Re: [PATCH] Document Linux's memory barriers [try #5]

From: Linus Torvalds
Date: Thu Mar 16 2006 - 12:15:56 EST

On Thu, 16 Mar 2006, David Howells wrote:
> More or less, I think; but Nick has raised a good point about whether I should
> be mentioning the existence of things like caching at all in the document,
> except to say that memory barriers can't be assumed to have any effect on them.
> The problem is that if I don't mention caches, I get lots of arguments about
> the effects locking primitives have (since in modern CPUs these happen within
> the caches and not much within memory). I can't just say these things affect
> memory because it's just not necessarily true:-/

Well, the argument I have against mentioning caches is that cache
coherency order is _not_ the only thing that is relevant. As already
mentioned, data speculation can cause exactly the same "non-causal" effect
as cache update ordering, so from a _conceptual_ standpoint, the cache is
really just one implementation detail in the much bigger picture of
buffering and speculative work re-ordering operations...

So it might be a good idea to first explain the memory barriers in a more
abstract sense without talking about what exactly goes on, and then have
the section that gives _examples_ of what the CPU actually is doing that
causes these barriers to be needed. And make it clear that the examples
are just that - examples.

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