[concurrency-interest] JLS 17.7 Non-atomic treatment of double and long : Android

Tim Halloran hallorant at gmail.com
Mon Apr 29 16:54:53 EDT 2013

Thanks Aleksey, can you elaborate on how to reproduce this in the wild?

Are you thinking of writing xFFFFFFFF_FFFFFFFF (or some known bit
pattern/long value) and x00000000_0000000 (another known bit pattern/long
value) from two different threads and seeing if you get x00000000_FFFFFFFF
or xFFFFFFFF_00000000 -- or did you have another scheme in mind.  Best.

On Mon, Apr 29, 2013 at 4:42 PM, Aleksey Shipilev <
aleksey.shipilev at oracle.com> wrote:

> On 04/30/2013 12:27 AM, Tim Halloran wrote:
> > Does anyone know if Android's Dalvik goes with the recommendation in JLS
> > 17.7 and avoids splitting 64-bit values. I'm pretty sure most other VMs
> do.
> You might safely assume ARM does not have full-width 64-bit reads/writes
> (it can emulate atomic read/write with LL/SC-ed loop), so I'm pretty
> sure non-volatile longs are not atomic on most mobile platforms.
> > After some searching I couldn't find any information and it seems pretty
> > hard to write code to "test" for this on the VM.
> It is actually one of the simplest tests for concurrency, and simple
> enough to reproduce in the wild.
> -Aleksey.
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