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Date:   Thu, 18 Dec 2014 20:06:20 +0100
From:   Ralf Baechle <ralf@linux-mips.org>
To:     David Daney <ddaney.cavm@gmail.com>
Cc:     Markos Chandras <markos.chandras@imgtec.com>,
        linux-mips@linux-mips.org
Subject: Re: [PATCH RFC 24/67] MIPS: asm: spinlock: Replace sub instruction
 with addiu
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On Thu, Dec 18, 2014 at 10:53:05AM -0800, David Daney wrote:

> On 12/18/2014 07:09 AM, Markos Chandras wrote:
> >sub $reg, imm is not a real MIPS instruction. The assembler replaces
> >that with 'addi $reg, -imm'.
> 
> That is a bug right there.  We cannot have faulting instructions like this
> in the kernel.

The instruction is meant to kill the kernel if a readlock gets unlocked
more often than it was taken.  Think of it as an efficient method of
implementing a BUG_ON() for this condition.

I've only seen that overflow exception once and honestly, the primary
reason was my desparate search for a rarely used CPU feature.  So if
this ADDI is repaced by an ADDIU I won't be sad.

  Ralf