[concurrency-interest] JSR-133 Cookbook and exit monitor

Oleksandr Otenko oleksandr.otenko at oracle.com
Thu Oct 9 09:04:37 EDT 2014


It seems sensible to coarsen the lock by consuming any sequence of 
instructions that will cost less than lock release + acquire.

It makes sense to coarsen the lock by extending the section until a lock 
release for another lock, if the sequence of instructions is short enough.

Alex

On 08/10/2014 19:23, Vitaly Davidovich wrote:
> That's fine -- if hardware wants to reorder, no issues (and it 
> typically does that for efficiency anyway).  My curiosity is about 
> compiler-only transformation, as I'm interested in seeing what that 
> code would look like.
>
> On Wed, Oct 8, 2014 at 2:12 PM, Hans Boehm <boehm at acm.org 
> <mailto:boehm at acm.org>> wrote:
>
>     The original transformation, moving a trailing "y = 43" into a
>     critical section, can typically happen on a weakly ordered
>     architecture like ARM as a result of hardware reordering.   On
>     such an architecture, an ordinary lock release is essentially a
>     fence followed by an unordered CAS (or on ARMv8, a CAS with
>     release semantics).  There is nothing to prevent a later store to
>     y from becoming visible before the CAS releasing the lock.  No
>     compiler transformations required ...
>
>     Hans
>
>     On Wed, Oct 8, 2014 at 7:34 AM, Vitaly Davidovich
>     <vitalyd at gmail.com <mailto:vitalyd at gmail.com>> wrote:
>
>         Yes, as Doug pointed out, the "roach motel" semantics is just
>         an allowed reordering -- compiler will hopefully not stuff
>         more things into a critical section unless it has strong
>         evidence that it's worthwhile.  I'd actually be curious to
>         hear from anyone that's seen such a transformation.
>
>
>
>
>
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