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

[Segher Boessenkool <segher@kernel.crashing.org>]

>>> No it does not have any volatile semantics. atomic_dec() can be 
>>> reordered
>>> at will by the compiler within the current basic unit if you do not 
>>> add a
>>> barrier.
>>
>> "volatile" has nothing to do with reordering.
>
> If you're talking of "volatile" the type-qualifier keyword, then
> http://lkml.org/lkml/2007/8/16/231 (and sub-thread below it) shows
> otherwise.

I'm not sure what in that mail you mean, but anyway...

Yes, of course, the fact that "volatile" creates a side effect
prevents certain things from being reordered wrt the atomic_dec();
but the atomic_dec() has a side effect *already* so the volatile
doesn't change anything.

>> atomic_dec() writes
>> to memory, so it _does_ have "volatile semantics", implicitly, as
>> long as the compiler cannot optimise the atomic variable away
>> completely -- any store counts as a side effect.
>
> I don't think an atomic_dec() implemented as an inline "asm volatile"
> or one that uses a "forget" macro would have the same re-ordering
> guarantees as an atomic_dec() that uses a volatile access cast.

The "asm volatile" implementation does have exactly the same
reordering guarantees as the "volatile cast" thing, if that is
implemented by GCC in the "obvious" way.  Even a "plain" asm()
will do the same.


Segher