Satyam Sharma wrote:
On Fri, 17 Aug 2007, Nick Piggin wrote:
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.
That's not obviously just taste to me. Not when the primitive has many
(perhaps, the majority) of uses that do not require said barriers. And
this is not solely about the code generation (which, as Paul says, is
relatively minor even on x86). I prefer people to think explicitly
about barriers in their lockless code.
I'm not saying "taste matters aren't important" (they are), but I'm really
skeptical if most folks would find the former tasteful.
So I /do/ have better taste than most folks? Thanks! :-)
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.
That is _not_ the point (of why a volatile atomic_read is good) because
there
has already been an alternative posted that better conforms with Linux
barrier
API and is much more widely useful and more usable. If you are so
worried about
barrier() being too heavyweight, then you're off to a poor start by
wanting to
add a few K of kernel text by making atomic_read volatile.
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).
If you follow the discussion.... You were thinking of a reason why the
semantics *should* be changed or added, and I was rebutting your argument
that it must be used when a full barrier() is too heavy (ie. by pointing
out that order() has superior semantics anyway).
Why do I keep repeating the same things? I'll not continue bloating this
thread until a new valid point comes up...
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