On 11/27/2014 04:00 AM, David Holmes wrote:
Can I make an observation about acquire() and release() - to me they are
meaningless when considered in isolation. Given their definitions they allow
anything to move into a region bounded by acquire() and release(), then you
can effectively move the whole program into the region and thus the
acquire() and release() do not constrain any reorderings.
acquire() and
release() only make sense when their own movement is constrained with
respect to something else - such as lock acquisition/release, or when
combined with specific load/store actions.
...or another acquire/release region?
Regards, Peter
David
Martin Buchholz writes:
On Tue, Nov 25, 2014 at 6:04 AM, Paul Sandoz
<paul.san...@oracle.com> wrote:
Hi Martin,
Thanks for looking into this.
1141 * Currently hotspot's implementation of a Java
language-level volatile
1142 * store has the same effect as a storeFence followed
by a relaxed store,
1143 * although that may be a little stronger than needed.
IIUC to emulate hotpot's volatile store you will need to say
that a fullFence immediately follows the relaxed store.
Right - I've been groking that.
The bit that always confuses me about release and acquire is
ordering is restricted to one direction, as talked about in
orderAccess.hpp [1]. So for a release, accesses prior to the
release cannot move below it, but accesses succeeding the release
can move above it. And that seems to apply to Unsafe.storeFence
[2] (acting like a monitor exit). Is that contrary to C++ release
fences where ordering is restricted both to prior and succeeding
accesses? [3]
So what about the following?
a = r1; // Cannot move below the fence
Unsafe.storeFence();
b = r2; // Can move above the fence?
I think the hotspot docs need to be more precise about when they're
talking about movement of stores and when about loads.
// release. I.e., subsequent memory accesses may float above the
// release, but prior ones may not float below it.
As I've said elsewhere, the above makes no sense without restricting
the type of access.
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