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

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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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