Re: [PATCH 0/24] make atomic_read() behave consistently across allarchitectures

[Satyam Sharma]

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).
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/