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Date: Wed, 04 Jan 2006 00:25:02 +1100
From: Nick Piggin <nickpiggin@yahoo.com.au>
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To: Ingo Molnar <mingo@elte.hu>
CC: lkml <linux-kernel@vger.kernel.org>, Linus Torvalds <torvalds@osdl.org>,
       Andrew Morton <akpm@osdl.org>, Arjan van de Ven <arjan@infradead.org>,
       Nicolas Pitre <nico@cam.org>, Jes Sorensen <jes@trained-monkey.org>,
       Al Viro <viro@ftp.linux.org.uk>, Oleg Nesterov <oleg@tv-sign.ru>,
       David Howells <dhowells@redhat.com>,
       Alan Cox <alan@lxorguk.ukuu.org.uk>,
       Christoph Hellwig <hch@infradead.org>, Andi Kleen <ak@suse.de>,
       Russell King <rmk+lkml@arm.linux.org.uk>
Subject: Re: [patch 04/19] mutex subsystem, add include/asm-i386/mutex.h
References: <20060103100737.GD23289@elte.hu>
In-Reply-To: <20060103100737.GD23289@elte.hu>
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Ingo Molnar wrote:
> From: Arjan van de Ven <arjan@infradead.org>
> 
> 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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