linux-kernel - Re: [PATCH 7/10] local

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Message-ID: <20070124104313.GB22579@iram.es>
Date:	Wed, 24 Jan 2007 11:43:13 +0100
From:	Gabriel Paubert <paubert@...m.es>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Andrew Morton <akpm@...l.org>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	linux-kernel@...r.kernel.org,
	"Martin J. Bligh" <mbligh@...igh.org>,
	Christoph Hellwig <hch@...radead.org>, linuxppc-dev@...abs.org,
	Douglas Niehaus <niehaus@...s.ku.edu>,
	Ingo Molnar <mingo@...hat.com>, ltt-dev@...fik.org,
	systemtap@...rces.redhat.com, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 7/10] local_t : powerpc

On Wed, Jan 24, 2007 at 08:08:12PM +1100, Paul Mackerras wrote:
> Mathieu Desnoyers writes:
> 
> > +static __inline__ int local_dec_if_positive(local_t *l)
> > +{
> > +	int t;
> > +
> > +	__asm__ __volatile__(
> > +"1:	lwarx	%0,0,%1		# local_dec_if_positive\n\
> > +	addic.	%0,%0,-1\n\
> > +	blt-	2f\n"
> > +	PPC405_ERR77(0,%1)
> > +"	stwcx.	%0,0,%1\n\
> > +	bne-	1b"
> 
> This has the same bugs that we fixed recently in atomic_dec_if_positive;
> first, on 64-bit machines, the lwarx will zero-extend the word loaded
> from memory, and so the result of the addic will be negative only if
> the word was originally 0.  Secondly, even on 32-bit machines,
> 0x80000000 will be considered positive since decrementing it gives
> 0x7fffffff, which is positive.
> 
> > +/* Use these for per-cpu local_t variables: on some archs they are
> > + * much more efficient than these naive implementations.  Note they take
> > + * a variable, not an address.
> > + *
> > + * This could be done better if we moved the per cpu data directly
> > + * after GS.
> > + */
> 
> What's "GS"?  Does this comment really apply on powerpc?
> 

1) It's an (application visible) i386/x86_64 segment register used
   to make memory addressing more confusing.
2) Because of 1), obviously not ;-)

	Gabriel
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