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Date: Mon, 21 Jan 2002 08:36:26 +1100
From: Peter Jeremy <peter.jeremy@alcatel.com.au>
Subject: Re: 64 bit counters again
In-reply-to: <200201202125.g0KLPqf31593@khavrinen.lcs.mit.edu>; from
 wollman@khavrinen.lcs.mit.edu on Sun, Jan 20, 2002 at 04:25:52PM -0500
To: Garrett Wollman <wollman@khavrinen.lcs.mit.edu>
Cc: Michal Mertl <mime@traveller.cz>, arch@FreeBSD.ORG
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On 2002-Jan-20 16:25:52 -0500, Garrett Wollman <wollman@khavrinen.lcs.mit.edu> wrote:
>The ultimate upshot of this is that it does not make sense to expose
>low-level primitives (such as compare-exchange or LL/SC) directly in
>the API.  This is because each processor architecture will have its
>own requirements for coherency, in some cases requiring specialized
>instructions which a compiler would not normally generate, and often
>requiring memory barriers inside the spin loop.

Agreed.

>However, we can easily define a set of basic operations which every
>architecture we currently support can implement reliably and
>coherently without locking, including incrementing counters and
                    ^^^^^^^ protecting the operation by a separate
                            mutex or semaphore-style lock.
>updating some kinds of linked lists.

Some form of inter-processor locking is required, but the low=level
primitives implicitly achieve this.

We already have a MI API for much of this in /sys/<arch>/include/atomic.h

Peter

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