_logical_cpuid is new, this is ensure it won't
>> go beyond NR_CPUS.
> If the above condition can happen, the original code should have had a
> similar check as above, right?  Sure, max_logical_cpuid is a new thing
> but that doesn't seem to change whether the above condition can happen
> or not, no?

Right, indeed. It is in

generic_processor_info()
|--> if (num_processors >= nr_cpu_ids)

Will remove my new added check.

Thanks.

>
> Thanks.
>

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