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Message-ID: <alpine.LFD.0.999.0708171612440.3666@enigma.security.iitk.ac.in>
Date: Fri, 17 Aug 2007 16:25:14 +0530 (IST)
From: Satyam Sharma <satyam@...radead.org>
To: Nick Piggin <piggin@...erone.com.au>
cc: Herbert Xu <herbert@...dor.apana.org.au>,
Paul Mackerras <paulus@...ba.org>,
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Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all
architectures
On Fri, 17 Aug 2007, Nick Piggin wrote:
> Satyam Sharma wrote:
> > [...]
> > The point is about *author expecations*. If people do expect atomic_read()
> > (or a variant thereof) to have volatile semantics, why not give them such
> > a variant?
>
> Because they should be thinking about them in terms of barriers, over
> which the compiler / CPU is not to reorder accesses or cache memory
> operations, rather than "special" "volatile" accesses.
This is obviously just a taste thing. Whether to have that forget(x)
barrier as something author should explicitly sprinkle appropriately
in appropriate places in the code by himself or use a primitive that
includes it itself.
I'm not saying "taste matters aren't important" (they are), but I'm really
skeptical if most folks would find the former tasteful.
> > And by the way, the point is *also* about the fact that cpu_relax(), as
> > of today, implies a full memory clobber, which is not what a lot of such
> > loops want. (due to stuff mentioned elsewhere, summarized in that summary)
>
> That's not the point,
That's definitely the point, why not. This is why "barrier()", being
heavy-handed, is not the best option.
> because as I also mentioned, the logical extention
> to Linux's barrier API to handle this is the order(x) macro. Again, not
> special volatile accessors.
Sure, that forget(x) macro _is_ proposed to be made part of the generic
API. Doesn't explain why not to define/use primitives that has volatility
semantics in itself, though (taste matters apart).
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