Messages in this thread | | | Date | Tue, 18 Feb 2014 09:44:48 -0800 | Subject | Re: [RFC][PATCH 0/5] arch: atomic rework | From | Linus Torvalds <> |
| |
On Tue, Feb 18, 2014 at 8:17 AM, Torvald Riegel <triegel@redhat.com> 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
| |