Re: [Qemu-devel] Java volatile vs. C11 seq_cst (was Re: [PATCH v2 1/2] add a header file for atomic operations)

[Torvald Riegel <triegel@redhat.com>]

On Tue, 2013-06-18 at 18:08 +0200, Paolo Bonzini wrote:
> Il 18/06/2013 16:50, Paul E. McKenney ha scritto:
> > PS:  Nevertheless, I personally prefer the C++ formulation, but that is
> >      only because I stand with one foot in theory and the other in
> >      practice.  If I were a pure practitioner, I would probably strongly
> >      prefer the Java formulation.
> 
> Awesome answer, and this last paragraph sums it up pretty well.

I disagree that for non-Java code the Java model should be better.  Both
C11 and C++11 use the same model, and I don't see a reason to not use it
if you're writing C/C++ code anyway.

The C++ model is definitely useful for practitioners; just because it
uses seq-cst memory order as safe default doesn't mean that programmers
that can deal with weaker ordering guarantees can't make use of those
weaker ones.
I thought Paul was referring to seq-cst as default; if that wasn't the
point he wanted to make, I actually don't understand his theory/practice
comparison (never mind that whenever you need to reason about concurrent
stuff, having a solid formal framework as the one by the Cambridge group
is definitely helpful).  Seq-cst and acq-rel are just different
guarantees -- this doesn't mean that one is better than the other; you
need to understand anyway what you're doing and which one you need.
Often, ensuring a synchronized-with edge by pairing release/acquire will
be sufficient, but that doesn't say anything about the Java vs. C/C++
model.

> That was basically my understanding, too.  I still do not completely 
> get the relationship between Java semantics and ACQ_REL, but I can 
> sidestep the issue for adding portable atomics to QEMU.  QEMU 
> developers and Linux developers have some overlap, and Java volatiles 
> are simple to understand in terms of memory barriers (which Linux 
> uses); hence, I'll treat ourselves as pure practitioners.

I don't think that this is the conclusion here.  I strongly suggest to
just go with the C11/C++11 model, instead of rolling your own or trying
to replicate the Java model.  That would also allow you to just point to
the C11 model and any information / tutorials about it instead of having
to document your own (see the patch), and you can make use of any
(future) tool support (e.g., race detectors).

> I will just not use __atomic_load/__atomic_store to implement the 
> primitives, and always express them in terms of memory barriers.

Why?  (If there's some QEMU-specific reason, just let me know; I know
little about QEMU..)
I would assume that using the __atomic* builtins is just fine if they're
available.


Torvald