linux-kernel - Re: Why Semaphore Hardware-Dependent?

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Message-ID: <21013.1156870656@warthog.cambridge.redhat.com>
Date:	Tue, 29 Aug 2006 17:57:36 +0100
From:	David Howells <dhowells@...hat.com>
To:	Christoph Lameter <clameter@....com>
Cc:	David Howells <dhowells@...hat.com>,
	Ralf Baechle <ralf@...ux-mips.org>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Arjan van de Ven <arjan@...radead.org>,
	Dong Feng <middle.fengdong@...il.com>, ak@...e.de,
	Paul Mackerras <paulus@...ba.org>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: Why Semaphore Hardware-Dependent? 

Christoph Lameter <clameter@....com> wrote:

> > Some of these have LL/SC or equivalent instead, but ARM5 and before, FRV,
> > M68K before 68020 to name but a few.
> 
> This is all pretty ancient hardware, right? And they are mostly single 
> processor so no need to worry about concurrency. Just disable interrupts.

No, they're not all ancient h/w, and "just disabling interrupts" can be really
expensive.

> > And anything that implements CMPXCHG with spinlocks is a really bad
> > candidate for CMPXCHG-based rwsems.
> 
> Those will optimize out if it is a single processor configuration.

Not necessarily.  Consider preemption.

David
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