linux-kernel - Re: [PATCH 1/2] lib: Provide generic atomic64

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Message-ID: <18996.60235.178618.531664@cargo.ozlabs.ibm.com>
Date:	Sun, 14 Jun 2009 22:21:31 +1000
From:	Paul Mackerras <paulus@...ba.org>
To:	Avi Kivity <avi@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	benh@...nel.crashing.org, akpm@...ux-foundation.org,
	linuxppc-dev@...abs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] lib: Provide generic atomic64_t implementation

Avi Kivity writes:

> An alternative implementation using 64-bit cmpxchg will recover most of 
> the costs of hashed spinlocks.  I assume most serious 32-bit 
> architectures have them?

Have a 64-bit cmpxchg, you mean?  x86 is the only one I know of, and
it already has an atomic64_t implementation using cmpxchg8b (or
whatever it's called).

My thinking is that the 32-bit non-x86 architectures will be mostly
UP, so the overhead is just an interrupt enable/restore.  Those that
are SMP I would expect to be small SMP -- mostly just 2 cpus and maybe
a few 4-way systems.

Paul.
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