linux-kernel - Re: [PATCH] percpu: fix unit_map[] verification in pcpu_setup_first

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Date:	Tue, 29 Sep 2009 01:53:06 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Christoph Lameter <cl@...ux-foundation.org>
CC:	Tony Vroon <tony@...x.net>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] percpu: fix unit_map[] verification in pcpu_setup_first_chunk()

Christoph Lameter wrote:
> On Tue, 29 Sep 2009, Tejun Heo wrote:
> 
>> Christoph, can you please elaborate why nr_cpu_ids < NR_CPUS is
>> broken?
> 
> If its just an internal number that becomes larger than NR_CPUS then
> everything is fine.
> 
> But if a real cpu id (returned by smp_processor_id()) gets larger than
> NR_CPUS then we get into trouble with the parts of the kernel that index
> by cpu id.

It's a unit number.  Each cpu has exactly one unit number assigned to
it but there can be holes, so unit number can legally go over both
nr_cpu_ids and NR_CPUS, so yeap, it's just an internal number used
inside percpu allocator.

Tony, the original patch should be correct.  Please verify.

Thanks.

-- 
tejun
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