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

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

> Of course, since *normal* accesses aren't necessarily limited wrt
> re-ordering, the question then becomes one of "with regard to *what* 
> does
> it limit re-ordering?".
>
> A C compiler that re-orders two different volatile accesses that have a
> sequence point in between them is pretty clearly a buggy compiler. So 
> at a
> minimum, it limits re-ordering wrt other volatiles (assuming sequence
> points exists). It also means that the compiler cannot move it
> speculatively across conditionals, but other than that it's starting to
> get fuzzy.

This is actually really well-defined in C, not fuzzy at all.
"Volatile accesses" are a side effect, and no side effects can
be reordered with respect to sequence points.  The side effects
that matter in the kernel environment are: 1) accessing a volatile
object; 2) modifying an object; 3) volatile asm(); 4) calling a
function that does any of these.

We certainly should avoid volatile whenever possible, but "because
it's fuzzy wrt reordering" is not a reason -- all alternatives have
exactly the same issues.


Segher

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