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Date:   Wed, 21 Jan 2015 00:58:48 +0000 (GMT)
From:   "Maciej W. Rozycki" <macro@linux-mips.org>
To:     Ralf Baechle <ralf@linux-mips.org>
cc:     David Daney <ddaney.cavm@gmail.com>,
        Markos Chandras <Markos.Chandras@imgtec.com>,
        linux-mips@linux-mips.org,
        Matthew Fortune <Matthew.Fortune@imgtec.com>
Subject: Re: [PATCH RFC v2 24/70] MIPS: asm: spinlock: Replace sub instruction
 with addiu
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Ralf,

> > According to a comment on another thread from Ralf, this has been observed
> > in the wild only once.  We can simplify the code and remove that comment.
> > Why not just use the ADDIU and be done with it?
> > 
> > There are many locking and atomic primitives that don't have any such error
> > checking.  What makes the read lock so special that it needs this extra
> > protection?
> 
> Because I was desparate to find a use for the signed add ;-)
> 
> Honestly, it's nice to have such a safeguard if it's available at no
> runtime overhead at all but these days are such nice lock debugging tools
> that the loss won't be missed.  So (cut'n'paste):
> 
> Why not just use the ADDIU and be done with it?

 Given David's comment I meant to defer to you on this as the originator 
of this bit, but since you have spoken, I think we've come to a conclusion 
now. :)

  Maciej