1 /*
   2  * Copyright (c) 2018, 2019, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.
   8  *
   9  * This code is distributed in the hope that it will be useful, but WITHOUT
  10  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  11  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  12  * version 2 for more details (a copy is included in the LICENSE file that
  13  * accompanied this code).
  14  *
  15  * You should have received a copy of the GNU General Public License version
  16  * 2 along with this work; if not, write to the Free Software Foundation,
  17  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  18  *
  19  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  20  * or visit www.oracle.com if you need additional information or have any
  21  * questions.
  22  *
  23  */
  24 
  25 #include "precompiled.hpp"
  26 #include "gc/shared/oopStorage.inline.hpp"
  27 #include "gc/shared/oopStorageParState.inline.hpp"
  28 #include "logging/log.hpp"
  29 #include "logging/logStream.hpp"
  30 #include "memory/allocation.inline.hpp"
  31 #include "runtime/atomic.hpp"
  32 #include "runtime/globals.hpp"
  33 #include "runtime/handles.inline.hpp"
  34 #include "runtime/interfaceSupport.inline.hpp"
  35 #include "runtime/mutex.hpp"
  36 #include "runtime/mutexLocker.hpp"
  37 #include "runtime/orderAccess.hpp"
  38 #include "runtime/os.hpp"
  39 #include "runtime/safepoint.hpp"
  40 #include "runtime/stubRoutines.hpp"
  41 #include "runtime/thread.hpp"
  42 #include "utilities/align.hpp"
  43 #include "utilities/count_trailing_zeros.hpp"
  44 #include "utilities/debug.hpp"
  45 #include "utilities/globalDefinitions.hpp"
  46 #include "utilities/macros.hpp"
  47 #include "utilities/ostream.hpp"
  48 
  49 OopStorage::AllocationListEntry::AllocationListEntry() : _prev(NULL), _next(NULL) {}
  50 
  51 OopStorage::AllocationListEntry::~AllocationListEntry() {
  52   assert(_prev == NULL, "deleting attached block");
  53   assert(_next == NULL, "deleting attached block");
  54 }
  55 
  56 OopStorage::AllocationList::AllocationList() : _head(NULL), _tail(NULL) {}
  57 
  58 OopStorage::AllocationList::~AllocationList() {
  59   // ~OopStorage() empties its lists before destroying them.
  60   assert(_head == NULL, "deleting non-empty block list");
  61   assert(_tail == NULL, "deleting non-empty block list");
  62 }
  63 
  64 void OopStorage::AllocationList::push_front(const Block& block) {
  65   const Block* old = _head;
  66   if (old == NULL) {
  67     assert(_tail == NULL, "invariant");
  68     _head = _tail = █
  69   } else {
  70     block.allocation_list_entry()._next = old;
  71     old->allocation_list_entry()._prev = █
  72     _head = █
  73   }
  74 }
  75 
  76 void OopStorage::AllocationList::push_back(const Block& block) {
  77   const Block* old = _tail;
  78   if (old == NULL) {
  79     assert(_head == NULL, "invariant");
  80     _head = _tail = █
  81   } else {
  82     old->allocation_list_entry()._next = █
  83     block.allocation_list_entry()._prev = old;
  84     _tail = █
  85   }
  86 }
  87 
  88 void OopStorage::AllocationList::unlink(const Block& block) {
  89   const AllocationListEntry& block_entry = block.allocation_list_entry();
  90   const Block* prev_blk = block_entry._prev;
  91   const Block* next_blk = block_entry._next;
  92   block_entry._prev = NULL;
  93   block_entry._next = NULL;
  94   if ((prev_blk == NULL) && (next_blk == NULL)) {
  95     assert(_head == &block, "invariant");
  96     assert(_tail == &block, "invariant");
  97     _head = _tail = NULL;
  98   } else if (prev_blk == NULL) {
  99     assert(_head == &block, "invariant");
 100     next_blk->allocation_list_entry()._prev = NULL;
 101     _head = next_blk;
 102   } else if (next_blk == NULL) {
 103     assert(_tail == &block, "invariant");
 104     prev_blk->allocation_list_entry()._next = NULL;
 105     _tail = prev_blk;
 106   } else {
 107     next_blk->allocation_list_entry()._prev = prev_blk;
 108     prev_blk->allocation_list_entry()._next = next_blk;
 109   }
 110 }
 111 
 112 OopStorage::ActiveArray::ActiveArray(size_t size) :
 113   _size(size),
 114   _block_count(0),
 115   _refcount(0)
 116 {}
 117 
 118 OopStorage::ActiveArray::~ActiveArray() {
 119   assert(_refcount == 0, "precondition");
 120 }
 121 
 122 OopStorage::ActiveArray* OopStorage::ActiveArray::create(size_t size, AllocFailType alloc_fail) {
 123   size_t size_in_bytes = blocks_offset() + sizeof(Block*) * size;
 124   void* mem = NEW_C_HEAP_ARRAY3(char, size_in_bytes, mtGC, CURRENT_PC, alloc_fail);
 125   if (mem == NULL) return NULL;
 126   return new (mem) ActiveArray(size);
 127 }
 128 
 129 void OopStorage::ActiveArray::destroy(ActiveArray* ba) {
 130   ba->~ActiveArray();
 131   FREE_C_HEAP_ARRAY(char, ba);
 132 }
 133 
 134 size_t OopStorage::ActiveArray::size() const {
 135   return _size;
 136 }
 137 
 138 size_t OopStorage::ActiveArray::block_count() const {
 139   return _block_count;
 140 }
 141 
 142 size_t OopStorage::ActiveArray::block_count_acquire() const {
 143   return OrderAccess::load_acquire(&_block_count);
 144 }
 145 
 146 void OopStorage::ActiveArray::increment_refcount() const {
 147   int new_value = Atomic::add(1, &_refcount);
 148   assert(new_value >= 1, "negative refcount %d", new_value - 1);
 149 }
 150 
 151 bool OopStorage::ActiveArray::decrement_refcount() const {
 152   int new_value = Atomic::sub(1, &_refcount);
 153   assert(new_value >= 0, "negative refcount %d", new_value);
 154   return new_value == 0;
 155 }
 156 
 157 bool OopStorage::ActiveArray::push(Block* block) {
 158   size_t index = _block_count;
 159   if (index < _size) {
 160     block->set_active_index(index);
 161     *block_ptr(index) = block;
 162     // Use a release_store to ensure all the setup is complete before
 163     // making the block visible.
 164     OrderAccess::release_store(&_block_count, index + 1);
 165     return true;
 166   } else {
 167     return false;
 168   }
 169 }
 170 
 171 void OopStorage::ActiveArray::remove(Block* block) {
 172   assert(_block_count > 0, "array is empty");
 173   size_t index = block->active_index();
 174   assert(*block_ptr(index) == block, "block not present");
 175   size_t last_index = _block_count - 1;
 176   Block* last_block = *block_ptr(last_index);
 177   last_block->set_active_index(index);
 178   *block_ptr(index) = last_block;
 179   _block_count = last_index;
 180 }
 181 
 182 void OopStorage::ActiveArray::copy_from(const ActiveArray* from) {
 183   assert(_block_count == 0, "array must be empty");
 184   size_t count = from->_block_count;
 185   assert(count <= _size, "precondition");
 186   Block* const* from_ptr = from->block_ptr(0);
 187   Block** to_ptr = block_ptr(0);
 188   for (size_t i = 0; i < count; ++i) {
 189     Block* block = *from_ptr++;
 190     assert(block->active_index() == i, "invariant");
 191     *to_ptr++ = block;
 192   }
 193   _block_count = count;
 194 }
 195 
 196 // Blocks start with an array of BitsPerWord oop entries.  That array
 197 // is divided into conceptual BytesPerWord sections of BitsPerByte
 198 // entries.  Blocks are allocated aligned on section boundaries, for
 199 // the convenience of mapping from an entry to the containing block;
 200 // see block_for_ptr().  Aligning on section boundary rather than on
 201 // the full _data wastes a lot less space, but makes for a bit more
 202 // work in block_for_ptr().
 203 
 204 const unsigned section_size = BitsPerByte;
 205 const unsigned section_count = BytesPerWord;
 206 const unsigned block_alignment = sizeof(oop) * section_size;
 207 
 208 OopStorage::Block::Block(const OopStorage* owner, void* memory) :
 209   _data(),
 210   _allocated_bitmask(0),
 211   _owner_address(reinterpret_cast<intptr_t>(owner)),
 212   _memory(memory),
 213   _active_index(0),
 214   _allocation_list_entry(),
 215   _deferred_updates_next(NULL),
 216   _release_refcount(0)
 217 {
 218   STATIC_ASSERT(_data_pos == 0);
 219   STATIC_ASSERT(section_size * section_count == ARRAY_SIZE(_data));
 220   assert(offset_of(Block, _data) == _data_pos, "invariant");
 221   assert(owner != NULL, "NULL owner");
 222   assert(is_aligned(this, block_alignment), "misaligned block");
 223 }
 224 
 225 OopStorage::Block::~Block() {
 226   assert(_release_refcount == 0, "deleting block while releasing");
 227   assert(_deferred_updates_next == NULL, "deleting block with deferred update");
 228   // Clear fields used by block_for_ptr and entry validation, which
 229   // might help catch bugs.  Volatile to prevent dead-store elimination.
 230   const_cast<uintx volatile&>(_allocated_bitmask) = 0;
 231   const_cast<intptr_t volatile&>(_owner_address) = 0;
 232 }
 233 
 234 size_t OopStorage::Block::allocation_size() {
 235   // _data must be first member, so aligning Block aligns _data.
 236   STATIC_ASSERT(_data_pos == 0);
 237   return sizeof(Block) + block_alignment - sizeof(void*);
 238 }
 239 
 240 size_t OopStorage::Block::allocation_alignment_shift() {
 241   return exact_log2(block_alignment);
 242 }
 243 
 244 inline bool is_full_bitmask(uintx bitmask) { return ~bitmask == 0; }
 245 inline bool is_empty_bitmask(uintx bitmask) { return bitmask == 0; }
 246 
 247 bool OopStorage::Block::is_full() const {
 248   return is_full_bitmask(allocated_bitmask());
 249 }
 250 
 251 bool OopStorage::Block::is_empty() const {
 252   return is_empty_bitmask(allocated_bitmask());
 253 }
 254 
 255 uintx OopStorage::Block::bitmask_for_entry(const oop* ptr) const {
 256   return bitmask_for_index(get_index(ptr));
 257 }
 258 
 259 // An empty block is not yet deletable if either:
 260 // (1) There is a release() operation currently operating on it.
 261 // (2) It is in the deferred updates list.
 262 // For interaction with release(), these must follow the empty check,
 263 // and the order of these checks is important.
 264 bool OopStorage::Block::is_safe_to_delete() const {
 265   assert(is_empty(), "precondition");
 266   OrderAccess::loadload();
 267   return (OrderAccess::load_acquire(&_release_refcount) == 0) &&
 268          (OrderAccess::load_acquire(&_deferred_updates_next) == NULL);
 269 }
 270 
 271 OopStorage::Block* OopStorage::Block::deferred_updates_next() const {
 272   return _deferred_updates_next;
 273 }
 274 
 275 void OopStorage::Block::set_deferred_updates_next(Block* block) {
 276   _deferred_updates_next = block;
 277 }
 278 
 279 bool OopStorage::Block::contains(const oop* ptr) const {
 280   const oop* base = get_pointer(0);
 281   return (base <= ptr) && (ptr < (base + ARRAY_SIZE(_data)));
 282 }
 283 
 284 size_t OopStorage::Block::active_index() const {
 285   return _active_index;
 286 }
 287 
 288 void OopStorage::Block::set_active_index(size_t index) {
 289   _active_index = index;
 290 }
 291 
 292 size_t OopStorage::Block::active_index_safe(const Block* block) {
 293   STATIC_ASSERT(sizeof(intptr_t) == sizeof(block->_active_index));
 294   assert(CanUseSafeFetchN(), "precondition");
 295   return SafeFetchN((intptr_t*)&block->_active_index, 0);
 296 }
 297 
 298 unsigned OopStorage::Block::get_index(const oop* ptr) const {
 299   assert(contains(ptr), PTR_FORMAT " not in block " PTR_FORMAT, p2i(ptr), p2i(this));
 300   return static_cast<unsigned>(ptr - get_pointer(0));
 301 }
 302 
 303 oop* OopStorage::Block::allocate() {
 304   // Use CAS loop because release may change bitmask outside of lock.
 305   uintx allocated = allocated_bitmask();
 306   while (true) {
 307     assert(!is_full_bitmask(allocated), "attempt to allocate from full block");
 308     unsigned index = count_trailing_zeros(~allocated);
 309     uintx new_value = allocated | bitmask_for_index(index);
 310     uintx fetched = Atomic::cmpxchg(new_value, &_allocated_bitmask, allocated);
 311     if (fetched == allocated) {
 312       return get_pointer(index); // CAS succeeded; return entry for index.
 313     }
 314     allocated = fetched;       // CAS failed; retry with latest value.
 315   }
 316 }
 317 
 318 OopStorage::Block* OopStorage::Block::new_block(const OopStorage* owner) {
 319   // _data must be first member: aligning block => aligning _data.
 320   STATIC_ASSERT(_data_pos == 0);
 321   size_t size_needed = allocation_size();
 322   void* memory = NEW_C_HEAP_ARRAY_RETURN_NULL(char, size_needed, mtGC);
 323   if (memory == NULL) {
 324     return NULL;
 325   }
 326   void* block_mem = align_up(memory, block_alignment);
 327   assert(sizeof(Block) + pointer_delta(block_mem, memory, 1) <= size_needed,
 328          "allocated insufficient space for aligned block");
 329   return ::new (block_mem) Block(owner, memory);
 330 }
 331 
 332 void OopStorage::Block::delete_block(const Block& block) {
 333   void* memory = block._memory;
 334   block.Block::~Block();
 335   FREE_C_HEAP_ARRAY(char, memory);
 336 }
 337 
 338 // This can return a false positive if ptr is not contained by some
 339 // block.  For some uses, it is a precondition that ptr is valid,
 340 // e.g. contained in some block in owner's _active_array.  Other uses
 341 // require additional validation of the result.
 342 OopStorage::Block*
 343 OopStorage::Block::block_for_ptr(const OopStorage* owner, const oop* ptr) {
 344   assert(CanUseSafeFetchN(), "precondition");
 345   STATIC_ASSERT(_data_pos == 0);
 346   // Const-ness of ptr is not related to const-ness of containing block.
 347   // Blocks are allocated section-aligned, so get the containing section.
 348   oop* section_start = align_down(const_cast<oop*>(ptr), block_alignment);
 349   // Start with a guess that the containing section is the last section,
 350   // so the block starts section_count-1 sections earlier.
 351   oop* section = section_start - (section_size * (section_count - 1));
 352   // Walk up through the potential block start positions, looking for
 353   // the owner in the expected location.  If we're below the actual block
 354   // start position, the value at the owner position will be some oop
 355   // (possibly NULL), which can never match the owner.
 356   intptr_t owner_addr = reinterpret_cast<intptr_t>(owner);
 357   for (unsigned i = 0; i < section_count; ++i, section += section_size) {
 358     Block* candidate = reinterpret_cast<Block*>(section);
 359     if (SafeFetchN(&candidate->_owner_address, 0) == owner_addr) {
 360       return candidate;
 361     }
 362   }
 363   return NULL;
 364 }
 365 
 366 //////////////////////////////////////////////////////////////////////////////
 367 // Allocation
 368 //
 369 // Allocation involves the _allocation_list, which contains a subset of the
 370 // blocks owned by a storage object.  This is a doubly-linked list, linked
 371 // through dedicated fields in the blocks.  Full blocks are removed from this
 372 // list, though they are still present in the _active_array.  Empty blocks are
 373 // kept at the end of the _allocation_list, to make it easy for empty block
 374 // deletion to find them.
 375 //
 376 // allocate(), and delete_empty_blocks() lock the
 377 // _allocation_mutex while performing any list and array modifications.
 378 //
 379 // allocate() and release() update a block's _allocated_bitmask using CAS
 380 // loops.  This prevents loss of updates even though release() performs
 381 // its updates without any locking.
 382 //
 383 // allocate() obtains the entry from the first block in the _allocation_list,
 384 // and updates that block's _allocated_bitmask to indicate the entry is in
 385 // use.  If this makes the block full (all entries in use), the block is
 386 // removed from the _allocation_list so it won't be considered by future
 387 // allocations until some entries in it are released.
 388 //
 389 // release() is performed lock-free. (Note: This means it can't notify the
 390 // service thread of pending cleanup work.  It must be lock-free because
 391 // it is called in all kinds of contexts where even quite low ranked locks
 392 // may be held.)  release() first looks up the block for
 393 // the entry, using address alignment to find the enclosing block (thereby
 394 // avoiding iteration over the _active_array).  Once the block has been
 395 // determined, its _allocated_bitmask needs to be updated, and its position in
 396 // the _allocation_list may need to be updated.  There are two cases:
 397 //
 398 // (a) If the block is neither full nor would become empty with the release of
 399 // the entry, only its _allocated_bitmask needs to be updated.  But if the CAS
 400 // update fails, the applicable case may change for the retry.
 401 //
 402 // (b) Otherwise, the _allocation_list also needs to be modified.  This requires
 403 // locking the _allocation_mutex.  To keep the release() operation lock-free,
 404 // rather than updating the _allocation_list itself, it instead performs a
 405 // lock-free push of the block onto the _deferred_updates list.  Entries on
 406 // that list are processed by allocate() and delete_empty_blocks(), while
 407 // they already hold the necessary lock.  That processing makes the block's
 408 // list state consistent with its current _allocated_bitmask.  The block is
 409 // added to the _allocation_list if not already present and the bitmask is not
 410 // full.  The block is moved to the end of the _allocation_list if the bitmask
 411 // is empty, for ease of empty block deletion processing.
 412 
 413 oop* OopStorage::allocate() {
 414   MutexLocker ml(_allocation_mutex, Mutex::_no_safepoint_check_flag);
 415 
 416   Block* block = block_for_allocation();
 417   if (block == NULL) return NULL; // Block allocation failed.
 418   assert(!block->is_full(), "invariant");
 419   if (block->is_empty()) {
 420     // Transitioning from empty to not empty.
 421     log_trace(oopstorage, blocks)("%s: block not empty " PTR_FORMAT, name(), p2i(block));
 422   }
 423   oop* result = block->allocate();
 424   assert(result != NULL, "allocation failed");
 425   assert(!block->is_empty(), "postcondition");
 426   Atomic::inc(&_allocation_count); // release updates outside lock.
 427   if (block->is_full()) {
 428     // Transitioning from not full to full.
 429     // Remove full blocks from consideration by future allocates.
 430     log_trace(oopstorage, blocks)("%s: block full " PTR_FORMAT, name(), p2i(block));
 431     _allocation_list.unlink(*block);
 432   }
 433   log_trace(oopstorage, ref)("%s: allocated " PTR_FORMAT, name(), p2i(result));
 434   return result;
 435 }
 436 
 437 bool OopStorage::try_add_block() {
 438   assert_lock_strong(_allocation_mutex);
 439   Block* block;
 440   {
 441     MutexUnlocker ul(_allocation_mutex, Mutex::_no_safepoint_check_flag);
 442     block = Block::new_block(this);
 443   }
 444   if (block == NULL) return false;
 445 
 446   // Add new block to the _active_array, growing if needed.
 447   if (!_active_array->push(block)) {
 448     if (expand_active_array()) {
 449       guarantee(_active_array->push(block), "push failed after expansion");
 450     } else {
 451       log_debug(oopstorage, blocks)("%s: failed active array expand", name());
 452       Block::delete_block(*block);
 453       return false;
 454     }
 455   }
 456   // Add to end of _allocation_list.  The mutex release allowed other
 457   // threads to add blocks to the _allocation_list.  We prefer to
 458   // allocate from non-empty blocks, to allow empty blocks to be
 459   // deleted.  But we don't bother notifying about the empty block
 460   // because we're (probably) about to allocate an entry from it.
 461   _allocation_list.push_back(*block);
 462   log_debug(oopstorage, blocks)("%s: new block " PTR_FORMAT, name(), p2i(block));
 463   return true;
 464 }
 465 
 466 OopStorage::Block* OopStorage::block_for_allocation() {
 467   assert_lock_strong(_allocation_mutex);
 468   while (true) {
 469     // Use the first block in _allocation_list for the allocation.
 470     Block* block = _allocation_list.head();
 471     if (block != NULL) {
 472       return block;
 473     } else if (reduce_deferred_updates()) {
 474       // Might have added a block to the _allocation_list, so retry.
 475     } else if (try_add_block()) {
 476       // Successfully added a new block to the list, so retry.
 477       assert(_allocation_list.chead() != NULL, "invariant");
 478     } else if (_allocation_list.chead() != NULL) {
 479       // Trying to add a block failed, but some other thread added to the
 480       // list while we'd dropped the lock over the new block allocation.
 481     } else if (!reduce_deferred_updates()) { // Once more before failure.
 482       // Attempt to add a block failed, no other thread added a block,
 483       // and no deferred updated added a block, then allocation failed.
 484       log_info(oopstorage, blocks)("%s: failed block allocation", name());
 485       return NULL;
 486     }
 487   }
 488 }
 489 
 490 // Create a new, larger, active array with the same content as the
 491 // current array, and then replace, relinquishing the old array.
 492 // Return true if the array was successfully expanded, false to
 493 // indicate allocation failure.
 494 bool OopStorage::expand_active_array() {
 495   assert_lock_strong(_allocation_mutex);
 496   ActiveArray* old_array = _active_array;
 497   size_t new_size = 2 * old_array->size();
 498   log_debug(oopstorage, blocks)("%s: expand active array " SIZE_FORMAT,
 499                                 name(), new_size);
 500   ActiveArray* new_array = ActiveArray::create(new_size, AllocFailStrategy::RETURN_NULL);
 501   if (new_array == NULL) return false;
 502   new_array->copy_from(old_array);
 503   replace_active_array(new_array);
 504   relinquish_block_array(old_array);
 505   return true;
 506 }
 507 
 508 // Make new_array the _active_array.  Increments new_array's refcount
 509 // to account for the new reference.  The assignment is atomic wrto
 510 // obtain_active_array; once this function returns, it is safe for the
 511 // caller to relinquish the old array.
 512 void OopStorage::replace_active_array(ActiveArray* new_array) {
 513   // Caller has the old array that is the current value of _active_array.
 514   // Update new_array refcount to account for the new reference.
 515   new_array->increment_refcount();
 516   // Install new_array, ensuring its initialization is complete first.
 517   OrderAccess::release_store(&_active_array, new_array);
 518   // Wait for any readers that could read the old array from _active_array.
 519   // Can't use GlobalCounter here, because this is called from allocate(),
 520   // which may be called in the scope of a GlobalCounter critical section
 521   // when inserting a StringTable entry.
 522   _protect_active.synchronize();
 523   // All obtain critical sections that could see the old array have
 524   // completed, having incremented the refcount of the old array.  The
 525   // caller can now safely relinquish the old array.
 526 }
 527 
 528 // Atomically (wrto replace_active_array) get the active array and
 529 // increment its refcount.  This provides safe access to the array,
 530 // even if an allocate operation expands and replaces the value of
 531 // _active_array.  The caller must relinquish the array when done
 532 // using it.
 533 OopStorage::ActiveArray* OopStorage::obtain_active_array() const {
 534   SingleWriterSynchronizer::CriticalSection cs(&_protect_active);
 535   ActiveArray* result = OrderAccess::load_acquire(&_active_array);
 536   result->increment_refcount();
 537   return result;
 538 }
 539 
 540 // Decrement refcount of array and destroy if refcount is zero.
 541 void OopStorage::relinquish_block_array(ActiveArray* array) const {
 542   if (array->decrement_refcount()) {
 543     assert(array != _active_array, "invariant");
 544     ActiveArray::destroy(array);
 545   }
 546 }
 547 
 548 class OopStorage::WithActiveArray : public StackObj {
 549   const OopStorage* _storage;
 550   ActiveArray* _active_array;
 551 
 552 public:
 553   WithActiveArray(const OopStorage* storage) :
 554     _storage(storage),
 555     _active_array(storage->obtain_active_array())
 556   {}
 557 
 558   ~WithActiveArray() {
 559     _storage->relinquish_block_array(_active_array);
 560   }
 561 
 562   ActiveArray& active_array() const {
 563     return *_active_array;
 564   }
 565 };
 566 
 567 OopStorage::Block* OopStorage::find_block_or_null(const oop* ptr) const {
 568   assert(ptr != NULL, "precondition");
 569   return Block::block_for_ptr(this, ptr);
 570 }
 571 
 572 static void log_release_transitions(uintx releasing,
 573                                     uintx old_allocated,
 574                                     const OopStorage* owner,
 575                                     const void* block) {
 576   LogTarget(Trace, oopstorage, blocks) lt;
 577   if (lt.is_enabled()) {
 578     LogStream ls(lt);
 579     if (is_full_bitmask(old_allocated)) {
 580       ls.print_cr("%s: block not full " PTR_FORMAT, owner->name(), p2i(block));
 581     }
 582     if (releasing == old_allocated) {
 583       ls.print_cr("%s: block empty " PTR_FORMAT, owner->name(), p2i(block));
 584     }
 585   }
 586 }
 587 
 588 void OopStorage::Block::release_entries(uintx releasing, OopStorage* owner) {
 589   assert(releasing != 0, "preconditon");
 590   // Prevent empty block deletion when transitioning to empty.
 591   Atomic::inc(&_release_refcount);
 592 
 593   // Atomically update allocated bitmask.
 594   uintx old_allocated = _allocated_bitmask;
 595   while (true) {
 596     assert((releasing & ~old_allocated) == 0, "releasing unallocated entries");
 597     uintx new_value = old_allocated ^ releasing;
 598     uintx fetched = Atomic::cmpxchg(new_value, &_allocated_bitmask, old_allocated);
 599     if (fetched == old_allocated) break; // Successful update.
 600     old_allocated = fetched;             // Retry with updated bitmask.
 601   }
 602 
 603   // Now that the bitmask has been updated, if we have a state transition
 604   // (updated bitmask is empty or old bitmask was full), atomically push
 605   // this block onto the deferred updates list.  Some future call to
 606   // reduce_deferred_updates will make any needed changes related to this
 607   // block and _allocation_list.  This deferral avoids _allocation_list
 608   // updates and the associated locking here.
 609   if ((releasing == old_allocated) || is_full_bitmask(old_allocated)) {
 610     // Log transitions.  Both transitions are possible in a single update.
 611     log_release_transitions(releasing, old_allocated, owner, this);
 612     // Attempt to claim responsibility for adding this block to the deferred
 613     // list, by setting the link to non-NULL by self-looping.  If this fails,
 614     // then someone else has made such a claim and the deferred update has not
 615     // yet been processed and will include our change, so we don't need to do
 616     // anything further.
 617     if (Atomic::replace_if_null(this, &_deferred_updates_next)) {
 618       // Successfully claimed.  Push, with self-loop for end-of-list.
 619       Block* head = owner->_deferred_updates;
 620       while (true) {
 621         _deferred_updates_next = (head == NULL) ? this : head;
 622         Block* fetched = Atomic::cmpxchg(this, &owner->_deferred_updates, head);
 623         if (fetched == head) break; // Successful update.
 624         head = fetched;             // Retry with updated head.
 625       }
 626       // Only request cleanup for to-empty transitions, not for from-full.
 627       // There isn't any rush to process from-full transitions.  Allocation
 628       // will reduce deferrals before allocating new blocks, so may process
 629       // some.  And the service thread will drain the entire deferred list
 630       // if there are any pending to-empty transitions.
 631       if (releasing == old_allocated) {
 632         owner->record_needs_cleanup();
 633       }
 634       log_trace(oopstorage, blocks)("%s: deferred update " PTR_FORMAT,
 635                                     owner->name(), p2i(this));
 636     }
 637   }
 638   // Release hold on empty block deletion.
 639   Atomic::dec(&_release_refcount);
 640 }
 641 
 642 // Process one available deferred update.  Returns true if one was processed.
 643 bool OopStorage::reduce_deferred_updates() {
 644   assert_lock_strong(_allocation_mutex);
 645   // Atomically pop a block off the list, if any available.
 646   // No ABA issue because this is only called by one thread at a time.
 647   // The atomicity is wrto pushes by release().
 648   Block* block = OrderAccess::load_acquire(&_deferred_updates);
 649   while (true) {
 650     if (block == NULL) return false;
 651     // Try atomic pop of block from list.
 652     Block* tail = block->deferred_updates_next();
 653     if (block == tail) tail = NULL; // Handle self-loop end marker.
 654     Block* fetched = Atomic::cmpxchg(tail, &_deferred_updates, block);
 655     if (fetched == block) break; // Update successful.
 656     block = fetched;             // Retry with updated block.
 657   }
 658   block->set_deferred_updates_next(NULL); // Clear tail after updating head.
 659   // Ensure bitmask read after pop is complete, including clearing tail, for
 660   // ordering with release().  Without this, we may be processing a stale
 661   // bitmask state here while blocking a release() operation from recording
 662   // the deferred update needed for its bitmask change.
 663   OrderAccess::fence();
 664   // Process popped block.
 665   uintx allocated = block->allocated_bitmask();
 666 
 667   // Make membership in list consistent with bitmask state.
 668   if ((_allocation_list.ctail() != NULL) &&
 669       ((_allocation_list.ctail() == block) ||
 670        (_allocation_list.next(*block) != NULL))) {
 671     // Block is in the _allocation_list.
 672     assert(!is_full_bitmask(allocated), "invariant");
 673   } else if (!is_full_bitmask(allocated)) {
 674     // Block is not in the _allocation_list, but now should be.
 675     _allocation_list.push_front(*block);
 676   } // Else block is full and not in list, which is correct.
 677 
 678   // Move empty block to end of list, for possible deletion.
 679   if (is_empty_bitmask(allocated)) {
 680     _allocation_list.unlink(*block);
 681     _allocation_list.push_back(*block);
 682   }
 683 
 684   log_trace(oopstorage, blocks)("%s: processed deferred update " PTR_FORMAT,
 685                                 name(), p2i(block));
 686   return true;              // Processed one pending update.
 687 }
 688 
 689 inline void check_release_entry(const oop* entry) {
 690   assert(entry != NULL, "Releasing NULL");
 691   assert(*entry == NULL, "Releasing uncleared entry: " PTR_FORMAT, p2i(entry));
 692 }
 693 
 694 void OopStorage::release(const oop* ptr) {
 695   check_release_entry(ptr);
 696   Block* block = find_block_or_null(ptr);
 697   assert(block != NULL, "%s: invalid release " PTR_FORMAT, name(), p2i(ptr));
 698   log_trace(oopstorage, ref)("%s: released " PTR_FORMAT, name(), p2i(ptr));
 699   block->release_entries(block->bitmask_for_entry(ptr), this);
 700   Atomic::dec(&_allocation_count);
 701 }
 702 
 703 void OopStorage::release(const oop* const* ptrs, size_t size) {
 704   size_t i = 0;
 705   while (i < size) {
 706     check_release_entry(ptrs[i]);
 707     Block* block = find_block_or_null(ptrs[i]);
 708     assert(block != NULL, "%s: invalid release " PTR_FORMAT, name(), p2i(ptrs[i]));
 709     log_trace(oopstorage, ref)("%s: released " PTR_FORMAT, name(), p2i(ptrs[i]));
 710     size_t count = 0;
 711     uintx releasing = 0;
 712     for ( ; i < size; ++i) {
 713       const oop* entry = ptrs[i];
 714       check_release_entry(entry);
 715       // If entry not in block, finish block and resume outer loop with entry.
 716       if (!block->contains(entry)) break;
 717       // Add entry to releasing bitmap.
 718       log_trace(oopstorage, ref)("%s: released " PTR_FORMAT, name(), p2i(entry));
 719       uintx entry_bitmask = block->bitmask_for_entry(entry);
 720       assert((releasing & entry_bitmask) == 0,
 721              "Duplicate entry: " PTR_FORMAT, p2i(entry));
 722       releasing |= entry_bitmask;
 723       ++count;
 724     }
 725     // Release the contiguous entries that are in block.
 726     block->release_entries(releasing, this);
 727     Atomic::sub(count, &_allocation_count);
 728   }
 729 }
 730 
 731 const size_t initial_active_array_size = 8;
 732 
 733 OopStorage::OopStorage(const char* name,
 734                        Mutex* allocation_mutex,
 735                        Mutex* active_mutex) :
 736   _name(os::strdup(name)),
 737   _active_array(ActiveArray::create(initial_active_array_size)),
 738   _allocation_list(),
 739   _deferred_updates(NULL),
 740   _allocation_mutex(allocation_mutex),
 741   _active_mutex(active_mutex),
 742   _allocation_count(0),
 743   _concurrent_iteration_count(0),
 744   _needs_cleanup(false)
 745 {
 746   _active_array->increment_refcount();
 747   assert(_active_mutex->rank() < _allocation_mutex->rank(),
 748          "%s: active_mutex must have lower rank than allocation_mutex", _name);
 749   assert(Service_lock->rank() < _active_mutex->rank(),
 750          "%s: active_mutex must have higher rank than Service_lock", _name);
 751   assert(_active_mutex->_safepoint_check_required == Mutex::_safepoint_check_never,
 752          "%s: active mutex requires never safepoint check", _name);
 753   assert(_allocation_mutex->_safepoint_check_required == Mutex::_safepoint_check_never,
 754          "%s: allocation mutex requires never safepoint check", _name);
 755 }
 756 
 757 void OopStorage::delete_empty_block(const Block& block) {
 758   assert(block.is_empty(), "discarding non-empty block");
 759   log_debug(oopstorage, blocks)("%s: delete empty block " PTR_FORMAT, name(), p2i(&block));
 760   Block::delete_block(block);
 761 }
 762 
 763 OopStorage::~OopStorage() {
 764   Block* block;
 765   while ((block = _deferred_updates) != NULL) {
 766     _deferred_updates = block->deferred_updates_next();
 767     block->set_deferred_updates_next(NULL);
 768   }
 769   while ((block = _allocation_list.head()) != NULL) {
 770     _allocation_list.unlink(*block);
 771   }
 772   bool unreferenced = _active_array->decrement_refcount();
 773   assert(unreferenced, "deleting storage while _active_array is referenced");
 774   for (size_t i = _active_array->block_count(); 0 < i; ) {
 775     block = _active_array->at(--i);
 776     Block::delete_block(*block);
 777   }
 778   ActiveArray::destroy(_active_array);
 779   os::free(const_cast<char*>(_name));
 780 }
 781 
 782 // Managing service thread notifications.
 783 //
 784 // We don't want cleanup work to linger indefinitely, but we also don't want
 785 // to run the service thread too often.  We're also very limited in what we
 786 // can do in a release operation, where cleanup work is created.
 787 //
 788 // When a release operation changes a block's state to empty, it records the
 789 // need for cleanup in both the associated storage object and in the global
 790 // request state.  A safepoint cleanup task notifies the service thread when
 791 // there may be cleanup work for any storage object, based on the global
 792 // request state.  But that notification is deferred if the service thread
 793 // has run recently, and we also avoid duplicate notifications.  The service
 794 // thread updates the timestamp and resets the state flags on every iteration.
 795 
 796 // Global cleanup request state.
 797 static volatile bool needs_cleanup_requested = false;
 798 
 799 // Flag for avoiding duplicate notifications.
 800 static bool needs_cleanup_triggered = false;
 801 
 802 // Time after which a notification can be made.
 803 static jlong cleanup_trigger_permit_time = 0;
 804 
 805 // Minimum time since last service thread check before notification is
 806 // permitted.  The value of 500ms was an arbitrary choice; frequent, but not
 807 // too frequent.
 808 const jlong cleanup_trigger_defer_period = 500 * NANOSECS_PER_MILLISEC;
 809 
 810 void OopStorage::trigger_cleanup_if_needed() {
 811   MonitorLocker ml(Service_lock, Monitor::_no_safepoint_check_flag);
 812   if (Atomic::load(&needs_cleanup_requested) &&
 813       !needs_cleanup_triggered &&
 814       (os::javaTimeNanos() > cleanup_trigger_permit_time)) {
 815     needs_cleanup_triggered = true;
 816     ml.notify_all();
 817   }
 818 }
 819 
 820 bool OopStorage::has_cleanup_work_and_reset() {
 821   assert_lock_strong(Service_lock);
 822   cleanup_trigger_permit_time =
 823     os::javaTimeNanos() + cleanup_trigger_defer_period;
 824   needs_cleanup_triggered = false;
 825   // Set the request flag false and return its old value.
 826   // Needs to be atomic to avoid dropping a concurrent request.
 827   // Can't use Atomic::xchg, which may not support bool.
 828   return Atomic::cmpxchg(false, &needs_cleanup_requested, true);
 829 }
 830 
 831 // Record that cleanup is needed, without notifying the Service thread.
 832 // Used by release(), where we can't lock even Service_lock.
 833 void OopStorage::record_needs_cleanup() {
 834   // Set local flag first, else service thread could wake up and miss
 835   // the request.  This order may instead (rarely) unnecessarily notify.
 836   OrderAccess::release_store(&_needs_cleanup, true);
 837   OrderAccess::release_store_fence(&needs_cleanup_requested, true);
 838 }
 839 
 840 bool OopStorage::delete_empty_blocks() {
 841   // Service thread might have oopstorage work, but not for this object.
 842   // Check for deferred updates even though that's not a service thread
 843   // trigger; since we're here, we might as well process them.
 844   if (!OrderAccess::load_acquire(&_needs_cleanup) &&
 845       (OrderAccess::load_acquire(&_deferred_updates) == NULL)) {
 846     return false;
 847   }
 848 
 849   MutexLocker ml(_allocation_mutex, Mutex::_no_safepoint_check_flag);
 850 
 851   // Clear the request before processing.
 852   OrderAccess::release_store_fence(&_needs_cleanup, false);
 853 
 854   // Other threads could be adding to the empty block count or the
 855   // deferred update list while we're working.  Set an upper bound on
 856   // how many updates we'll process and blocks we'll try to release,
 857   // so other threads can't cause an unbounded stay in this function.
 858   // We add a bit of slop because the reduce_deferred_updates clause
 859   // can cause blocks to be double counted.  If there are few blocks
 860   // and many of them are deferred and empty, we might hit the limit
 861   // and spin the caller without doing very much work.  Otherwise,
 862   // we don't normally hit the limit anyway, instead running out of
 863   // work to do.
 864   size_t limit = block_count() + 10;
 865 
 866   for (size_t i = 0; i < limit; ++i) {
 867     // Process deferred updates, which might make empty blocks available.
 868     // Continue checking once deletion starts, since additional updates
 869     // might become available while we're working.
 870     if (reduce_deferred_updates()) {
 871       // Be safepoint-polite while looping.
 872       MutexUnlocker ul(_allocation_mutex, Mutex::_no_safepoint_check_flag);
 873       ThreadBlockInVM tbiv(JavaThread::current());
 874     } else {
 875       Block* block = _allocation_list.tail();
 876       if ((block == NULL) || !block->is_empty()) {
 877         return false;
 878       } else if (!block->is_safe_to_delete()) {
 879         // Look for other work while waiting for block to be deletable.
 880         break;
 881       }
 882 
 883       // Try to delete the block.  First, try to remove from _active_array.
 884       {
 885         MutexLocker aml(_active_mutex, Mutex::_no_safepoint_check_flag);
 886         // Don't interfere with an active concurrent iteration.
 887         // Instead, give up immediately.  There is more work to do,
 888         // but don't re-notify, to avoid useless spinning of the
 889         // service thread.  Instead, iteration completion notifies.
 890         if (_concurrent_iteration_count > 0) return true;
 891         _active_array->remove(block);
 892       }
 893       // Remove block from _allocation_list and delete it.
 894       _allocation_list.unlink(*block);
 895       // Be safepoint-polite while deleting and looping.
 896       MutexUnlocker ul(_allocation_mutex, Mutex::_no_safepoint_check_flag);
 897       delete_empty_block(*block);
 898       ThreadBlockInVM tbiv(JavaThread::current());
 899     }
 900   }
 901   // Exceeded work limit or can't delete last block.  This will
 902   // cause the service thread to loop, giving other subtasks an
 903   // opportunity to run too.  There's no need for a notification,
 904   // because we are part of the service thread (unless gtesting).
 905   record_needs_cleanup();
 906   return true;
 907 }
 908 
 909 OopStorage::EntryStatus OopStorage::allocation_status(const oop* ptr) const {
 910   const Block* block = find_block_or_null(ptr);
 911   if (block != NULL) {
 912     // Prevent block deletion and _active_array modification.
 913     MutexLocker ml(_allocation_mutex, Mutex::_no_safepoint_check_flag);
 914     // Block could be a false positive, so get index carefully.
 915     size_t index = Block::active_index_safe(block);
 916     if ((index < _active_array->block_count()) &&
 917         (block == _active_array->at(index)) &&
 918         block->contains(ptr)) {
 919       if ((block->allocated_bitmask() & block->bitmask_for_entry(ptr)) != 0) {
 920         return ALLOCATED_ENTRY;
 921       } else {
 922         return UNALLOCATED_ENTRY;
 923       }
 924     }
 925   }
 926   return INVALID_ENTRY;
 927 }
 928 
 929 size_t OopStorage::allocation_count() const {
 930   return _allocation_count;
 931 }
 932 
 933 size_t OopStorage::block_count() const {
 934   WithActiveArray wab(this);
 935   // Count access is racy, but don't care.
 936   return wab.active_array().block_count();
 937 }
 938 
 939 size_t OopStorage::total_memory_usage() const {
 940   size_t total_size = sizeof(OopStorage);
 941   total_size += strlen(name()) + 1;
 942   total_size += sizeof(ActiveArray);
 943   WithActiveArray wab(this);
 944   const ActiveArray& blocks = wab.active_array();
 945   // Count access is racy, but don't care.
 946   total_size += blocks.block_count() * Block::allocation_size();
 947   total_size += blocks.size() * sizeof(Block*);
 948   return total_size;
 949 }
 950 
 951 // Parallel iteration support
 952 
 953 uint OopStorage::BasicParState::default_estimated_thread_count(bool concurrent) {
 954   uint configured = concurrent ? ConcGCThreads : ParallelGCThreads;
 955   return MAX2(1u, configured);  // Never estimate zero threads.
 956 }
 957 
 958 OopStorage::BasicParState::BasicParState(const OopStorage* storage,
 959                                          uint estimated_thread_count,
 960                                          bool concurrent) :
 961   _storage(storage),
 962   _active_array(_storage->obtain_active_array()),
 963   _block_count(0),              // initialized properly below
 964   _next_block(0),
 965   _estimated_thread_count(estimated_thread_count),
 966   _concurrent(concurrent)
 967 {
 968   assert(estimated_thread_count > 0, "estimated thread count must be positive");
 969   update_concurrent_iteration_count(1);
 970   // Get the block count *after* iteration state updated, so concurrent
 971   // empty block deletion is suppressed and can't reduce the count.  But
 972   // ensure the count we use was written after the block with that count
 973   // was fully initialized; see ActiveArray::push.
 974   _block_count = _active_array->block_count_acquire();
 975 }
 976 
 977 OopStorage::BasicParState::~BasicParState() {
 978   _storage->relinquish_block_array(_active_array);
 979   update_concurrent_iteration_count(-1);
 980   if (_concurrent) {
 981     // We may have deferred some cleanup work.
 982     const_cast<OopStorage*>(_storage)->record_needs_cleanup();
 983   }
 984 }
 985 
 986 void OopStorage::BasicParState::update_concurrent_iteration_count(int value) {
 987   if (_concurrent) {
 988     MutexLocker ml(_storage->_active_mutex, Mutex::_no_safepoint_check_flag);
 989     _storage->_concurrent_iteration_count += value;
 990     assert(_storage->_concurrent_iteration_count >= 0, "invariant");
 991   }
 992 }
 993 
 994 bool OopStorage::BasicParState::claim_next_segment(IterationData* data) {
 995   data->_processed += data->_segment_end - data->_segment_start;
 996   size_t start = OrderAccess::load_acquire(&_next_block);
 997   if (start >= _block_count) {
 998     return finish_iteration(data); // No more blocks available.
 999   }
1000   // Try to claim several at a time, but not *too* many.  We want to
1001   // avoid deciding there are many available and selecting a large
1002   // quantity, get delayed, and then end up claiming most or all of
1003   // the remaining largish amount of work, leaving nothing for other
1004   // threads to do.  But too small a step can lead to contention
1005   // over _next_block, esp. when the work per block is small.
1006   size_t max_step = 10;
1007   size_t remaining = _block_count - start;
1008   size_t step = MIN2(max_step, 1 + (remaining / _estimated_thread_count));
1009   // Atomic::add with possible overshoot.  This can perform better
1010   // than a CAS loop on some platforms when there is contention.
1011   // We can cope with the uncertainty by recomputing start/end from
1012   // the result of the add, and dealing with potential overshoot.
1013   size_t end = Atomic::add(step, &_next_block);
1014   // _next_block may have changed, so recompute start from result of add.
1015   start = end - step;
1016   // _next_block may have changed so much that end has overshot.
1017   end = MIN2(end, _block_count);
1018   // _next_block may have changed so much that even start has overshot.
1019   if (start < _block_count) {
1020     // Record claimed segment for iteration.
1021     data->_segment_start = start;
1022     data->_segment_end = end;
1023     return true;                // Success.
1024   } else {
1025     // No more blocks to claim.
1026     return finish_iteration(data);
1027   }
1028 }
1029 
1030 bool OopStorage::BasicParState::finish_iteration(const IterationData* data) const {
1031   log_info(oopstorage, blocks, stats)
1032           ("Parallel iteration on %s: blocks = " SIZE_FORMAT
1033            ", processed = " SIZE_FORMAT " (%2.f%%)",
1034            _storage->name(), _block_count, data->_processed,
1035            percent_of(data->_processed, _block_count));
1036   return false;
1037 }
1038 
1039 const char* OopStorage::name() const { return _name; }
1040 
1041 #ifndef PRODUCT
1042 
1043 void OopStorage::print_on(outputStream* st) const {
1044   size_t allocations = _allocation_count;
1045   size_t blocks = _active_array->block_count();
1046 
1047   double data_size = section_size * section_count;
1048   double alloc_percentage = percent_of((double)allocations, blocks * data_size);
1049 
1050   st->print("%s: " SIZE_FORMAT " entries in " SIZE_FORMAT " blocks (%.F%%), " SIZE_FORMAT " bytes",
1051             name(), allocations, blocks, alloc_percentage, total_memory_usage());
1052   if (_concurrent_iteration_count > 0) {
1053     st->print(", concurrent iteration active");
1054   }
1055 }
1056 
1057 #endif // !PRODUCT