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

[Satyam Sharma <satyam@infradead.org>]

On Fri, 17 Aug 2007, Nick Piggin wrote:

> 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).

See, you do *require* people to have to repeat the same things to you!

As has been written about enough times already, and if you followed the
discussion on this thread, I am *not* proposing that atomic_read()'s
semantics be changed to have any extra barriers. What is proposed is a
different atomic_read_xxx() variant thereof, that those can use who do
want that.

Now whether to have a kind of barrier ("volatile", whatever) in the
atomic_read_xxx() itself, or whether to make the code writer himself to
explicitly write the order(x) appropriately in appropriate places in the
code _is_ a matter of taste.


> > That's definitely the point, why not. This is why "barrier()", being
> > heavy-handed, is not the best option.
> 
> That is _not_ the point [...]

Again, you're requiring me to repeat things that were already made evident
on this thread (if you follow it).

This _is_ the point, because a lot of loops out there (too many of them,
I WILL NOT bother citing file_name:line_number) end up having to use a
barrier just because they're using a loop-exit-condition that depends
on a value returned by atomic_read(). It would be good for them if they
used an atomic_read_xxx() primitive that gave these "volatility" semantics
without junking compiler optimizations for other memory references.

> because there has already been an alternative posted

Whether that alternative (explicitly using forget(x), or wrappers thereof,
such as the "order_atomic" you proposed) is better than other alternatives
(such as atomic_read_xxx() which includes the volatility behaviour in
itself) is still open, and precisely what we started discussing just one
mail back.

(The above was also mostly stuff I had to repeated for you, sadly.)

> that better conforms with Linux barrier
> API and is much more widely useful and more usable.

I don't think so.

(Now *this* _is_ the "taste-dependent matter" that I mentioned earlier.)

> 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.

Repeating myself, for the N'th time, NO, I DON'T want to make atomic_read
have "volatile" semantics.

> > > 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).

Amazing. Either you have reading comprehension problems, or else, please
try reading this thread (or at least this sub-thread) again. I don't want
_you_ blaming _me_ for having to repeat things to you all over again.