--- a/src/hotspot/share/classfile/classLoaderDataGraph.cpp Thu Jun 06 20:19:03 2019 +0200
+++ b/src/hotspot/share/classfile/classLoaderDataGraph.cpp Thu Jun 06 15:09:17 2019 -0400
@@ -38,6 +38,7 @@
#include "runtime/atomic.hpp"
#include "runtime/handles.inline.hpp"
#include "runtime/mutex.hpp"
+#include "runtime/orderAccess.hpp"
#include "runtime/safepoint.hpp"
#include "runtime/safepointVerifiers.hpp"
#include "utilities/growableArray.hpp"
@@ -48,8 +49,17 @@
volatile size_t ClassLoaderDataGraph::_num_instance_classes = 0;
void ClassLoaderDataGraph::clear_claimed_marks() {
- assert_locked_or_safepoint_weak(ClassLoaderDataGraph_lock);
- for (ClassLoaderData* cld = _head; cld != NULL; cld = cld->next()) {
+ // The claimed marks of the CLDs in the ClassLoaderDataGraph are cleared
+ // outside a safepoint and without locking the ClassLoaderDataGraph_lock.
+ // This is required to avoid a deadlock between concurrent GC threads and safepointing.
+ //
+ // We need to make sure that the CLD contents are fully visible to the
+ // reader thread. This is accomplished by acquire/release of the _head,
+ // and is sufficient.
+ //
+ // Any ClassLoaderData added after or during walking the list are prepended to
+ // _head. Their claim mark need not be handled here.
+ for (ClassLoaderData* cld = OrderAccess::load_acquire(&_head); cld != NULL; cld = cld->next()) {
cld->clear_claim();
}
}
@@ -169,7 +179,7 @@
}
// GC root of class loader data created.
-ClassLoaderData* ClassLoaderDataGraph::_head = NULL;
+ClassLoaderData* volatile ClassLoaderDataGraph::_head = NULL;
ClassLoaderData* ClassLoaderDataGraph::_unloading = NULL;
ClassLoaderData* ClassLoaderDataGraph::_saved_unloading = NULL;
ClassLoaderData* ClassLoaderDataGraph::_saved_head = NULL;
@@ -205,7 +215,7 @@
// First install the new CLD to the Graph.
cld->set_next(_head);
- _head = cld;
+ OrderAccess::release_store(&_head, cld);
// Next associate with the class_loader.
if (!is_unsafe_anonymous) {