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

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Message-ID: <17847.8700.118329.777644@cargo.ozlabs.ibm.com>
Date:	Wed, 24 Jan 2007 20:08:12 +1100
From:	Paul Mackerras <paulus@...ba.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	linux-kernel@...r.kernel.org, Andrew Morton <akpm@...l.org>,
	Ingo Molnar <mingo@...hat.com>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Christoph Hellwig <hch@...radead.org>,
	"Martin J. Bligh" <mbligh@...igh.org>, linuxppc-dev@...abs.org,
	Douglas Niehaus <niehaus@...s.ku.edu>, ltt-dev@...fik.org,
	systemtap@...rces.redhat.com, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 7/10] local_t : powerpc

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?

Paul.
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