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/*
* Copyright (c) 1998, 2022, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*
*/
#include "precompiled.hpp"
#include "classfile/vmSymbols.hpp"
#include "jfr/jfrEvents.hpp"
#include "logging/log.hpp"
#include "logging/logStream.hpp"
#include "memory/allocation.inline.hpp"
#include "memory/padded.hpp"
#include "memory/resourceArea.hpp"
#include "memory/universe.hpp"
#include "oops/markWord.hpp"
#include "oops/oop.inline.hpp"
#include "runtime/atomic.hpp"
#include "runtime/frame.inline.hpp"
#include "runtime/handles.inline.hpp"
#include "runtime/handshake.hpp"
#include "runtime/interfaceSupport.inline.hpp"
#include "runtime/javaThread.hpp"
#include "runtime/mutexLocker.hpp"
#include "runtime/objectMonitor.hpp"
#include "runtime/objectMonitor.inline.hpp"
#include "runtime/os.inline.hpp"
#include "runtime/osThread.hpp"
#include "runtime/perfData.hpp"
#include "runtime/safepointMechanism.inline.hpp"
#include "runtime/safepointVerifiers.hpp"
#include "runtime/sharedRuntime.hpp"
#include "runtime/stubRoutines.hpp"
#include "runtime/synchronizer.hpp"
#include "runtime/threads.hpp"
#include "runtime/timer.hpp"
#include "runtime/vframe.hpp"
#include "runtime/vmThread.hpp"
#include "utilities/align.hpp"
#include "utilities/dtrace.hpp"
#include "utilities/events.hpp"
#include "utilities/linkedlist.hpp"
#include "utilities/preserveException.hpp"
class ObjectMonitorsHashtable::PtrList :
public LinkedListImpl<ObjectMonitor*,
AnyObj::C_HEAP, mtThread,
AllocFailStrategy::RETURN_NULL> {};
class CleanupObjectMonitorsHashtable: StackObj {
public:
bool do_entry(void*& key, ObjectMonitorsHashtable::PtrList*& list) {
list->clear(); // clear the LinkListNodes
delete list; // then delete the LinkedList
return true;
}
};
ObjectMonitorsHashtable::~ObjectMonitorsHashtable() {
CleanupObjectMonitorsHashtable cleanup;
_ptrs->unlink(&cleanup); // cleanup the LinkedLists
delete _ptrs; // then delete the hash table
}
void ObjectMonitorsHashtable::add_entry(void* key, ObjectMonitor* om) {
ObjectMonitorsHashtable::PtrList* list = get_entry(key);
if (list == nullptr) {
// Create new list and add it to the hash table:
list = new (mtThread) ObjectMonitorsHashtable::PtrList;
add_entry(key, list);
}
list->add(om); // Add the ObjectMonitor to the list.
_om_count++;
}
bool ObjectMonitorsHashtable::has_entry(void* key, ObjectMonitor* om) {
ObjectMonitorsHashtable::PtrList* list = get_entry(key);
if (list == nullptr || list->find(om) == nullptr) {
return false;
}
return true;
}
void MonitorList::add(ObjectMonitor* m) {
ObjectMonitor* head;
do {
head = Atomic::load(&_head);
m->set_next_om(head);
} while (Atomic::cmpxchg(&_head, head, m) != head);
size_t count = Atomic::add(&_count, 1u);
if (count > max()) {
Atomic::inc(&_max);
}
}
size_t MonitorList::count() const {
return Atomic::load(&_count);
}
size_t MonitorList::max() const {
return Atomic::load(&_max);
}
// Walk the in-use list and unlink (at most MonitorDeflationMax) deflated
// ObjectMonitors. Returns the number of unlinked ObjectMonitors.
size_t MonitorList::unlink_deflated(Thread* current, LogStream* ls,
elapsedTimer* timer_p,
GrowableArray<ObjectMonitor*>* unlinked_list) {
size_t unlinked_count = 0;
ObjectMonitor* prev = NULL;
ObjectMonitor* head = Atomic::load_acquire(&_head);
ObjectMonitor* m = head;
// The in-use list head can be NULL during the final audit.
while (m != NULL) {
if (m->is_being_async_deflated()) {
// Find next live ObjectMonitor.
ObjectMonitor* next = m;
do {
ObjectMonitor* next_next = next->next_om();
unlinked_count++;
unlinked_list->append(next);
next = next_next;
if (unlinked_count >= (size_t)MonitorDeflationMax) {
// Reached the max so bail out on the gathering loop.
break;
}
} while (next != NULL && next->is_being_async_deflated());
if (prev == NULL) {
ObjectMonitor* prev_head = Atomic::cmpxchg(&_head, head, next);
if (prev_head != head) {
// Find new prev ObjectMonitor that just got inserted.
for (ObjectMonitor* n = prev_head; n != m; n = n->next_om()) {
prev = n;
}
prev->set_next_om(next);
}
} else {
prev->set_next_om(next);
}
if (unlinked_count >= (size_t)MonitorDeflationMax) {
// Reached the max so bail out on the searching loop.
break;
}
m = next;
} else {
prev = m;
m = m->next_om();
}
if (current->is_Java_thread()) {
// A JavaThread must check for a safepoint/handshake and honor it.
ObjectSynchronizer::chk_for_block_req(JavaThread::cast(current), "unlinking",
"unlinked_count", unlinked_count,
ls, timer_p);
}
}
Atomic::sub(&_count, unlinked_count);
return unlinked_count;
}
MonitorList::Iterator MonitorList::iterator() const {
return Iterator(Atomic::load_acquire(&_head));
}
ObjectMonitor* MonitorList::Iterator::next() {
ObjectMonitor* current = _current;
_current = current->next_om();
return current;
}
// The "core" versions of monitor enter and exit reside in this file.
// The interpreter and compilers contain specialized transliterated
// variants of the enter-exit fast-path operations. See c2_MacroAssembler_x86.cpp
// fast_lock(...) for instance. If you make changes here, make sure to modify the
// interpreter, and both C1 and C2 fast-path inline locking code emission.
//
// -----------------------------------------------------------------------------
#ifdef DTRACE_ENABLED
// Only bother with this argument setup if dtrace is available
// TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
#define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \
char* bytes = NULL; \
int len = 0; \
jlong jtid = SharedRuntime::get_java_tid(thread); \
Symbol* klassname = obj->klass()->name(); \
if (klassname != NULL) { \
bytes = (char*)klassname->bytes(); \
len = klassname->utf8_length(); \
}
#define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
{ \
if (DTraceMonitorProbes) { \
DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
HOTSPOT_MONITOR_WAIT(jtid, \
(uintptr_t)(monitor), bytes, len, (millis)); \
} \
}
#define HOTSPOT_MONITOR_PROBE_notify HOTSPOT_MONITOR_NOTIFY
#define HOTSPOT_MONITOR_PROBE_notifyAll HOTSPOT_MONITOR_NOTIFYALL
#define HOTSPOT_MONITOR_PROBE_waited HOTSPOT_MONITOR_WAITED
#define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
{ \
if (DTraceMonitorProbes) { \
DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
HOTSPOT_MONITOR_PROBE_##probe(jtid, /* probe = waited */ \
(uintptr_t)(monitor), bytes, len); \
} \
}
#else // ndef DTRACE_ENABLED
#define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;}
#define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;}
#endif // ndef DTRACE_ENABLED
// This exists only as a workaround of dtrace bug 6254741
int dtrace_waited_probe(ObjectMonitor* monitor, Handle obj, JavaThread* thr) {
DTRACE_MONITOR_PROBE(waited, monitor, obj(), thr);
return 0;
}
static const int NINFLATIONLOCKS = 256;
static PlatformMutex* gInflationLocks[NINFLATIONLOCKS];
void ObjectSynchronizer::initialize() {
for (int i = 0; i < NINFLATIONLOCKS; i++) {
gInflationLocks[i] = new PlatformMutex();
}
// Start the ceiling with the estimate for one thread.
set_in_use_list_ceiling(AvgMonitorsPerThreadEstimate);
}
MonitorList ObjectSynchronizer::_in_use_list;
// monitors_used_above_threshold() policy is as follows:
//
// The ratio of the current _in_use_list count to the ceiling is used
// to determine if we are above MonitorUsedDeflationThreshold and need
// to do an async monitor deflation cycle. The ceiling is increased by
// AvgMonitorsPerThreadEstimate when a thread is added to the system
// and is decreased by AvgMonitorsPerThreadEstimate when a thread is
// removed from the system.
//
// Note: If the _in_use_list max exceeds the ceiling, then
// monitors_used_above_threshold() will use the in_use_list max instead
// of the thread count derived ceiling because we have used more
// ObjectMonitors than the estimated average.
//
// Note: If deflate_idle_monitors() has NoAsyncDeflationProgressMax
// no-progress async monitor deflation cycles in a row, then the ceiling
// is adjusted upwards by monitors_used_above_threshold().
//
// Start the ceiling with the estimate for one thread in initialize()
// which is called after cmd line options are processed.
static size_t _in_use_list_ceiling = 0;
bool volatile ObjectSynchronizer::_is_async_deflation_requested = false;
bool volatile ObjectSynchronizer::_is_final_audit = false;
jlong ObjectSynchronizer::_last_async_deflation_time_ns = 0;
static uintx _no_progress_cnt = 0;
// =====================> Quick functions
// The quick_* forms are special fast-path variants used to improve
// performance. In the simplest case, a "quick_*" implementation could
// simply return false, in which case the caller will perform the necessary
// state transitions and call the slow-path form.
// The fast-path is designed to handle frequently arising cases in an efficient
// manner and is just a degenerate "optimistic" variant of the slow-path.
// returns true -- to indicate the call was satisfied.
// returns false -- to indicate the call needs the services of the slow-path.
// A no-loitering ordinance is in effect for code in the quick_* family
// operators: safepoints or indefinite blocking (blocking that might span a
// safepoint) are forbidden. Generally the thread_state() is _in_Java upon
// entry.
//
// Consider: An interesting optimization is to have the JIT recognize the
// following common idiom:
// synchronized (someobj) { .... ; notify(); }
// That is, we find a notify() or notifyAll() call that immediately precedes
// the monitorexit operation. In that case the JIT could fuse the operations
// into a single notifyAndExit() runtime primitive.
bool ObjectSynchronizer::quick_notify(oopDesc* obj, JavaThread* current, bool all) {
assert(current->thread_state() == _thread_in_Java, "invariant");
NoSafepointVerifier nsv;
if (obj == NULL) return false; // slow-path for invalid obj
const markWord mark = obj->mark();
if (mark.has_locker() && current->is_lock_owned((address)mark.locker())) {
// Degenerate notify
// stack-locked by caller so by definition the implied waitset is empty.
return true;
}
if (mark.has_monitor()) {
ObjectMonitor* const mon = mark.monitor();
assert(mon->object() == oop(obj), "invariant");
if (mon->owner() != current) return false; // slow-path for IMS exception
if (mon->first_waiter() != NULL) {
// We have one or more waiters. Since this is an inflated monitor
// that we own, we can transfer one or more threads from the waitset
// to the entrylist here and now, avoiding the slow-path.
if (all) {
DTRACE_MONITOR_PROBE(notifyAll, mon, obj, current);
} else {
DTRACE_MONITOR_PROBE(notify, mon, obj, current);
}
int free_count = 0;
do {
mon->INotify(current);
++free_count;
} while (mon->first_waiter() != NULL && all);
OM_PERFDATA_OP(Notifications, inc(free_count));
}
return true;
}
// other IMS exception states take the slow-path
return false;
}
// The LockNode emitted directly at the synchronization site would have
// been too big if it were to have included support for the cases of inflated
// recursive enter and exit, so they go here instead.
// Note that we can't safely call AsyncPrintJavaStack() from within
// quick_enter() as our thread state remains _in_Java.
bool ObjectSynchronizer::quick_enter(oop obj, JavaThread* current,
BasicLock * lock) {
assert(current->thread_state() == _thread_in_Java, "invariant");
NoSafepointVerifier nsv;
if (obj == NULL) return false; // Need to throw NPE
if (obj->klass()->is_value_based()) {
return false;
}
const markWord mark = obj->mark();
if (mark.has_monitor()) {
ObjectMonitor* const m = mark.monitor();
// An async deflation or GC can race us before we manage to make
// the ObjectMonitor busy by setting the owner below. If we detect
// that race we just bail out to the slow-path here.
if (m->object_peek() == NULL) {
return false;
}
JavaThread* const owner = (JavaThread*) m->owner_raw();
// Lock contention and Transactional Lock Elision (TLE) diagnostics
// and observability
// Case: light contention possibly amenable to TLE
// Case: TLE inimical operations such as nested/recursive synchronization
if (owner == current) {
m->_recursions++;
current->inc_held_monitor_count();
return true;
}
// This Java Monitor is inflated so obj's header will never be
// displaced to this thread's BasicLock. Make the displaced header
// non-NULL so this BasicLock is not seen as recursive nor as
// being locked. We do this unconditionally so that this thread's
// BasicLock cannot be mis-interpreted by any stack walkers. For
// performance reasons, stack walkers generally first check for
// stack-locking in the object's header, the second check is for
// recursive stack-locking in the displaced header in the BasicLock,
// and last are the inflated Java Monitor (ObjectMonitor) checks.
lock->set_displaced_header(markWord::unused_mark());
if (owner == NULL && m->try_set_owner_from(NULL, current) == NULL) {
assert(m->_recursions == 0, "invariant");
current->inc_held_monitor_count();
return true;
}
}
// Note that we could inflate in quick_enter.
// This is likely a useful optimization
// Critically, in quick_enter() we must not:
// -- block indefinitely, or
// -- reach a safepoint
return false; // revert to slow-path
}
// Handle notifications when synchronizing on value based classes
void ObjectSynchronizer::handle_sync_on_value_based_class(Handle obj, JavaThread* current) {
frame last_frame = current->last_frame();
bool bcp_was_adjusted = false;
// Don't decrement bcp if it points to the frame's first instruction. This happens when
// handle_sync_on_value_based_class() is called because of a synchronized method. There
// is no actual monitorenter instruction in the byte code in this case.
if (last_frame.is_interpreted_frame() &&
(last_frame.interpreter_frame_method()->code_base() < last_frame.interpreter_frame_bcp())) {
// adjust bcp to point back to monitorenter so that we print the correct line numbers
last_frame.interpreter_frame_set_bcp(last_frame.interpreter_frame_bcp() - 1);
bcp_was_adjusted = true;
}
if (DiagnoseSyncOnValueBasedClasses == FATAL_EXIT) {
ResourceMark rm(current);
stringStream ss;
current->print_stack_on(&ss);
char* base = (char*)strstr(ss.base(), "at");
char* newline = (char*)strchr(ss.base(), '\n');
if (newline != NULL) {
*newline = '\0';
}
fatal("Synchronizing on object " INTPTR_FORMAT " of klass %s %s", p2i(obj()), obj->klass()->external_name(), base);
} else {
assert(DiagnoseSyncOnValueBasedClasses == LOG_WARNING, "invalid value for DiagnoseSyncOnValueBasedClasses");
ResourceMark rm(current);
Log(valuebasedclasses) vblog;
vblog.info("Synchronizing on object " INTPTR_FORMAT " of klass %s", p2i(obj()), obj->klass()->external_name());
if (current->has_last_Java_frame()) {
LogStream info_stream(vblog.info());
current->print_stack_on(&info_stream);
} else {
vblog.info("Cannot find the last Java frame");
}
EventSyncOnValueBasedClass event;
if (event.should_commit()) {
event.set_valueBasedClass(obj->klass());
event.commit();
}
}
if (bcp_was_adjusted) {
last_frame.interpreter_frame_set_bcp(last_frame.interpreter_frame_bcp() + 1);
}
}
static bool useHeavyMonitors() {
#if defined(X86) || defined(AARCH64) || defined(PPC64) || defined(RISCV64)
return UseHeavyMonitors;
#else
return false;
#endif
}
// -----------------------------------------------------------------------------
// Monitor Enter/Exit
// The interpreter and compiler assembly code tries to lock using the fast path
// of this algorithm. Make sure to update that code if the following function is
// changed. The implementation is extremely sensitive to race condition. Be careful.
void ObjectSynchronizer::enter(Handle obj, BasicLock* lock, JavaThread* current) {
if (obj->klass()->is_value_based()) {
handle_sync_on_value_based_class(obj, current);
}
current->inc_held_monitor_count();
if (!useHeavyMonitors()) {
markWord mark = obj->mark();
if (mark.is_neutral()) {
// Anticipate successful CAS -- the ST of the displaced mark must
// be visible <= the ST performed by the CAS.
lock->set_displaced_header(mark);
if (mark == obj()->cas_set_mark(markWord::from_pointer(lock), mark)) {
return;
}
// Fall through to inflate() ...
} else if (mark.has_locker() &&
current->is_lock_owned((address)mark.locker())) {
assert(lock != mark.locker(), "must not re-lock the same lock");
assert(lock != (BasicLock*)obj->mark().value(), "don't relock with same BasicLock");
lock->set_displaced_header(markWord::from_pointer(NULL));
return;
}
// The object header will never be displaced to this lock,
// so it does not matter what the value is, except that it
// must be non-zero to avoid looking like a re-entrant lock,
// and must not look locked either.
lock->set_displaced_header(markWord::unused_mark());
} else if (VerifyHeavyMonitors) {
guarantee(!obj->mark().has_locker(), "must not be stack-locked");
}
// An async deflation can race after the inflate() call and before
// enter() can make the ObjectMonitor busy. enter() returns false if
// we have lost the race to async deflation and we simply try again.
while (true) {
ObjectMonitor* monitor = inflate(current, obj(), inflate_cause_monitor_enter);
if (monitor->enter(current)) {
return;
}
}
}
void ObjectSynchronizer::exit(oop object, BasicLock* lock, JavaThread* current) {
current->dec_held_monitor_count();
if (!useHeavyMonitors()) {
markWord mark = object->mark();
markWord dhw = lock->displaced_header();
if (dhw.value() == 0) {
// If the displaced header is NULL, then this exit matches up with
// a recursive enter. No real work to do here except for diagnostics.
#ifndef PRODUCT
if (mark != markWord::INFLATING()) {
// Only do diagnostics if we are not racing an inflation. Simply
// exiting a recursive enter of a Java Monitor that is being
// inflated is safe; see the has_monitor() comment below.
assert(!mark.is_neutral(), "invariant");
assert(!mark.has_locker() ||
current->is_lock_owned((address)mark.locker()), "invariant");
if (mark.has_monitor()) {
// The BasicLock's displaced_header is marked as a recursive
// enter and we have an inflated Java Monitor (ObjectMonitor).
// This is a special case where the Java Monitor was inflated
// after this thread entered the stack-lock recursively. When a
// Java Monitor is inflated, we cannot safely walk the Java
// Monitor owner's stack and update the BasicLocks because a
// Java Monitor can be asynchronously inflated by a thread that
// does not own the Java Monitor.
ObjectMonitor* m = mark.monitor();
assert(m->object()->mark() == mark, "invariant");
assert(m->is_entered(current), "invariant");
}
}
#endif
return;
}
if (mark == markWord::from_pointer(lock)) {
// If the object is stack-locked by the current thread, try to
// swing the displaced header from the BasicLock back to the mark.
assert(dhw.is_neutral(), "invariant");
if (object->cas_set_mark(dhw, mark) == mark) {
return;
}
}
} else if (VerifyHeavyMonitors) {
guarantee(!object->mark().has_locker(), "must not be stack-locked");
}
// We have to take the slow-path of possible inflation and then exit.
// The ObjectMonitor* can't be async deflated until ownership is
// dropped inside exit() and the ObjectMonitor* must be !is_busy().
ObjectMonitor* monitor = inflate(current, object, inflate_cause_vm_internal);
monitor->exit(current);
}
// -----------------------------------------------------------------------------
// Class Loader support to workaround deadlocks on the class loader lock objects
// Also used by GC
// complete_exit()/reenter() are used to wait on a nested lock
// i.e. to give up an outer lock completely and then re-enter
// Used when holding nested locks - lock acquisition order: lock1 then lock2
// 1) complete_exit lock1 - saving recursion count
// 2) wait on lock2
// 3) when notified on lock2, unlock lock2
// 4) reenter lock1 with original recursion count
// 5) lock lock2
// NOTE: must use heavy weight monitor to handle complete_exit/reenter()
intx ObjectSynchronizer::complete_exit(Handle obj, JavaThread* current) {
// The ObjectMonitor* can't be async deflated until ownership is
// dropped inside exit() and the ObjectMonitor* must be !is_busy().
ObjectMonitor* monitor = inflate(current, obj(), inflate_cause_vm_internal);
intx recur_count = monitor->complete_exit(current);
current->dec_held_monitor_count(recur_count + 1);
return recur_count;
}
// NOTE: must use heavy weight monitor to handle complete_exit/reenter()
void ObjectSynchronizer::reenter(Handle obj, intx recursions, JavaThread* current) {
// An async deflation can race after the inflate() call and before
// reenter() -> enter() can make the ObjectMonitor busy. reenter() ->
// enter() returns false if we have lost the race to async deflation
// and we simply try again.
while (true) {
ObjectMonitor* monitor = inflate(current, obj(), inflate_cause_vm_internal);
if (monitor->reenter(recursions, current)) {
current->inc_held_monitor_count(recursions + 1);
return;
}
}
}
// -----------------------------------------------------------------------------
// JNI locks on java objects
// NOTE: must use heavy weight monitor to handle jni monitor enter
void ObjectSynchronizer::jni_enter(Handle obj, JavaThread* current) {
if (obj->klass()->is_value_based()) {
handle_sync_on_value_based_class(obj, current);
}
// the current locking is from JNI instead of Java code
current->set_current_pending_monitor_is_from_java(false);
// An async deflation can race after the inflate() call and before
// enter() can make the ObjectMonitor busy. enter() returns false if
// we have lost the race to async deflation and we simply try again.
while (true) {
ObjectMonitor* monitor = inflate(current, obj(), inflate_cause_jni_enter);
if (monitor->enter(current)) {
current->inc_held_monitor_count(1, true);
break;
}
}
current->set_current_pending_monitor_is_from_java(true);
}
// NOTE: must use heavy weight monitor to handle jni monitor exit
void ObjectSynchronizer::jni_exit(oop obj, TRAPS) {
JavaThread* current = THREAD;
// The ObjectMonitor* can't be async deflated until ownership is
// dropped inside exit() and the ObjectMonitor* must be !is_busy().
ObjectMonitor* monitor = inflate(current, obj, inflate_cause_jni_exit);
// If this thread has locked the object, exit the monitor. We
// intentionally do not use CHECK on check_owner because we must exit the
// monitor even if an exception was already pending.
if (monitor->check_owner(THREAD)) {
monitor->exit(current);
current->dec_held_monitor_count(1, true);
}
}
// -----------------------------------------------------------------------------
// Internal VM locks on java objects
// standard constructor, allows locking failures
ObjectLocker::ObjectLocker(Handle obj, JavaThread* thread) {
_thread = thread;
_thread->check_for_valid_safepoint_state();
_obj = obj;
if (_obj() != NULL) {
ObjectSynchronizer::enter(_obj, &_lock, _thread);
}
}
ObjectLocker::~ObjectLocker() {
if (_obj() != NULL) {
ObjectSynchronizer::exit(_obj(), &_lock, _thread);
}
}
// -----------------------------------------------------------------------------
// Wait/Notify/NotifyAll
// NOTE: must use heavy weight monitor to handle wait()
int ObjectSynchronizer::wait(Handle obj, jlong millis, TRAPS) {
JavaThread* current = THREAD;
if (millis < 0) {
THROW_MSG_0(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
}
// The ObjectMonitor* can't be async deflated because the _waiters
// field is incremented before ownership is dropped and decremented
// after ownership is regained.
ObjectMonitor* monitor = inflate(current, obj(), inflate_cause_wait);
DTRACE_MONITOR_WAIT_PROBE(monitor, obj(), current, millis);
monitor->wait(millis, true, THREAD); // Not CHECK as we need following code
// This dummy call is in place to get around dtrace bug 6254741. Once
// that's fixed we can uncomment the following line, remove the call
// and change this function back into a "void" func.
// DTRACE_MONITOR_PROBE(waited, monitor, obj(), THREAD);
int ret_code = dtrace_waited_probe(monitor, obj, THREAD);
return ret_code;
}
void ObjectSynchronizer::notify(Handle obj, TRAPS) {
JavaThread* current = THREAD;
markWord mark = obj->mark();
if (mark.has_locker() && current->is_lock_owned((address)mark.locker())) {
// Not inflated so there can't be any waiters to notify.
return;
}
// The ObjectMonitor* can't be async deflated until ownership is
// dropped by the calling thread.
ObjectMonitor* monitor = inflate(current, obj(), inflate_cause_notify);
monitor->notify(CHECK);
}
// NOTE: see comment of notify()
void ObjectSynchronizer::notifyall(Handle obj, TRAPS) {
JavaThread* current = THREAD;
markWord mark = obj->mark();
if (mark.has_locker() && current->is_lock_owned((address)mark.locker())) {
// Not inflated so there can't be any waiters to notify.
return;
}
// The ObjectMonitor* can't be async deflated until ownership is
// dropped by the calling thread.
ObjectMonitor* monitor = inflate(current, obj(), inflate_cause_notify);
monitor->notifyAll(CHECK);
}
// -----------------------------------------------------------------------------
// Hash Code handling
struct SharedGlobals {
char _pad_prefix[OM_CACHE_LINE_SIZE];
// This is a highly shared mostly-read variable.
// To avoid false-sharing it needs to be the sole occupant of a cache line.
volatile int stw_random;
DEFINE_PAD_MINUS_SIZE(1, OM_CACHE_LINE_SIZE, sizeof(volatile int));
// Hot RW variable -- Sequester to avoid false-sharing
volatile int hc_sequence;
DEFINE_PAD_MINUS_SIZE(2, OM_CACHE_LINE_SIZE, sizeof(volatile int));
};
static SharedGlobals GVars;
static markWord read_stable_mark(oop obj) {
markWord mark = obj->mark_acquire();
if (!mark.is_being_inflated()) {
return mark; // normal fast-path return
}
int its = 0;
for (;;) {
markWord mark = obj->mark_acquire();
if (!mark.is_being_inflated()) {
return mark; // normal fast-path return
}
// The object is being inflated by some other thread.
// The caller of read_stable_mark() must wait for inflation to complete.
// Avoid live-lock.
++its;
if (its > 10000 || !os::is_MP()) {
if (its & 1) {
os::naked_yield();
} else {
// Note that the following code attenuates the livelock problem but is not
// a complete remedy. A more complete solution would require that the inflating
// thread hold the associated inflation lock. The following code simply restricts
// the number of spinners to at most one. We'll have N-2 threads blocked
// on the inflationlock, 1 thread holding the inflation lock and using
// a yield/park strategy, and 1 thread in the midst of inflation.
// A more refined approach would be to change the encoding of INFLATING
// to allow encapsulation of a native thread pointer. Threads waiting for
// inflation to complete would use CAS to push themselves onto a singly linked
// list rooted at the markword. Once enqueued, they'd loop, checking a per-thread flag
// and calling park(). When inflation was complete the thread that accomplished inflation
// would detach the list and set the markword to inflated with a single CAS and
// then for each thread on the list, set the flag and unpark() the thread.
// Index into the lock array based on the current object address.
static_assert(is_power_of_2(NINFLATIONLOCKS), "must be");
int ix = (cast_from_oop<intptr_t>(obj) >> 5) & (NINFLATIONLOCKS-1);
int YieldThenBlock = 0;
assert(ix >= 0 && ix < NINFLATIONLOCKS, "invariant");
gInflationLocks[ix]->lock();
while (obj->mark_acquire() == markWord::INFLATING()) {
// Beware: naked_yield() is advisory and has almost no effect on some platforms
// so we periodically call current->_ParkEvent->park(1).
// We use a mixed spin/yield/block mechanism.
if ((YieldThenBlock++) >= 16) {
Thread::current()->_ParkEvent->park(1);
} else {
os::naked_yield();
}
}
gInflationLocks[ix]->unlock();
}
} else {
SpinPause(); // SMP-polite spinning
}
}
}
// hashCode() generation :
//
// Possibilities:
// * MD5Digest of {obj,stw_random}
// * CRC32 of {obj,stw_random} or any linear-feedback shift register function.
// * A DES- or AES-style SBox[] mechanism
// * One of the Phi-based schemes, such as:
// 2654435761 = 2^32 * Phi (golden ratio)
// HashCodeValue = ((uintptr_t(obj) >> 3) * 2654435761) ^ GVars.stw_random ;
// * A variation of Marsaglia's shift-xor RNG scheme.
// * (obj ^ stw_random) is appealing, but can result
// in undesirable regularity in the hashCode values of adjacent objects
// (objects allocated back-to-back, in particular). This could potentially
// result in hashtable collisions and reduced hashtable efficiency.
// There are simple ways to "diffuse" the middle address bits over the
// generated hashCode values:
static inline intptr_t get_next_hash(Thread* current, oop obj) {
intptr_t value = 0;
if (hashCode == 0) {
// This form uses global Park-Miller RNG.
// On MP system we'll have lots of RW access to a global, so the
// mechanism induces lots of coherency traffic.
value = os::random();
} else if (hashCode == 1) {
// This variation has the property of being stable (idempotent)
// between STW operations. This can be useful in some of the 1-0
// synchronization schemes.
intptr_t addr_bits = cast_from_oop<intptr_t>(obj) >> 3;
value = addr_bits ^ (addr_bits >> 5) ^ GVars.stw_random;
} else if (hashCode == 2) {
value = 1; // for sensitivity testing
} else if (hashCode == 3) {
value = ++GVars.hc_sequence;
} else if (hashCode == 4) {
value = cast_from_oop<intptr_t>(obj);
} else {
// Marsaglia's xor-shift scheme with thread-specific state
// This is probably the best overall implementation -- we'll
// likely make this the default in future releases.
unsigned t = current->_hashStateX;
t ^= (t << 11);
current->_hashStateX = current->_hashStateY;
current->_hashStateY = current->_hashStateZ;
current->_hashStateZ = current->_hashStateW;
unsigned v = current->_hashStateW;
v = (v ^ (v >> 19)) ^ (t ^ (t >> 8));
current->_hashStateW = v;
value = v;
}
value &= markWord::hash_mask;
if (value == 0) value = 0xBAD;
assert(value != markWord::no_hash, "invariant");
return value;
}
intptr_t ObjectSynchronizer::FastHashCode(Thread* current, oop obj) {
while (true) {
ObjectMonitor* monitor = NULL;
markWord temp, test;
intptr_t hash;
markWord mark = read_stable_mark(obj);
if (VerifyHeavyMonitors) {
assert(UseHeavyMonitors, "+VerifyHeavyMonitors requires +UseHeavyMonitors");
guarantee(!mark.has_locker(), "must not be stack locked");
}
if (mark.is_neutral()) { // if this is a normal header
hash = mark.hash();
if (hash != 0) { // if it has a hash, just return it
return hash;
}
hash = get_next_hash(current, obj); // get a new hash
temp = mark.copy_set_hash(hash); // merge the hash into header
// try to install the hash
test = obj->cas_set_mark(temp, mark);
if (test == mark) { // if the hash was installed, return it
return hash;
}
// Failed to install the hash. It could be that another thread
// installed the hash just before our attempt or inflation has
// occurred or... so we fall thru to inflate the monitor for
// stability and then install the hash.
} else if (mark.has_monitor()) {
monitor = mark.monitor();
temp = monitor->header();
assert(temp.is_neutral(), "invariant: header=" INTPTR_FORMAT, temp.value());
hash = temp.hash();
if (hash != 0) {
// It has a hash.
// Separate load of dmw/header above from the loads in
// is_being_async_deflated().
// dmw/header and _contentions may get written by different threads.
// Make sure to observe them in the same order when having several observers.
OrderAccess::loadload_for_IRIW();
if (monitor->is_being_async_deflated()) {
// But we can't safely use the hash if we detect that async
// deflation has occurred. So we attempt to restore the
// header/dmw to the object's header so that we only retry
// once if the deflater thread happens to be slow.
monitor->install_displaced_markword_in_object(obj);
continue;
}
return hash;
}
// Fall thru so we only have one place that installs the hash in
// the ObjectMonitor.
} else if (current->is_lock_owned((address)mark.locker())) {
// This is a stack lock owned by the calling thread so fetch the
// displaced markWord from the BasicLock on the stack.
temp = mark.displaced_mark_helper();
assert(temp.is_neutral(), "invariant: header=" INTPTR_FORMAT, temp.value());
hash = temp.hash();
if (hash != 0) { // if it has a hash, just return it
return hash;
}
// WARNING:
// The displaced header in the BasicLock on a thread's stack
// is strictly immutable. It CANNOT be changed in ANY cases.
// So we have to inflate the stack lock into an ObjectMonitor
// even if the current thread owns the lock. The BasicLock on
// a thread's stack can be asynchronously read by other threads
// during an inflate() call so any change to that stack memory
// may not propagate to other threads correctly.
}
// Inflate the monitor to set the hash.
// An async deflation can race after the inflate() call and before we
// can update the ObjectMonitor's header with the hash value below.
monitor = inflate(current, obj, inflate_cause_hash_code);
// Load ObjectMonitor's header/dmw field and see if it has a hash.
mark = monitor->header();
assert(mark.is_neutral(), "invariant: header=" INTPTR_FORMAT, mark.value());
hash = mark.hash();
if (hash == 0) { // if it does not have a hash
hash = get_next_hash(current, obj); // get a new hash
temp = mark.copy_set_hash(hash) ; // merge the hash into header
assert(temp.is_neutral(), "invariant: header=" INTPTR_FORMAT, temp.value());
uintptr_t v = Atomic::cmpxchg((volatile uintptr_t*)monitor->header_addr(), mark.value(), temp.value());
test = markWord(v);
if (test != mark) {
// The attempt to update the ObjectMonitor's header/dmw field
// did not work. This can happen if another thread managed to
// merge in the hash just before our cmpxchg().
// If we add any new usages of the header/dmw field, this code
// will need to be updated.
hash = test.hash();
assert(test.is_neutral(), "invariant: header=" INTPTR_FORMAT, test.value());
assert(hash != 0, "should only have lost the race to a thread that set a non-zero hash");
}
if (monitor->is_being_async_deflated()) {
// If we detect that async deflation has occurred, then we
// attempt to restore the header/dmw to the object's header
// so that we only retry once if the deflater thread happens
// to be slow.
monitor->install_displaced_markword_in_object(obj);
continue;
}
}
// We finally get the hash.
return hash;
}
}
bool ObjectSynchronizer::current_thread_holds_lock(JavaThread* current,
Handle h_obj) {
assert(current == JavaThread::current(), "Can only be called on current thread");
oop obj = h_obj();
markWord mark = read_stable_mark(obj);
// Uncontended case, header points to stack
if (mark.has_locker()) {
return current->is_lock_owned((address)mark.locker());
}
// Contended case, header points to ObjectMonitor (tagged pointer)
if (mark.has_monitor()) {
// The first stage of async deflation does not affect any field
// used by this comparison so the ObjectMonitor* is usable here.
ObjectMonitor* monitor = mark.monitor();
return monitor->is_entered(current) != 0;
}
// Unlocked case, header in place
assert(mark.is_neutral(), "sanity check");
return false;
}
JavaThread* ObjectSynchronizer::get_lock_owner(ThreadsList * t_list, Handle h_obj) {
oop obj = h_obj();
address owner = NULL;
markWord mark = read_stable_mark(obj);
// Uncontended case, header points to stack
if (mark.has_locker()) {
owner = (address) mark.locker();
}
// Contended case, header points to ObjectMonitor (tagged pointer)
else if (mark.has_monitor()) {
// The first stage of async deflation does not affect any field
// used by this comparison so the ObjectMonitor* is usable here.
ObjectMonitor* monitor = mark.monitor();
assert(monitor != NULL, "monitor should be non-null");
owner = (address) monitor->owner();
}
if (owner != NULL) {
// owning_thread_from_monitor_owner() may also return NULL here
return Threads::owning_thread_from_monitor_owner(t_list, owner);
}
// Unlocked case, header in place
// Cannot have assertion since this object may have been
// locked by another thread when reaching here.
// assert(mark.is_neutral(), "sanity check");
return NULL;
}
// Visitors ...
// Iterate ObjectMonitors where the owner == thread; this does NOT include
// ObjectMonitors where owner is set to a stack lock address in thread.
//
// This version of monitors_iterate() works with the in-use monitor list.
//
void ObjectSynchronizer::monitors_iterate(MonitorClosure* closure, JavaThread* thread) {
MonitorList::Iterator iter = _in_use_list.iterator();
while (iter.has_next()) {
ObjectMonitor* mid = iter.next();
if (mid->owner() != thread) {
// Not owned by the target thread and intentionally skips when owner
// is set to a stack lock address in the target thread.
continue;
}
if (!mid->is_being_async_deflated() && mid->object_peek() != NULL) {
// Only process with closure if the object is set.
// monitors_iterate() is only called at a safepoint or when the
// target thread is suspended or when the target thread is
// operating on itself. The current closures in use today are
// only interested in an owned ObjectMonitor and ownership
// cannot be dropped under the calling contexts so the
// ObjectMonitor cannot be async deflated.
closure->do_monitor(mid);
}
}
}
// This version of monitors_iterate() works with the specified linked list.
//
void ObjectSynchronizer::monitors_iterate(MonitorClosure* closure,
ObjectMonitorsHashtable::PtrList* list,
JavaThread* thread) {
typedef LinkedListIterator<ObjectMonitor*> ObjectMonitorIterator;
ObjectMonitorIterator iter(list->head());
while (!iter.is_empty()) {
ObjectMonitor* mid = *iter.next();
// Owner set to a stack lock address in thread should never be seen here:
assert(mid->owner() == thread, "must be");
if (!mid->is_being_async_deflated() && mid->object_peek() != NULL) {
// Only process with closure if the object is set.
// monitors_iterate() is only called at a safepoint or when the
// target thread is suspended or when the target thread is
// operating on itself. The current closures in use today are
// only interested in an owned ObjectMonitor and ownership
// cannot be dropped under the calling contexts so the
// ObjectMonitor cannot be async deflated.
closure->do_monitor(mid);
}
}
}
static bool monitors_used_above_threshold(MonitorList* list) {
if (MonitorUsedDeflationThreshold == 0) { // disabled case is easy
return false;
}
// Start with ceiling based on a per-thread estimate:
size_t ceiling = ObjectSynchronizer::in_use_list_ceiling();
size_t old_ceiling = ceiling;
if (ceiling < list->max()) {
// The max used by the system has exceeded the ceiling so use that:
ceiling = list->max();
}
size_t monitors_used = list->count();
if (monitors_used == 0) { // empty list is easy
return false;
}
if (NoAsyncDeflationProgressMax != 0 &&
_no_progress_cnt >= NoAsyncDeflationProgressMax) {
float remainder = (100.0 - MonitorUsedDeflationThreshold) / 100.0;
size_t new_ceiling = ceiling + (ceiling * remainder) + 1;
ObjectSynchronizer::set_in_use_list_ceiling(new_ceiling);
log_info(monitorinflation)("Too many deflations without progress; "
"bumping in_use_list_ceiling from " SIZE_FORMAT
" to " SIZE_FORMAT, old_ceiling, new_ceiling);
_no_progress_cnt = 0;
ceiling = new_ceiling;
}
// Check if our monitor usage is above the threshold:
size_t monitor_usage = (monitors_used * 100LL) / ceiling;
return int(monitor_usage) > MonitorUsedDeflationThreshold;
}
size_t ObjectSynchronizer::in_use_list_ceiling() {
return _in_use_list_ceiling;
}
void ObjectSynchronizer::dec_in_use_list_ceiling() {
Atomic::sub(&_in_use_list_ceiling, AvgMonitorsPerThreadEstimate);
}
void ObjectSynchronizer::inc_in_use_list_ceiling() {
Atomic::add(&_in_use_list_ceiling, AvgMonitorsPerThreadEstimate);
}
void ObjectSynchronizer::set_in_use_list_ceiling(size_t new_value) {
_in_use_list_ceiling = new_value;
}
bool ObjectSynchronizer::is_async_deflation_needed() {
if (is_async_deflation_requested()) {
// Async deflation request.
return true;
}
if (AsyncDeflationInterval > 0 &&
time_since_last_async_deflation_ms() > AsyncDeflationInterval &&
monitors_used_above_threshold(&_in_use_list)) {
// It's been longer than our specified deflate interval and there
// are too many monitors in use. We don't deflate more frequently
// than AsyncDeflationInterval (unless is_async_deflation_requested)
// in order to not swamp the MonitorDeflationThread.
return true;
}
return false;
}
bool ObjectSynchronizer::request_deflate_idle_monitors() {
JavaThread* current = JavaThread::current();
bool ret_code = false;
jlong last_time = last_async_deflation_time_ns();
set_is_async_deflation_requested(true);
{
MonitorLocker ml(MonitorDeflation_lock, Mutex::_no_safepoint_check_flag);
ml.notify_all();
}
const int N_CHECKS = 5;
for (int i = 0; i < N_CHECKS; i++) { // sleep for at most 5 seconds
if (last_async_deflation_time_ns() > last_time) {
log_info(monitorinflation)("Async Deflation happened after %d check(s).", i);
ret_code = true;
break;
}
{
// JavaThread has to honor the blocking protocol.
ThreadBlockInVM tbivm(current);
os::naked_short_sleep(999); // sleep for almost 1 second
}
}
if (!ret_code) {
log_info(monitorinflation)("Async Deflation DID NOT happen after %d checks.", N_CHECKS);
}
return ret_code;
}
jlong ObjectSynchronizer::time_since_last_async_deflation_ms() {
return (os::javaTimeNanos() - last_async_deflation_time_ns()) / (NANOUNITS / MILLIUNITS);
}
static void post_monitor_inflate_event(EventJavaMonitorInflate* event,
const oop obj,
ObjectSynchronizer::InflateCause cause) {
assert(event != NULL, "invariant");
event->set_monitorClass(obj->klass());
event->set_address((uintptr_t)(void*)obj);
event->set_cause((u1)cause);
event->commit();
}
// Fast path code shared by multiple functions
void ObjectSynchronizer::inflate_helper(oop obj) {
markWord mark = obj->mark_acquire();
if (mark.has_monitor()) {
ObjectMonitor* monitor = mark.monitor();
markWord dmw = monitor->header();
assert(dmw.is_neutral(), "sanity check: header=" INTPTR_FORMAT, dmw.value());
return;
}
(void)inflate(Thread::current(), obj, inflate_cause_vm_internal);
}
ObjectMonitor* ObjectSynchronizer::inflate(Thread* current, oop object,
const InflateCause cause) {
EventJavaMonitorInflate event;
for (;;) {
const markWord mark = object->mark_acquire();
// The mark can be in one of the following states:
// * Inflated - just return
// * Stack-locked - coerce it to inflated
// * INFLATING - busy wait for conversion to complete
// * Neutral - aggressively inflate the object.
// CASE: inflated
if (mark.has_monitor()) {
ObjectMonitor* inf = mark.monitor();
markWord dmw = inf->header();
assert(dmw.is_neutral(), "invariant: header=" INTPTR_FORMAT, dmw.value());
return inf;
}
// CASE: inflation in progress - inflating over a stack-lock.
// Some other thread is converting from stack-locked to inflated.
// Only that thread can complete inflation -- other threads must wait.
// The INFLATING value is transient.
// Currently, we spin/yield/park and poll the markword, waiting for inflation to finish.
// We could always eliminate polling by parking the thread on some auxiliary list.
if (mark == markWord::INFLATING()) {
read_stable_mark(object);
continue;
}
// CASE: stack-locked
// Could be stack-locked either by this thread or by some other thread.
//
// Note that we allocate the ObjectMonitor speculatively, _before_ attempting
// to install INFLATING into the mark word. We originally installed INFLATING,
// allocated the ObjectMonitor, and then finally STed the address of the
// ObjectMonitor into the mark. This was correct, but artificially lengthened
// the interval in which INFLATING appeared in the mark, thus increasing
// the odds of inflation contention.
LogStreamHandle(Trace, monitorinflation) lsh;
if (mark.has_locker()) {
ObjectMonitor* m = new ObjectMonitor(object);
// Optimistically prepare the ObjectMonitor - anticipate successful CAS
// We do this before the CAS in order to minimize the length of time
// in which INFLATING appears in the mark.
markWord cmp = object->cas_set_mark(markWord::INFLATING(), mark);
if (cmp != mark) {
delete m;
continue; // Interference -- just retry
}
// We've successfully installed INFLATING (0) into the mark-word.
// This is the only case where 0 will appear in a mark-word.
// Only the singular thread that successfully swings the mark-word
// to 0 can perform (or more precisely, complete) inflation.
//
// Why do we CAS a 0 into the mark-word instead of just CASing the
// mark-word from the stack-locked value directly to the new inflated state?
// Consider what happens when a thread unlocks a stack-locked object.
// It attempts to use CAS to swing the displaced header value from the
// on-stack BasicLock back into the object header. Recall also that the
// header value (hash code, etc) can reside in (a) the object header, or
// (b) a displaced header associated with the stack-lock, or (c) a displaced
// header in an ObjectMonitor. The inflate() routine must copy the header
// value from the BasicLock on the owner's stack to the ObjectMonitor, all
// the while preserving the hashCode stability invariants. If the owner
// decides to release the lock while the value is 0, the unlock will fail
// and control will eventually pass from slow_exit() to inflate. The owner
// will then spin, waiting for the 0 value to disappear. Put another way,
// the 0 causes the owner to stall if the owner happens to try to
// drop the lock (restoring the header from the BasicLock to the object)
// while inflation is in-progress. This protocol avoids races that might
// would otherwise permit hashCode values to change or "flicker" for an object.
// Critically, while object->mark is 0 mark.displaced_mark_helper() is stable.
// 0 serves as a "BUSY" inflate-in-progress indicator.
// fetch the displaced mark from the owner's stack.
// The owner can't die or unwind past the lock while our INFLATING
// object is in the mark. Furthermore the owner can't complete
// an unlock on the object, either.
markWord dmw = mark.displaced_mark_helper();
// Catch if the object's header is not neutral (not locked and
// not marked is what we care about here).
assert(dmw.is_neutral(), "invariant: header=" INTPTR_FORMAT, dmw.value());
// Setup monitor fields to proper values -- prepare the monitor
m->set_header(dmw);
// Optimization: if the mark.locker stack address is associated
// with this thread we could simply set m->_owner = current.
// Note that a thread can inflate an object
// that it has stack-locked -- as might happen in wait() -- directly
// with CAS. That is, we can avoid the xchg-NULL .... ST idiom.
m->set_owner_from(NULL, mark.locker());
// TODO-FIXME: assert BasicLock->dhw != 0.
// Must preserve store ordering. The monitor state must
// be stable at the time of publishing the monitor address.
guarantee(object->mark() == markWord::INFLATING(), "invariant");
// Release semantics so that above set_object() is seen first.
object->release_set_mark(markWord::encode(m));
// Once ObjectMonitor is configured and the object is associated
// with the ObjectMonitor, it is safe to allow async deflation:
_in_use_list.add(m);
// Hopefully the performance counters are allocated on distinct cache lines
// to avoid false sharing on MP systems ...
OM_PERFDATA_OP(Inflations, inc());
if (log_is_enabled(Trace, monitorinflation)) {
ResourceMark rm(current);
lsh.print_cr("inflate(has_locker): object=" INTPTR_FORMAT ", mark="
INTPTR_FORMAT ", type='%s'", p2i(object),
object->mark().value(), object->klass()->external_name());
}
if (event.should_commit()) {
post_monitor_inflate_event(&event, object, cause);
}
return m;
}
// CASE: neutral
// TODO-FIXME: for entry we currently inflate and then try to CAS _owner.
// If we know we're inflating for entry it's better to inflate by swinging a
// pre-locked ObjectMonitor pointer into the object header. A successful
// CAS inflates the object *and* confers ownership to the inflating thread.
// In the current implementation we use a 2-step mechanism where we CAS()
// to inflate and then CAS() again to try to swing _owner from NULL to current.
// An inflateTry() method that we could call from enter() would be useful.
// Catch if the object's header is not neutral (not locked and
// not marked is what we care about here).
assert(mark.is_neutral(), "invariant: header=" INTPTR_FORMAT, mark.value());
ObjectMonitor* m = new ObjectMonitor(object);
// prepare m for installation - set monitor to initial state
m->set_header(mark);
if (object->cas_set_mark(markWord::encode(m), mark) != mark) {
delete m;
m = NULL;
continue;
// interference - the markword changed - just retry.
// The state-transitions are one-way, so there's no chance of
// live-lock -- "Inflated" is an absorbing state.
}
// Once the ObjectMonitor is configured and object is associated
// with the ObjectMonitor, it is safe to allow async deflation:
_in_use_list.add(m);
// Hopefully the performance counters are allocated on distinct
// cache lines to avoid false sharing on MP systems ...
OM_PERFDATA_OP(Inflations, inc());
if (log_is_enabled(Trace, monitorinflation)) {
ResourceMark rm(current);
lsh.print_cr("inflate(neutral): object=" INTPTR_FORMAT ", mark="
INTPTR_FORMAT ", type='%s'", p2i(object),
object->mark().value(), object->klass()->external_name());
}
if (event.should_commit()) {
post_monitor_inflate_event(&event, object, cause);
}
return m;
}
}
void ObjectSynchronizer::chk_for_block_req(JavaThread* current, const char* op_name,
const char* cnt_name, size_t cnt,
LogStream* ls, elapsedTimer* timer_p) {
if (!SafepointMechanism::should_process(current)) {
return;
}
// A safepoint/handshake has started.
if (ls != NULL) {
timer_p->stop();
ls->print_cr("pausing %s: %s=" SIZE_FORMAT ", in_use_list stats: ceiling="
SIZE_FORMAT ", count=" SIZE_FORMAT ", max=" SIZE_FORMAT,
op_name, cnt_name, cnt, in_use_list_ceiling(),
_in_use_list.count(), _in_use_list.max());
}
{
// Honor block request.
ThreadBlockInVM tbivm(current);
}
if (ls != NULL) {
ls->print_cr("resuming %s: in_use_list stats: ceiling=" SIZE_FORMAT
", count=" SIZE_FORMAT ", max=" SIZE_FORMAT, op_name,
in_use_list_ceiling(), _in_use_list.count(), _in_use_list.max());
timer_p->start();
}
}
// Walk the in-use list and deflate (at most MonitorDeflationMax) idle
// ObjectMonitors. Returns the number of deflated ObjectMonitors.
//
// If table != nullptr, we gather owned ObjectMonitors indexed by the
// owner in the table. Please note that ObjectMonitors where the owner
// is set to a stack lock address are NOT associated with the JavaThread
// that holds that stack lock. All of the current consumers of
// ObjectMonitorsHashtable info only care about JNI locked monitors and
// those do not have the owner set to a stack lock address.
//
size_t ObjectSynchronizer::deflate_monitor_list(Thread* current, LogStream* ls,
elapsedTimer* timer_p,
ObjectMonitorsHashtable* table) {
MonitorList::Iterator iter = _in_use_list.iterator();
size_t deflated_count = 0;
while (iter.has_next()) {
if (deflated_count >= (size_t)MonitorDeflationMax) {
break;
}
ObjectMonitor* mid = iter.next();
if (mid->deflate_monitor()) {
deflated_count++;
} else if (table != nullptr) {
// The caller is interested in the owned ObjectMonitors. This does
// not include when owner is set to a stack lock address in thread.
// This also does not capture unowned ObjectMonitors that cannot be
// deflated because of a waiter.
void* key = mid->owner();
// Since deflate_idle_monitors() and deflate_monitor_list() can be
// called more than once, we have to make sure the entry has not
// already been added.
if (key != nullptr && !table->has_entry(key, mid)) {
table->add_entry(key, mid);
}
}
if (current->is_Java_thread()) {
// A JavaThread must check for a safepoint/handshake and honor it.
chk_for_block_req(JavaThread::cast(current), "deflation", "deflated_count",
deflated_count, ls, timer_p);
}
}
return deflated_count;
}
class HandshakeForDeflation : public HandshakeClosure {
public:
HandshakeForDeflation() : HandshakeClosure("HandshakeForDeflation") {}
void do_thread(Thread* thread) {
log_trace(monitorinflation)("HandshakeForDeflation::do_thread: thread="
INTPTR_FORMAT, p2i(thread));
}
};
// This function is called by the MonitorDeflationThread to deflate
// ObjectMonitors. It is also called via do_final_audit_and_print_stats()
// and VM_ThreadDump::doit() by the VMThread.
size_t ObjectSynchronizer::deflate_idle_monitors(ObjectMonitorsHashtable* table) {
Thread* current = Thread::current();
if (current->is_Java_thread()) {
// The async deflation request has been processed.
_last_async_deflation_time_ns = os::javaTimeNanos();
set_is_async_deflation_requested(false);
}
LogStreamHandle(Debug, monitorinflation) lsh_debug;
LogStreamHandle(Info, monitorinflation) lsh_info;
LogStream* ls = NULL;
if (log_is_enabled(Debug, monitorinflation)) {
ls = &lsh_debug;
} else if (log_is_enabled(Info, monitorinflation)) {
ls = &lsh_info;
}
elapsedTimer timer;
if (ls != NULL) {
ls->print_cr("begin deflating: in_use_list stats: ceiling=" SIZE_FORMAT ", count=" SIZE_FORMAT ", max=" SIZE_FORMAT,
in_use_list_ceiling(), _in_use_list.count(), _in_use_list.max());
timer.start();
}
// Deflate some idle ObjectMonitors.
size_t deflated_count = deflate_monitor_list(current, ls, &timer, table);
size_t unlinked_count = 0;
size_t deleted_count = 0;
if (deflated_count > 0 || is_final_audit()) {
// There are ObjectMonitors that have been deflated or this is the
// final audit and all the remaining ObjectMonitors have been
// deflated, BUT the MonitorDeflationThread blocked for the final
// safepoint during unlinking.
// Unlink deflated ObjectMonitors from the in-use list.
ResourceMark rm;
GrowableArray<ObjectMonitor*> delete_list((int)deflated_count);
unlinked_count = _in_use_list.unlink_deflated(current, ls, &timer, &delete_list);
if (current->is_Java_thread()) {
if (ls != NULL) {
timer.stop();
ls->print_cr("before handshaking: unlinked_count=" SIZE_FORMAT
", in_use_list stats: ceiling=" SIZE_FORMAT ", count="
SIZE_FORMAT ", max=" SIZE_FORMAT,
unlinked_count, in_use_list_ceiling(),
_in_use_list.count(), _in_use_list.max());
}
// A JavaThread needs to handshake in order to safely free the
// ObjectMonitors that were deflated in this cycle.
HandshakeForDeflation hfd_hc;
Handshake::execute(&hfd_hc);
if (ls != NULL) {
ls->print_cr("after handshaking: in_use_list stats: ceiling="
SIZE_FORMAT ", count=" SIZE_FORMAT ", max=" SIZE_FORMAT,
in_use_list_ceiling(), _in_use_list.count(), _in_use_list.max());
timer.start();
}
}
// After the handshake, safely free the ObjectMonitors that were
// deflated in this cycle.
for (ObjectMonitor* monitor: delete_list) {
delete monitor;
deleted_count++;
if (current->is_Java_thread()) {
// A JavaThread must check for a safepoint/handshake and honor it.
chk_for_block_req(JavaThread::cast(current), "deletion", "deleted_count",
deleted_count, ls, &timer);
}
}
assert(unlinked_count == deleted_count, "must be");
}
if (ls != NULL) {
timer.stop();
if (deflated_count != 0 || unlinked_count != 0 || log_is_enabled(Debug, monitorinflation)) {
ls->print_cr("deflated_count=" SIZE_FORMAT ", {unlinked,deleted}_count=" SIZE_FORMAT " monitors in %3.7f secs",
deflated_count, unlinked_count, timer.seconds());
}
ls->print_cr("end deflating: in_use_list stats: ceiling=" SIZE_FORMAT ", count=" SIZE_FORMAT ", max=" SIZE_FORMAT,
in_use_list_ceiling(), _in_use_list.count(), _in_use_list.max());
if (table != nullptr) {
ls->print_cr("ObjectMonitorsHashtable: key_count=" SIZE_FORMAT ", om_count=" SIZE_FORMAT,
table->key_count(), table->om_count());
}
}
OM_PERFDATA_OP(MonExtant, set_value(_in_use_list.count()));
OM_PERFDATA_OP(Deflations, inc(deflated_count));
GVars.stw_random = os::random();
if (deflated_count != 0) {
_no_progress_cnt = 0;
} else {
_no_progress_cnt++;
}
return deflated_count;
}
// Monitor cleanup on JavaThread::exit
// Iterate through monitor cache and attempt to release thread's monitors
class ReleaseJavaMonitorsClosure: public MonitorClosure {
private:
JavaThread* _thread;
public:
ReleaseJavaMonitorsClosure(JavaThread* thread) : _thread(thread) {}
void do_monitor(ObjectMonitor* mid) {
intx rec = mid->complete_exit(_thread);
_thread->dec_held_monitor_count(rec + 1);
}
};
// Release all inflated monitors owned by current thread. Lightweight monitors are
// ignored. This is meant to be called during JNI thread detach which assumes
// all remaining monitors are heavyweight. All exceptions are swallowed.
// Scanning the extant monitor list can be time consuming.
// A simple optimization is to add a per-thread flag that indicates a thread
// called jni_monitorenter() during its lifetime.
//
// Instead of NoSafepointVerifier it might be cheaper to
// use an idiom of the form:
// auto int tmp = SafepointSynchronize::_safepoint_counter ;
// <code that must not run at safepoint>
// guarantee (((tmp ^ _safepoint_counter) | (tmp & 1)) == 0) ;
// Since the tests are extremely cheap we could leave them enabled
// for normal product builds.
void ObjectSynchronizer::release_monitors_owned_by_thread(JavaThread* current) {
assert(current == JavaThread::current(), "must be current Java thread");
NoSafepointVerifier nsv;
ReleaseJavaMonitorsClosure rjmc(current);
ObjectSynchronizer::monitors_iterate(&rjmc, current);
assert(!current->has_pending_exception(), "Should not be possible");
current->clear_pending_exception();
assert(current->held_monitor_count() == 0, "Should not be possible");
// All monitors (including entered via JNI) have been unlocked above, so we need to clear jni count.
current->clear_jni_monitor_count();
}
const char* ObjectSynchronizer::inflate_cause_name(const InflateCause cause) {
switch (cause) {
case inflate_cause_vm_internal: return "VM Internal";
--> --------------------
--> maximum size reached
--> --------------------
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