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:	Sat, 26 Sep 2015 13:56:22 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Tang Chen <tangchen@...fujitsu.com>
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
Subject: Re: [PATCH v2 5/7] x86, acpi, cpu-hotplug: Introduce
 apicid_to_cpuid[] array to store persistent cpuid <-> apicid mapping.

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?

Thanks.

-- 
tejun
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