How to use 'lock' prefix to make instruction atomic (in amulti-threaded program)

#11 09-07-2008, 07:33 PM

In article <20080824134656.2f1018d1@thor.crayne.org>,
spamtrap@crayne.org says...
> On Sun, 24 Aug 2008 20:21:26 +0300
> Phil Carmody <thefatphil_demunged@yahoo.co.uk> wrote:
>
> > I've known architectures
> > where the two halves of the fetch from memory (or should I say
> > two fetches from memory) would leave room for a race condition,
> > and currently have no reason to believe that x86 would be any
> > different.
>
> In the 8086/8088 days, this was a potential problem, but since the
> introduction of cache memory, the designers have provided ways to
> synchronize the caches on two or more processors, and all reads,
> and most writes, are atomic. Only those instructions which internally
> perform a read->update->write cycle can make use of the LOCK prefix,
> although there may be assemblers which will silently discard the
> prefix when it is used inappropriately. This has been true since at
> least the introduction of the 80386.

This isn't really true, but it can look that way most of the time. It's
all really a question of HOW badly misaligned an item can get before it
gets read and/or written in a non-atomic fashion.

On a reasonably current machine, if an item crosses a cache line
boundary, it won't be read/written atomically.

--
Later,
Jerry.

The universe is a figment of its own imagination.

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