[concurrency-interest] DirectByteBuffers and reachabilityFence
David M. Lloyd
david.lloyd at redhat.com
Tue Dec 8 10:22:49 EST 2015
On 12/08/2015 08:40 AM, Vitaly Davidovich wrote:
> The lifetime, natural or otherwise, of an instance does not survive
> until an instance method returns because, a lot of the time, that
> instance method is inlined.
> You're talking about optimization here (inlining); by "natural" I meant
> the naive/no optimization case (e.g. interpreter, debugger attached
> w/breakpoint in method, etc).
> It's not just HotSpot, though: some VMs are even more aggressive, and
> Which java VMs are these? Just curious.
> we have seen finalizers executed even before constructors have
> completed. And that is allowed by the specification.
> Ok, but that's beside the point, really. Surely if compiler can
> optimize and arrange for liveness to allow for it, then it's a good
> thing it does that. My point isn't that this cannot happen due to spec,
> but rather that in places like DBB where `this` is used after the Unsafe
> call the compiler has to schedule things differently in order to reduce
> lifetime. And my point is that compilers generally tend to be cautious
> in doing things that may break code. This is the practical aspect we
> were referring to - it's actual humans writing these optimizations, and
> they're sensitive to breaking code, particularly in java.
> Theoretically, yes, anything is possible.
> It's already broken.
> Sure. Now try to submit a patch to Hotspot that will break this case,
> even if allowed by spec, and see how far you get :).
If you're talking about simply observing the effects of an object being
collected while method invocations on that object are still in flight,
see this article:
We have run into this issue numerous times in various situations, which
is why we're happy to see reachabilityFence() come into being. So yes,
it's already broken.
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