Re: [RFC][PATCH 0/5] arch: atomic rework

[Linus Torvalds]

On Tue, Feb 18, 2014 at 8:17 AM, Torvald Riegel <triegel@xxxxxxxxxx> wrote:
>>
>>   "Consume operation: no reads in the current thread dependent on the
>> value currently loaded can be reordered before this load"
>
> I can't remember seeing that language in the standard (ie, C or C++).
> Where is this from?

That's just for googling for explanations. I do have some old standard
draft, but that doesn't have any concise definitions anywhere that I
could find.

>> and it could make a compiler writer say that value speculation is
>> still valid, if you do it like this (with "ptr" being the atomic
>> variable):
>>
>>   value = ptr->val;
>
> I assume the load from ptr has mo_consume ordering?

Yes.

>> into
>>
>>   tmp = ptr;
>>   value = speculated.value;
>>   if (unlikely(tmp != &speculated))
>>     value = tmp->value;
>>
>> which is still bogus. The load of "ptr" does happen before the load of
>> "value = speculated->value" in the instruction stream, but it would
>> still result in the CPU possibly moving the value read before the
>> pointer read at least on ARM and power.
>
> And surprise, in the C/C++ model the load from ptr is sequenced-before
> the load from speculated, but there's no ordering constraint on the
> reads-from relation for the value load if you use mo_consume on the ptr
> load.  Thus, the transformed code has less ordering constraints than the
> original code, and we arrive at the same outcome.

Ok, good.

> The standard is clear on what's required.  I strongly suggest reading
> the formalization of the memory model by Batty et al.

Can you point to it? Because I can find a draft standard, and it sure
as hell does *not* contain any clarity of the model. It has a *lot* of
verbiage, but it's pretty much impossible to actually understand, even
for somebody who really understands memory ordering.

             Linus
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