> memory_order_release meaningful, but somewhat different from the
non-fence
it would be nice to have release fence with an artificial dependency
to define a set of actually release stores and not constraining other
subsequent stores (and the order of release stores with respect to
each other), e.g.:
// set multiple flags each indicating 'release' without imposing
// ordering on 'release' stores respect to each other and not
// constraining other subsequent stores
.
.
.
if (atomic_thread_fence(memory_order_release)) {
flag1.store(READY, memory_order_relaxed);
flag2.store(READY, memory_order_relaxed);
}
regards,
alexander.
Hans Boehm <bo...@acm.org>@cs.oswego.edu on 29.11.2014 05:56:04
Sent by: concurrency-interest-boun...@cs.oswego.edu
To: Peter Levart <peter.lev...@gmail.com>
cc: Vladimir Kozlov <vladimir.koz...@oracle.com>, concurrency-interest
<concurrency-inter...@cs.oswego.edu>, Martin Buchholz
<marti...@google.com>, core-libs-dev
<core-libs-dev@openjdk.java.net>, dhol...@ieee.org
Subject: Re: [concurrency-interest] RFR: 8065804: JEP 171:
Clarifications/corrections for fence intrinsics
I basically agree with David's observation. However the C++
atomic_thread_fence(memory_order_acquire)
actually has somewhat different semantics from load(memory_order_acquire).
It basically ensures that prior atomic loads L are not reordered with later
(i.e. following the fence in program order) loads and stores, making it
something like a LoadLoad|LoadStore fence. Thus the fence orders two sets
of operations where the acquire load orders a single operation with respect
to a set. This makes the fence versions of memory_order_acquire and
memory_order_release meaningful, but somewhat different from the non-fence
versions. The terminology is probably not great, but that seems to be the
most common usage now.
On Wed, Nov 26, 2014 at 11:48 PM, Peter Levart <peter.lev...@gmail.com>
wrote:
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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