Re: [PATCH 7/10] local_t : powerpc

[Paul Mackerras <paulus@samba.org>]

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.