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

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

>>>> The "asm volatile" implementation does have exactly the same
>>>> reordering guarantees as the "volatile cast" thing,
>>>
>>> I don't think so.
>>
>> "asm volatile" creates a side effect.
>
> Yeah.
>
>> Side effects aren't
>> allowed to be reordered wrt sequence points.
>
> Yeah.
>
>> This is exactly
>> the same reason as why "volatile accesses" cannot be reordered.
>
> No, the code in that sub-thread I earlier pointed you at *WAS* written
> such that there was a sequence point after all the uses of that 
> volatile
> access cast macro, and _therefore_ we were safe from re-ordering
> (behaviour guaranteed by the C standard).

And exactly the same is true for the "asm" version.

> Now, one cannot fantasize that "volatile asms" are also sequence 
> points.

Sure you can do that.  I don't though.

> In fact such an argument would be sadly mistaken, because "sequence
> points" are defined by the C standard and it'd be horribly wrong to
> even _try_ claiming that the C standard knows about "volatile asms".

That's nonsense.  GCC can extend the C standard any way they
bloody well please -- witness the fact that they added an
extra class of side effects...

>>> Read the relevant GCC documentation.
>>
>> I did, yes.
>
> No, you didn't read:
>
> http://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html
>
> Read the bit about the need for artificial dependencies, and the 
> example
> given there:
>
> 	asm volatile("mtfsf 255,%0" : : "f" (fpenv));
> 	sum = x + y;
>
> The docs explicitly say the addition can be moved before the "volatile
> asm". Hopefully, as you know, (x + y) is an C "expression" and hence
> a "sequence point" as defined by the standard.

The _end of a full expression_ is a sequence point, not every
expression.  And that is irrelevant here anyway.

It is perfectly fine to compute x+y any time before the
assignment; the C compiler is allowed to compute it _after_
the assignment even, if it could figure out how ;-)

x+y does not contain a side effect, you know.

> I know there is also stuff written about "side-effects" there which
> _could_ give the same semantic w.r.t. sequence points as the volatile
> access casts,

s/could/does/

> but hey, it's GCC's own documentation, you obviously can't
> find fault with _me_ if there's wrong stuff written in there. Say that
> to GCC ...

There's nothing wrong there.

> See, "volatile" C keyword, for all it's ill-definition and dodgy
> semantics, is still at least given somewhat of a treatment in the C
> standard (whose quality is ... ummm, sadly not always good and clear,
> but unsurprisingly, still about 5,482 orders-of-magnitude times
> better than GCC docs).

If you find any problems/shortcomings in the GCC documentation,
please file a PR, don't go whine on some unrelated mailing lists.
Thank you.

> Semantics of "volatile" as applies to inline
> asm, OTOH? You're completely relying on the compiler for that ...

Yes, and?  GCC promises the behaviour it has documented.

>>> [ of course, if the (latest) GCC documentation is *yet again*
>>>   wrong, then alright, not much I can do about it, is there. ]
>>
>> There was (and is) nothing wrong about the "+m" documentation, if
>> that is what you are talking about.  It could be extended now, to
>> allow "+m" -- but that takes more than just "fixing" the 
>> documentation.
>
> No, there was (and is) _everything_ wrong about the "+" documentation 
> as
> applies to memory-constrained operands. I don't give a whit if it's
> some workaround in their gimplifier, or the other, that makes it 
> possible
> to use "+m" (like the current kernel code does). The docs suggest
> otherwise, so there's obviously a clear disconnect between the docs and
> actual GCC behaviour.

The documentation simply doesn't say "+m" is allowed.  The code to
allow it was added for the benefit of people who do not read the
documentation.  Documentation for "+m" might get added later if it
is decided this [the code, not the documentation] is a sane thing
to have (which isn't directly obvious).

> [ You seem to often take issue with _amazingly_ petty and pedantic 
> things,
>   by the way :-) ]

If you're talking details, you better get them right.  Handwaving is
fine with me as long as you're not purporting you're not.

And I simply cannot stand false assertions.

You can always ignore me if _you_ take issue with _that_ :-)


Segher