linux-kernel - Re: semaphore: lockless fastpath using atomic_{inc,dec}

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Message-ID: <20080709085018.3a76d1e0@infradead.org>
Date:	Wed, 9 Jul 2008 08:50:18 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	Bruno Santos <bsantos@...it.pt>
Cc:	linux-kernel@...r.kernel.org,
	Christoph Lameter <cl@...ux-foundation.org>
Subject: Re: semaphore: lockless fastpath using atomic_{inc,dec}_return

On Wed, 09 Jul 2008 16:39:43 +0100
Bruno Santos <bsantos@...it.pt> wrote:

> Hi,
> 
>  >hi,
>  >
>  >not to ruin the party but... how is this lockless? An atomic
>  >variable is every bit a "lock" as a spinlock is... and very much
>  >equally expensive as well for most cases ;-(
> 
> Perhaps not the best the choice of words, I should have omitted the
> word lockless. But it seems my understanding of lockless and yours is
> different. And indeed, it's very expensive as a spinlock, but
> comparatively, is only one operation, that if successful doesn't have
> to lock and then unlock (that's why I called it lockless ...).

ok I only come from an Intel/x86 background, where unlock is basically
free, and the "lock" is exactly the same cost as an atomic op.
(in fact, an atomic op and a lock are the exact same code.. you're just
open coding it)

> The mutex takes the same approach, however it uses it's own flavour
> of atomic ops. What I'm really interested is if this brings any
> benefit in terms of performance.

on x86... I would highly doubt it since you have the same number of
atomic operations. (it's not the lock that is expensive. ever. it's
always the fact that a lock implies an atomic operation that makes it
expensive)
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