linux-kernel - PROBLEM: Off-by-one bug on sun4u SMP

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Message-ID: <AANLkTimw2Hzv1WIJXehqMo4450ftRJWwefQipDM9Zxb4@mail.gmail.com>
Date:	Thu, 10 Jun 2010 11:05:09 -0700
From:	Ryan Davis <iconoclasmandheresy@...il.com>
To:	linux-kernel <linux-kernel@...r.kernel.org>
Subject: PROBLEM: Off-by-one bug on sun4u SMP

On at least some sun4u machines, the CPU numbering starts at one not 0.
This causes an off-by-one bug as other parts of the code assume
zero-based numbering.

If you set max-cpus in the kernel config to the actual number of CPUs,
the last CPU will be ignored and unused.
This is because the CPU numbering starts at 1 but the code to check
against max-cpus assumes a zero-based numbering.

On my computer: Sun Ultra 60 2x450mhz Ultrasparc.
Building with max-cpus of 2 ignores the second cpu because 2 (one
based cpu number) >= 2 (zero based max cpus)
Rebuilding with a larger max-cpus is a workaround but non optimal.
---
Those who will not reason, are bigots, those who cannot, are fools,
and those who dare not, are slaves.
 -- Lord Byron
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