Re: [patch 04/19] mutex subsystem, add include/asm-i386/mutex.h

[Nick Piggin]

Ingo Molnar wrote:
> From: Arjan van de Ven <arjan@xxxxxxxxxxxxx>
>
> add the i386 version of mutex.h, optimized in assembly.

> +static inline int
> +__mutex_fastpath_trylock(atomic_t *count, int (*fail_fn)(atomic_t *))
> +{
> +	/*
> +	 * We have two variants here. The cmpxchg based one is the best one
> +	 * because it never induce a false contention state.  It is included
> +	 * here because architectures using the inc/dec algorithms over the
> +	 * xchg ones are much more likely to support cmpxchg natively.
> +	 *
> +	 * If not we fall back to the spinlock based variant - that is
> +	 * just as efficient (and simpler) as a 'destructive' probing of
> +	 * the mutex state would be.
> +	 */
> +#ifdef __HAVE_ARCH_CMPXCHG
> +	if (likely(atomic_cmpxchg(count, 1, 0)) == 1)
> +		return 1;
> +	return 0;
> +#else
> +	return fail_fn(count);
> +#endif
> +}

asm-i386 version I think really should just use atomic_cmpxchg unconditionally,
because otherwise an i386 compatible kernel will not use cmpxchg even when
running on 486+ (not sure how important that is these days, but still...).

--
SUSE Labs, Novell Inc.

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