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8385903: G1: G1CollectionSet::_num_regions needs to be Atomic

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Reviewed-by: stefank
This commit is contained in:
Thomas Schatzl 2026-06-29 08:52:38 +00:00
parent 5f1355b085
commit 78112cbcaa
2 changed files with 13 additions and 12 deletions
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16
src/hotspot/share/gc/g1/g1CollectionSet.cpp
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@ -33,7 +33,6 @@
#include "gc/g1/g1ParScanThreadState.hpp"
#include "gc/g1/g1Policy.hpp"
#include "logging/logStream.hpp"
#include "runtime/orderAccess.hpp"
#include "utilities/debug.hpp"
#include "utilities/globalDefinitions.hpp"
@ -128,8 +127,11 @@ void G1CollectionSet::add_old_region(G1HeapRegion* hr) {
_g1h->register_old_collection_set_region_with_region_attr(hr);
assert(num_regions() < _max_num_regions, "Collection set now larger than maximum size.");
_regions[_num_regions++] = hr->hrm_index();
uint local_num_regions = num_regions();
assert(local_num_regions < _max_num_regions, "Collection set now larger than maximum size.");
_regions[local_num_regions] = hr->hrm_index();
_num_regions.store_relaxed(local_num_regions + 1);
_num_initial_old_regions++;
_g1h->old_set_remove(hr);
@ -162,14 +164,13 @@ void G1CollectionSet::stop_incremental_building() {
void G1CollectionSet::clear() {
assert_at_safepoint_on_vm_thread();
_num_regions = 0;
_num_regions.store_relaxed(0);
_groups.clear();
assert(_optional_groups.length() == 0, "must be");
}
void G1CollectionSet::iterate(G1HeapRegionClosure* cl) const {
uint len = _num_regions;
OrderAccess::loadload();
uint len = _num_regions.load_acquire();
for (uint i = 0; i < len; i++) {
G1HeapRegion* r = _g1h->region_at(_regions[i]);
@ -233,8 +234,7 @@ void G1CollectionSet::add_young_region_common(G1HeapRegion* hr) {
_regions[index] = hr->hrm_index();
// Concurrent readers must observe the store of the value in the array before an
// update to the _num_regions field.
OrderAccess::storestore();
_num_regions++;
_num_regions.fetch_then_add(1u, memory_order_release);
}
void G1CollectionSet::add_survivor_regions(G1HeapRegion* hr) {

9
src/hotspot/share/gc/g1/g1CollectionSet.hpp
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@ -26,6 +26,7 @@
#define SHARE_GC_G1_G1COLLECTIONSET_HPP
#include "gc/g1/g1CollectionSetCandidates.hpp"
#include "runtime/atomic.hpp"
#include "utilities/debug.hpp"
#include "utilities/globalDefinitions.hpp"
@ -142,14 +143,14 @@ class G1CollectionSet {
// All regions in _regions below _num_regions are assumed to be part of the
// collection set.
// We assume that at any time there is at most only one writer and (one or more)
// concurrent readers. This means synchronization using storestore and loadload
// barriers on the writer and reader respectively only are sufficient.
// concurrent readers. This means synchronization using release and acquire
// on the writer and reader respectively only are sufficient.
//
// This corresponds to the regions referenced by the candidate groups further below.
uint* _regions;
uint _max_num_regions;
volatile uint _num_regions;
Atomic<uint> _num_regions;
// Old gen groups selected for evacuation.
G1CSetCandidateGroupList _groups;
@ -285,7 +286,7 @@ public:
// Returns the number of regions in the current collection set increment.
uint num_regions_in_increment() const { return num_regions() - _regions_inc_part_start; }
// Returns the total number of regions in the current collection set.
uint num_regions() const { return _num_regions; }
uint num_regions() const { return _num_regions.load_relaxed(); }
// Iterate over the entire collection set (all increments calculated so far), applying
// the given G1HeapRegionClosure on all of the regions.