419 // add ourselves to superklass' subklass list
420 InstanceKlass* super = superklass();
421 if (super == NULL) return; // special case: class Object
422 assert((!super->is_interface() // interfaces cannot be supers
423 && (super->superklass() == NULL || !is_interface())),
424 "an interface can only be a subklass of Object");
425
426 // Make sure there is no stale subklass head
427 super->clean_subklass();
428
429 for (;;) {
430 Klass* prev_first_subklass = Atomic::load_acquire(&_super->_subklass);
431 if (prev_first_subklass != NULL) {
432 // set our sibling to be the superklass' previous first subklass
433 assert(prev_first_subklass->is_loader_alive(), "May not attach not alive klasses");
434 set_next_sibling(prev_first_subklass);
435 }
436 // Note that the prev_first_subklass is always alive, meaning no sibling_next links
437 // are ever created to not alive klasses. This is an important invariant of the lock-free
438 // cleaning protocol, that allows us to safely unlink dead klasses from the sibling list.
439 if (Atomic::cmpxchg(this, &super->_subklass, prev_first_subklass) == prev_first_subklass) {
440 return;
441 }
442 }
443 debug_only(verify();)
444 }
445
446 void Klass::clean_subklass() {
447 for (;;) {
448 // Need load_acquire, due to contending with concurrent inserts
449 Klass* subklass = Atomic::load_acquire(&_subklass);
450 if (subklass == NULL || subklass->is_loader_alive()) {
451 return;
452 }
453 // Try to fix _subklass until it points at something not dead.
454 Atomic::cmpxchg(subklass->next_sibling(), &_subklass, subklass);
455 }
456 }
457
458 void Klass::clean_weak_klass_links(bool unloading_occurred, bool clean_alive_klasses) {
459 if (!ClassUnloading || !unloading_occurred) {
460 return;
461 }
462
463 Klass* root = SystemDictionary::Object_klass();
464 Stack<Klass*, mtGC> stack;
465
466 stack.push(root);
467 while (!stack.is_empty()) {
468 Klass* current = stack.pop();
469
470 assert(current->is_loader_alive(), "just checking, this should be live");
471
472 // Find and set the first alive subklass
473 Klass* sub = current->subklass(true);
474 current->clean_subklass();
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419 // add ourselves to superklass' subklass list
420 InstanceKlass* super = superklass();
421 if (super == NULL) return; // special case: class Object
422 assert((!super->is_interface() // interfaces cannot be supers
423 && (super->superklass() == NULL || !is_interface())),
424 "an interface can only be a subklass of Object");
425
426 // Make sure there is no stale subklass head
427 super->clean_subklass();
428
429 for (;;) {
430 Klass* prev_first_subklass = Atomic::load_acquire(&_super->_subklass);
431 if (prev_first_subklass != NULL) {
432 // set our sibling to be the superklass' previous first subklass
433 assert(prev_first_subklass->is_loader_alive(), "May not attach not alive klasses");
434 set_next_sibling(prev_first_subklass);
435 }
436 // Note that the prev_first_subklass is always alive, meaning no sibling_next links
437 // are ever created to not alive klasses. This is an important invariant of the lock-free
438 // cleaning protocol, that allows us to safely unlink dead klasses from the sibling list.
439 if (Atomic::cmpxchg(&super->_subklass, prev_first_subklass, this) == prev_first_subklass) {
440 return;
441 }
442 }
443 debug_only(verify();)
444 }
445
446 void Klass::clean_subklass() {
447 for (;;) {
448 // Need load_acquire, due to contending with concurrent inserts
449 Klass* subklass = Atomic::load_acquire(&_subklass);
450 if (subklass == NULL || subklass->is_loader_alive()) {
451 return;
452 }
453 // Try to fix _subklass until it points at something not dead.
454 Atomic::cmpxchg(&_subklass, subklass, subklass->next_sibling());
455 }
456 }
457
458 void Klass::clean_weak_klass_links(bool unloading_occurred, bool clean_alive_klasses) {
459 if (!ClassUnloading || !unloading_occurred) {
460 return;
461 }
462
463 Klass* root = SystemDictionary::Object_klass();
464 Stack<Klass*, mtGC> stack;
465
466 stack.push(root);
467 while (!stack.is_empty()) {
468 Klass* current = stack.pop();
469
470 assert(current->is_loader_alive(), "just checking, this should be live");
471
472 // Find and set the first alive subklass
473 Klass* sub = current->subklass(true);
474 current->clean_subklass();
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