/* * 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. *
*/
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 returntrue;
}
};
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++;
}
void MonitorList::add(ObjectMonitor* m) {
ObjectMonitor* head; do {
head = Atomic::load(&_head);
m->set_next_om(head);
} while (Atomic::cmpxchg(&_head, head, m) != head);
// 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;
}
// 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(); \
}
// 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;
}
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; boolvolatile ObjectSynchronizer::_is_async_deflation_requested = false; boolvolatile 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) returnfalse; // 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. returntrue;
}
if (mark.has_monitor()) {
ObjectMonitor* const mon = mark.monitor();
assert(mon->object() == oop(obj), "invariant"); if (mon->owner() != current) returnfalse; // 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));
} returntrue;
}
// other IMS exception states take the slow-path returnfalse;
}
// 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) returnfalse; // Need to throw NPE
if (obj->klass()->is_value_based()) { returnfalse;
}
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) { returnfalse;
}
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(); returntrue;
}
// 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());
// 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
returnfalse; // 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);
}
}
// ----------------------------------------------------------------------------- // 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.
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() ...
} elseif (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());
} elseif (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;
}
}
}
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;
}
}
} elseif (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);
}
}
// ----------------------------------------------------------------------------- // 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);
}
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. volatileint stw_random;
DEFINE_PAD_MINUS_SIZE(1, OM_CACHE_LINE_SIZE, sizeof(volatileint)); // Hot RW variable -- Sequester to avoid false-sharing volatileint hc_sequence;
DEFINE_PAD_MINUS_SIZE(2, OM_CACHE_LINE_SIZE, sizeof(volatileint));
};
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:
staticinline 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();
} elseif (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;
} elseif (hashCode == 2) {
value = 1; // for sensitivity testing
} elseif (hashCode == 3) {
value = ++GVars.hc_sequence;
} elseif (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;
}
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.
} elseif (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.
} elseif (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"); returnfalse;
}
// Uncontended case, header points to stack if (mark.has_locker()) {
owner = (address) mark.locker();
}
// Contended case, header points to ObjectMonitor (tagged pointer) elseif (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);
}
}
}
staticbool monitors_used_above_threshold(MonitorList* list) { if (MonitorUsedDeflationThreshold == 0) { // disabled case is easy returnfalse;
} // 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 returnfalse;
} 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; returnint(monitor_usage) > MonitorUsedDeflationThreshold;
}
bool ObjectSynchronizer::is_async_deflation_needed() { if (is_async_deflation_requested()) { // Async deflation request. returntrue;
} 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. returntrue;
} returnfalse;
}
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();
} constint 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);
}
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: 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;
}
}
// 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++;
} elseif (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") {}
// 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;
} elseif (log_is_enabled(Info, monitorinflation)) {
ls = &lsh_info;
}
// 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.
// 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);
// 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");
}
// Iterate through monitor cache and attempt to release thread's monitors class ReleaseJavaMonitorsClosure: public MonitorClosure { private:
JavaThread* _thread;
// 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();
}
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