/*
* Copyright (c) 1998, 2018, 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 "logging/log.hpp"
#include "jfr/jfrEvents.hpp"
#include "memory/allocation.inline.hpp"
#include "memory/metaspaceShared.hpp"
#include "memory/padded.hpp"
#include "memory/resourceArea.hpp"
#include "oops/markOop.hpp"
#include "oops/oop.inline.hpp"
#include "runtime/atomic.hpp"
#include "runtime/biasedLocking.hpp"
#include "runtime/handles.inline.hpp"
#include "runtime/interfaceSupport.inline.hpp"
#include "runtime/mutexLocker.hpp"
#include "runtime/objectMonitor.hpp"
#include "runtime/objectMonitor.inline.hpp"
#include "runtime/osThread.hpp"
#include "runtime/safepointVerifiers.hpp"
#include "runtime/sharedRuntime.hpp"
#include "runtime/stubRoutines.hpp"
#include "runtime/synchronizer.hpp"
#include "runtime/thread.inline.hpp"
#include "runtime/vframe.hpp"
#include "runtime/vmThread.hpp"
#include "utilities/align.hpp"
#include "utilities/dtrace.hpp"
#include "utilities/events.hpp"
#include "utilities/preserveException.hpp"
// 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 i486.ad 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 = ((oop)(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, Thread* thr) {
DTRACE_MONITOR_PROBE(waited, monitor, obj(), thr);
return 0;
}
#define NINFLATIONLOCKS 256
static volatile intptr_t gInflationLocks[NINFLATIONLOCKS];
// global list of blocks of monitors
PaddedEnd<ObjectMonitor> * volatile ObjectSynchronizer::gBlockList = NULL;
// global monitor free list
ObjectMonitor * volatile ObjectSynchronizer::gFreeList = NULL;
// global monitor in-use list, for moribund threads,
// monitors they inflated need to be scanned for deflation
ObjectMonitor * volatile ObjectSynchronizer::gOmInUseList = NULL;
// count of entries in gOmInUseList
int ObjectSynchronizer::gOmInUseCount = 0;
static volatile intptr_t gListLock = 0; // protects global monitor lists
static volatile int gMonitorFreeCount = 0; // # on gFreeList
static volatile int gMonitorPopulation = 0; // # Extant -- in circulation
#define CHAINMARKER (cast_to_oop<intptr_t>(-1))
// =====================> 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, Thread * self, bool all) {
assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
assert(self->is_Java_thread(), "invariant");
assert(((JavaThread *) self)->thread_state() == _thread_in_Java, "invariant");
NoSafepointVerifier nsv;
if (obj == NULL) return false; // slow-path for invalid obj
const markOop mark = obj->mark();
if (mark->has_locker() && self->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(oopDesc::equals((oop) mon->object(), obj), "invariant");
if (mon->owner() != self) 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, self);
} else {
DTRACE_MONITOR_PROBE(notify, mon, obj, self);
}
int tally = 0;
do {
mon->INotify(self);
++tally;
} while (mon->first_waiter() != NULL && all);
OM_PERFDATA_OP(Notifications, inc(tally));
}
return true;
}
// biased locking and any 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, Thread * Self,
BasicLock * lock) {
assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
assert(Self->is_Java_thread(), "invariant");
assert(((JavaThread *) Self)->thread_state() == _thread_in_Java, "invariant");
NoSafepointVerifier nsv;
if (obj == NULL) return false; // Need to throw NPE
const markOop mark = obj->mark();
if (mark->has_monitor()) {
ObjectMonitor * const m = mark->monitor();
assert(oopDesc::equals((oop) m->object(), obj), "invariant");
Thread * const owner = (Thread *) m->_owner;
// 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 == Self) {
m->_recursions++;
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
// Biased Locking in the object's header, the second check is for
// stack-locking in the object's header, the third 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(markOopDesc::unused_mark());
if (owner == NULL && Atomic::replace_if_null(Self, &(m->_owner))) {
assert(m->_recursions == 0, "invariant");
assert(m->_owner == Self, "invariant");
return true;
}
}
// Note that we could inflate in quick_enter.
// This is likely a useful optimization
// Critically, in quick_enter() we must not:
// -- perform bias revocation, or
// -- block indefinitely, or
// -- reach a safepoint
return false; // revert to slow-path
}
// -----------------------------------------------------------------------------
// Fast Monitor Enter/Exit
// This the fast monitor enter. The interpreter and compiler use
// some assembly copies of this code. Make sure update those code
// if the following function is changed. The implementation is
// extremely sensitive to race condition. Be careful.
void ObjectSynchronizer::fast_enter(Handle obj, BasicLock* lock,
bool attempt_rebias, TRAPS) {
if (UseBiasedLocking) {
if (!SafepointSynchronize::is_at_safepoint()) {
BiasedLocking::Condition cond = BiasedLocking::revoke_and_rebias(obj, attempt_rebias, THREAD);
if (cond == BiasedLocking::BIAS_REVOKED_AND_REBIASED) {
return;
}
} else {
assert(!attempt_rebias, "can not rebias toward VM thread");
BiasedLocking::revoke_at_safepoint(obj);
}
assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
}
slow_enter(obj, lock, THREAD);
}
void ObjectSynchronizer::fast_exit(oop object, BasicLock* lock, TRAPS) {
markOop mark = object->mark();
// We cannot check for Biased Locking if we are racing an inflation.
assert(mark == markOopDesc::INFLATING() ||
!mark->has_bias_pattern(), "should not see bias pattern here");
markOop dhw = lock->displaced_header();
if (dhw == NULL) {
// 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 != markOopDesc::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() ||
THREAD->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(((oop)(m->object()))->mark() == mark, "invariant");
assert(m->is_entered(THREAD), "invariant");
}
}
#endif
return;
}
if (mark == (markOop) 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;
}
}
// We have to take the slow-path of possible inflation and then exit.
ObjectSynchronizer::inflate(THREAD,
object,
inflate_cause_vm_internal)->exit(true, THREAD);
}
// -----------------------------------------------------------------------------
// Interpreter/Compiler Slow Case
// This routine is used to handle interpreter/compiler slow case
// We don't need to use fast path here, because it must have been
// failed in the interpreter/compiler code.
void ObjectSynchronizer::slow_enter(Handle obj, BasicLock* lock, TRAPS) {
markOop mark = obj->mark();
assert(!mark->has_bias_pattern(), "should not see bias pattern here");
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((markOop) lock, mark)) {
return;
}
// Fall through to inflate() ...
} else if (mark->has_locker() &&
THREAD->is_lock_owned((address)mark->locker())) {
assert(lock != mark->locker(), "must not re-lock the same lock");
assert(lock != (BasicLock*)obj->mark(), "don't relock with same BasicLock");
lock->set_displaced_header(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(markOopDesc::unused_mark());
ObjectSynchronizer::inflate(THREAD,
obj(),
inflate_cause_monitor_enter)->enter(THREAD);
}
// This routine is used to handle interpreter/compiler slow case
// We don't need to use fast path here, because it must have
// failed in the interpreter/compiler code. Simply use the heavy
// weight monitor should be ok, unless someone find otherwise.
void ObjectSynchronizer::slow_exit(oop object, BasicLock* lock, TRAPS) {
fast_exit(object, lock, THREAD);
}
// -----------------------------------------------------------------------------
// 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()
intptr_t ObjectSynchronizer::complete_exit(Handle obj, TRAPS) {
if (UseBiasedLocking) {
BiasedLocking::revoke_and_rebias(obj, false, THREAD);
assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
}
ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD,
obj(),
inflate_cause_vm_internal);
return monitor->complete_exit(THREAD);
}
// NOTE: must use heavy weight monitor to handle complete_exit/reenter()
void ObjectSynchronizer::reenter(Handle obj, intptr_t recursion, TRAPS) {
if (UseBiasedLocking) {
BiasedLocking::revoke_and_rebias(obj, false, THREAD);
assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
}
ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD,
obj(),
inflate_cause_vm_internal);
monitor->reenter(recursion, THREAD);
}
// -----------------------------------------------------------------------------
// JNI locks on java objects
// NOTE: must use heavy weight monitor to handle jni monitor enter
void ObjectSynchronizer::jni_enter(Handle obj, TRAPS) {
// the current locking is from JNI instead of Java code
if (UseBiasedLocking) {
BiasedLocking::revoke_and_rebias(obj, false, THREAD);
assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
}
THREAD->set_current_pending_monitor_is_from_java(false);
ObjectSynchronizer::inflate(THREAD, obj(), inflate_cause_jni_enter)->enter(THREAD);
THREAD->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, Thread* THREAD) {
if (UseBiasedLocking) {
Handle h_obj(THREAD, obj);
BiasedLocking::revoke_and_rebias(h_obj, false, THREAD);
obj = h_obj();
}
assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD,
obj,
inflate_cause_jni_exit);
// If this thread has locked the object, exit the monitor. Note: can't use
// monitor->check(CHECK); must exit even if an exception is pending.
if (monitor->check(THREAD)) {
monitor->exit(true, THREAD);
}
}
// -----------------------------------------------------------------------------
// Internal VM locks on java objects
// standard constructor, allows locking failures
ObjectLocker::ObjectLocker(Handle obj, Thread* thread, bool doLock) {
_dolock = doLock;
_thread = thread;
debug_only(if (StrictSafepointChecks) _thread->check_for_valid_safepoint_state(false);)
_obj = obj;
if (_dolock) {
ObjectSynchronizer::fast_enter(_obj, &_lock, false, _thread);
}
}
ObjectLocker::~ObjectLocker() {
if (_dolock) {
ObjectSynchronizer::fast_exit(_obj(), &_lock, _thread);
}
}
// -----------------------------------------------------------------------------
// Wait/Notify/NotifyAll
// NOTE: must use heavy weight monitor to handle wait()
int ObjectSynchronizer::wait(Handle obj, jlong millis, TRAPS) {
if (UseBiasedLocking) {
BiasedLocking::revoke_and_rebias(obj, false, THREAD);
assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
}
if (millis < 0) {
THROW_MSG_0(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
}
ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD,
obj(),
inflate_cause_wait);
DTRACE_MONITOR_WAIT_PROBE(monitor, obj(), THREAD, millis);
monitor->wait(millis, true, THREAD);
// 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);
return dtrace_waited_probe(monitor, obj, THREAD);
}
void ObjectSynchronizer::waitUninterruptibly(Handle obj, jlong millis, TRAPS) {
if (UseBiasedLocking) {
BiasedLocking::revoke_and_rebias(obj, false, THREAD);
assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
}
if (millis < 0) {
THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
}
ObjectSynchronizer::inflate(THREAD,
obj(),
inflate_cause_wait)->wait(millis, false, THREAD);
}
void ObjectSynchronizer::notify(Handle obj, TRAPS) {
if (UseBiasedLocking) {
BiasedLocking::revoke_and_rebias(obj, false, THREAD);
assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
}
markOop mark = obj->mark();
if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
return;
}
ObjectSynchronizer::inflate(THREAD,
obj(),
inflate_cause_notify)->notify(THREAD);
}
// NOTE: see comment of notify()
void ObjectSynchronizer::notifyall(Handle obj, TRAPS) {
if (UseBiasedLocking) {
BiasedLocking::revoke_and_rebias(obj, false, THREAD);
assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
}
markOop mark = obj->mark();
if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
return;
}
ObjectSynchronizer::inflate(THREAD,
obj(),
inflate_cause_notify)->notifyAll(THREAD);
}
// -----------------------------------------------------------------------------
// Hash Code handling
//
// Performance concern:
// OrderAccess::storestore() calls release() which at one time stored 0
// into the global volatile OrderAccess::dummy variable. This store was
// unnecessary for correctness. Many threads storing into a common location
// causes considerable cache migration or "sloshing" on large SMP systems.
// As such, I avoided using OrderAccess::storestore(). In some cases
// OrderAccess::fence() -- which incurs local latency on the executing
// processor -- is a better choice as it scales on SMP systems.
//
// See http://blogs.oracle.com/dave/entry/biased_locking_in_hotspot for
// a discussion of coherency costs. Note that all our current reference
// platforms provide strong ST-ST order, so the issue is moot on IA32,
// x64, and SPARC.
//
// As a general policy we use "volatile" to control compiler-based reordering
// and explicit fences (barriers) to control for architectural reordering
// performed by the CPU(s) or platform.
struct SharedGlobals {
char _pad_prefix[DEFAULT_CACHE_LINE_SIZE];
// These are highly shared mostly-read variables.
// To avoid false-sharing they need to be the sole occupants of a cache line.
volatile int stwRandom;
volatile int stwCycle;
DEFINE_PAD_MINUS_SIZE(1, DEFAULT_CACHE_LINE_SIZE, sizeof(volatile int) * 2);
// Hot RW variable -- Sequester to avoid false-sharing
volatile int hcSequence;
DEFINE_PAD_MINUS_SIZE(2, DEFAULT_CACHE_LINE_SIZE, sizeof(volatile int));
};
static SharedGlobals GVars;
static int MonitorScavengeThreshold = 1000000;
static volatile int ForceMonitorScavenge = 0; // Scavenge required and pending
static markOop ReadStableMark(oop obj) {
markOop mark = obj->mark();
if (!mark->is_being_inflated()) {
return mark; // normal fast-path return
}
int its = 0;
for (;;) {
markOop mark = obj->mark();
if (!mark->is_being_inflated()) {
return mark; // normal fast-path return
}
// The object is being inflated by some other thread.
// The caller of ReadStableMark() must wait for inflation to complete.
// Avoid live-lock
// TODO: consider calling SafepointSynchronize::do_call_back() while
// spinning to see if there's a safepoint pending. If so, immediately
// yielding or blocking would be appropriate. Avoid spinning while
// there is a safepoint pending.
// TODO: add inflation contention performance counters.
// TODO: restrict the aggregate number of spinners.
++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.
// This is conceptually similar to muxAcquire-muxRelease, except that muxRelease
// wakes at most one thread whereas we need to wake the entire list.
int ix = (cast_from_oop<intptr_t>(obj) >> 5) & (NINFLATIONLOCKS-1);
int YieldThenBlock = 0;
assert(ix >= 0 && ix < NINFLATIONLOCKS, "invariant");
assert((NINFLATIONLOCKS & (NINFLATIONLOCKS-1)) == 0, "invariant");
Thread::muxAcquire(gInflationLocks + ix, "gInflationLock");
while (obj->mark() == markOopDesc::INFLATING()) {
// Beware: NakedYield() is advisory and has almost no effect on some platforms
// so we periodically call Self->_ParkEvent->park(1).
// We use a mixed spin/yield/block mechanism.
if ((YieldThenBlock++) >= 16) {
Thread::current()->_ParkEvent->park(1);
} else {
os::naked_yield();
}
}
Thread::muxRelease(gInflationLocks + ix);
}
} else {
SpinPause(); // SMP-polite spinning
}
}
}
// hashCode() generation :
//
// Possibilities:
// * MD5Digest of {obj,stwRandom}
// * CRC32 of {obj,stwRandom} 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.stwRandom ;
// * A variation of Marsaglia's shift-xor RNG scheme.
// * (obj ^ stwRandom) 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 * Self, 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 addrBits = cast_from_oop<intptr_t>(obj) >> 3;
value = addrBits ^ (addrBits >> 5) ^ GVars.stwRandom;
} else if (hashCode == 2) {
value = 1; // for sensitivity testing
} else if (hashCode == 3) {
value = ++GVars.hcSequence;
} 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 = Self->_hashStateX;
t ^= (t << 11);
Self->_hashStateX = Self->_hashStateY;
Self->_hashStateY = Self->_hashStateZ;
Self->_hashStateZ = Self->_hashStateW;
unsigned v = Self->_hashStateW;
v = (v ^ (v >> 19)) ^ (t ^ (t >> 8));
Self->_hashStateW = v;
value = v;
}
value &= markOopDesc::hash_mask;
if (value == 0) value = 0xBAD;
assert(value != markOopDesc::no_hash, "invariant");
return value;
}
intptr_t ObjectSynchronizer::FastHashCode(Thread * Self, oop obj) {
if (UseBiasedLocking) {
// NOTE: many places throughout the JVM do not expect a safepoint
// to be taken here, in particular most operations on perm gen
// objects. However, we only ever bias Java instances and all of
// the call sites of identity_hash that might revoke biases have
// been checked to make sure they can handle a safepoint. The
// added check of the bias pattern is to avoid useless calls to
// thread-local storage.
if (obj->mark()->has_bias_pattern()) {
// Handle for oop obj in case of STW safepoint
Handle hobj(Self, obj);
// Relaxing assertion for bug 6320749.
assert(Universe::verify_in_progress() ||
!SafepointSynchronize::is_at_safepoint(),
"biases should not be seen by VM thread here");
BiasedLocking::revoke_and_rebias(hobj, false, JavaThread::current());
obj = hobj();
assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
}
}
// hashCode() is a heap mutator ...
// Relaxing assertion for bug 6320749.
assert(Universe::verify_in_progress() || DumpSharedSpaces ||
!SafepointSynchronize::is_at_safepoint(), "invariant");
assert(Universe::verify_in_progress() || DumpSharedSpaces ||
Self->is_Java_thread() , "invariant");
assert(Universe::verify_in_progress() || DumpSharedSpaces ||
((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant");
ObjectMonitor* monitor = NULL;
markOop temp, test;
intptr_t hash;
markOop mark = ReadStableMark(obj);
// object should remain ineligible for biased locking
assert(!mark->has_bias_pattern(), "invariant");
if (mark->is_neutral()) {
hash = mark->hash(); // this is a normal header
if (hash) { // if it has hash, just return it
return hash;
}
hash = get_next_hash(Self, obj); // allocate a new hash code
temp = mark->copy_set_hash(hash); // merge the hash code into header
// use (machine word version) atomic operation to install the hash
test = obj->cas_set_mark(temp, mark);
if (test == mark) {
return hash;
}
// If atomic operation failed, we must inflate the header
// into heavy weight monitor. We could add more code here
// for fast path, but it does not worth the complexity.
} else if (mark->has_monitor()) {
monitor = mark->monitor();
temp = monitor->header();
assert(temp->is_neutral(), "invariant");
hash = temp->hash();
if (hash) {
return hash;
}
// Skip to the following code to reduce code size
} else if (Self->is_lock_owned((address)mark->locker())) {
temp = mark->displaced_mark_helper(); // this is a lightweight monitor owned
assert(temp->is_neutral(), "invariant");
hash = temp->hash(); // by current thread, check if the displaced
if (hash) { // header contains hash code
return hash;
}
// WARNING:
// The displaced header is strictly immutable.
// It can NOT be changed in ANY cases. So we have
// to inflate the header into heavyweight monitor
// even the current thread owns the lock. The reason
// is the BasicLock (stack slot) will be asynchronously
// read by other threads during the inflate() function.
// Any change to stack may not propagate to other threads
// correctly.
}
// Inflate the monitor to set hash code
monitor = ObjectSynchronizer::inflate(Self, obj, inflate_cause_hash_code);
// Load displaced header and check it has hash code
mark = monitor->header();
assert(mark->is_neutral(), "invariant");
hash = mark->hash();
if (hash == 0) {
hash = get_next_hash(Self, obj);
temp = mark->copy_set_hash(hash); // merge hash code into header
assert(temp->is_neutral(), "invariant");
test = Atomic::cmpxchg(temp, monitor->header_addr(), mark);
if (test != mark) {
// The only update to the header in the monitor (outside GC)
// is install the hash code. If someone add new usage of
// displaced header, please update this code
hash = test->hash();
assert(test->is_neutral(), "invariant");
assert(hash != 0, "Trivial unexpected object/monitor header usage.");
}
}
// We finally get the hash
return hash;
}
// Deprecated -- use FastHashCode() instead.
intptr_t ObjectSynchronizer::identity_hash_value_for(Handle obj) {
return FastHashCode(Thread::current(), obj());
}
bool ObjectSynchronizer::current_thread_holds_lock(JavaThread* thread,
Handle h_obj) {
if (UseBiasedLocking) {
BiasedLocking::revoke_and_rebias(h_obj, false, thread);
assert(!h_obj->mark()->has_bias_pattern(), "biases should be revoked by now");
}
assert(thread == JavaThread::current(), "Can only be called on current thread");
oop obj = h_obj();
markOop mark = ReadStableMark(obj);
// Uncontended case, header points to stack
if (mark->has_locker()) {
return thread->is_lock_owned((address)mark->locker());
}
// Contended case, header points to ObjectMonitor (tagged pointer)
if (mark->has_monitor()) {
ObjectMonitor* monitor = mark->monitor();
return monitor->is_entered(thread) != 0;
}
// Unlocked case, header in place
assert(mark->is_neutral(), "sanity check");
return false;
}
// Be aware of this method could revoke bias of the lock object.
// This method queries the ownership of the lock handle specified by 'h_obj'.
// If the current thread owns the lock, it returns owner_self. If no
// thread owns the lock, it returns owner_none. Otherwise, it will return
// owner_other.
ObjectSynchronizer::LockOwnership ObjectSynchronizer::query_lock_ownership
(JavaThread *self, Handle h_obj) {
// The caller must beware this method can revoke bias, and
// revocation can result in a safepoint.
assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
assert(self->thread_state() != _thread_blocked, "invariant");
// Possible mark states: neutral, biased, stack-locked, inflated
if (UseBiasedLocking && h_obj()->mark()->has_bias_pattern()) {
// CASE: biased
BiasedLocking::revoke_and_rebias(h_obj, false, self);
assert(!h_obj->mark()->has_bias_pattern(),
"biases should be revoked by now");
}
assert(self == JavaThread::current(), "Can only be called on current thread");
oop obj = h_obj();
markOop mark = ReadStableMark(obj);
// CASE: stack-locked. Mark points to a BasicLock on the owner's stack.
if (mark->has_locker()) {
return self->is_lock_owned((address)mark->locker()) ?
owner_self : owner_other;
}
// CASE: inflated. Mark (tagged pointer) points to an objectMonitor.
// The Object:ObjectMonitor relationship is stable as long as we're
// not at a safepoint.
if (mark->has_monitor()) {
void * owner = mark->monitor()->_owner;
if (owner == NULL) return owner_none;
return (owner == self ||
self->is_lock_owned((address)owner)) ? owner_self : owner_other;
}
// CASE: neutral
assert(mark->is_neutral(), "sanity check");
return owner_none; // it's unlocked
}
// FIXME: jvmti should call this
JavaThread* ObjectSynchronizer::get_lock_owner(ThreadsList * t_list, Handle h_obj) {
if (UseBiasedLocking) {
if (SafepointSynchronize::is_at_safepoint()) {
BiasedLocking::revoke_at_safepoint(h_obj);
} else {
BiasedLocking::revoke_and_rebias(h_obj, false, JavaThread::current());
}
assert(!h_obj->mark()->has_bias_pattern(), "biases should be revoked by now");
}
oop obj = h_obj();
address owner = NULL;
markOop mark = ReadStableMark(obj);
// Uncontended case, header points to stack
if (mark->has_locker()) {
owner = (address) mark->locker();
}
// Contended case, header points to ObjectMonitor (tagged pointer)
if (mark->has_monitor()) {
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 ...
void ObjectSynchronizer::monitors_iterate(MonitorClosure* closure) {
PaddedEnd<ObjectMonitor> * block = OrderAccess::load_acquire(&gBlockList);
while (block != NULL) {
assert(block->object() == CHAINMARKER, "must be a block header");
for (int i = _BLOCKSIZE - 1; i > 0; i--) {
ObjectMonitor* mid = (ObjectMonitor *)(block + i);
oop object = (oop)mid->object();
if (object != NULL) {
closure->do_monitor(mid);
}
}
block = (PaddedEnd<ObjectMonitor> *)block->FreeNext;
}
}
// Get the next block in the block list.
static inline PaddedEnd<ObjectMonitor>* next(PaddedEnd<ObjectMonitor>* block) {
assert(block->object() == CHAINMARKER, "must be a block header");
block = (PaddedEnd<ObjectMonitor>*) block->FreeNext;
assert(block == NULL || block->object() == CHAINMARKER, "must be a block header");
return block;
}
static bool monitors_used_above_threshold() {
if (gMonitorPopulation == 0) {
return false;
}
int monitors_used = gMonitorPopulation - gMonitorFreeCount;
int monitor_usage = (monitors_used * 100LL) / gMonitorPopulation;
return monitor_usage > MonitorUsedDeflationThreshold;
}
bool ObjectSynchronizer::is_cleanup_needed() {
if (MonitorUsedDeflationThreshold > 0) {
return monitors_used_above_threshold();
}
return false;
}
void ObjectSynchronizer::oops_do(OopClosure* f) {
// We only scan the global used list here (for moribund threads), and
// the thread-local monitors in Thread::oops_do().
global_used_oops_do(f);
}
void ObjectSynchronizer::global_used_oops_do(OopClosure* f) {
assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
list_oops_do(gOmInUseList, f);
}
void ObjectSynchronizer::thread_local_used_oops_do(Thread* thread, OopClosure* f) {
assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
list_oops_do(thread->omInUseList, f);
}
void ObjectSynchronizer::list_oops_do(ObjectMonitor* list, OopClosure* f) {
assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
ObjectMonitor* mid;
for (mid = list; mid != NULL; mid = mid->FreeNext) {
if (mid->object() != NULL) {
f->do_oop((oop*)mid->object_addr());
}
}
}
// -----------------------------------------------------------------------------
// ObjectMonitor Lifecycle
// -----------------------
// Inflation unlinks monitors from the global gFreeList and
// associates them with objects. Deflation -- which occurs at
// STW-time -- disassociates idle monitors from objects. Such
// scavenged monitors are returned to the gFreeList.
//
// The global list is protected by gListLock. All the critical sections
// are short and operate in constant-time.
//
// ObjectMonitors reside in type-stable memory (TSM) and are immortal.
//
// Lifecycle:
// -- unassigned and on the global free list
// -- unassigned and on a thread's private omFreeList
// -- assigned to an object. The object is inflated and the mark refers
// to the objectmonitor.
// Constraining monitor pool growth via MonitorBound ...
//
// The monitor pool is grow-only. We scavenge at STW safepoint-time, but the
// the rate of scavenging is driven primarily by GC. As such, we can find
// an inordinate number of monitors in circulation.
// To avoid that scenario we can artificially induce a STW safepoint
// if the pool appears to be growing past some reasonable bound.
// Generally we favor time in space-time tradeoffs, but as there's no
// natural back-pressure on the # of extant monitors we need to impose some
// type of limit. Beware that if MonitorBound is set to too low a value
// we could just loop. In addition, if MonitorBound is set to a low value
// we'll incur more safepoints, which are harmful to performance.
// See also: GuaranteedSafepointInterval
//
// The current implementation uses asynchronous VM operations.
static void InduceScavenge(Thread * Self, const char * Whence) {
// Induce STW safepoint to trim monitors
// Ultimately, this results in a call to deflate_idle_monitors() in the near future.
// More precisely, trigger an asynchronous STW safepoint as the number
// of active monitors passes the specified threshold.
// TODO: assert thread state is reasonable
if (ForceMonitorScavenge == 0 && Atomic::xchg (1, &ForceMonitorScavenge) == 0) {
// Induce a 'null' safepoint to scavenge monitors
// Must VM_Operation instance be heap allocated as the op will be enqueue and posted
// to the VMthread and have a lifespan longer than that of this activation record.
// The VMThread will delete the op when completed.
VMThread::execute(new VM_ScavengeMonitors());
}
}
ObjectMonitor* ObjectSynchronizer::omAlloc(Thread * Self) {
// A large MAXPRIVATE value reduces both list lock contention
// and list coherency traffic, but also tends to increase the
// number of objectMonitors in circulation as well as the STW
// scavenge costs. As usual, we lean toward time in space-time
// tradeoffs.
const int MAXPRIVATE = 1024;
for (;;) {
ObjectMonitor * m;
// 1: try to allocate from the thread's local omFreeList.
// Threads will attempt to allocate first from their local list, then
// from the global list, and only after those attempts fail will the thread
// attempt to instantiate new monitors. Thread-local free lists take
// heat off the gListLock and improve allocation latency, as well as reducing
// coherency traffic on the shared global list.
m = Self->omFreeList;
if (m != NULL) {
Self->omFreeList = m->FreeNext;
Self->omFreeCount--;
guarantee(m->object() == NULL, "invariant");
m->FreeNext = Self->omInUseList;
Self->omInUseList = m;
Self->omInUseCount++;
return m;
}
// 2: try to allocate from the global gFreeList
// CONSIDER: use muxTry() instead of muxAcquire().
// If the muxTry() fails then drop immediately into case 3.
// If we're using thread-local free lists then try
// to reprovision the caller's free list.
if (gFreeList != NULL) {
// Reprovision the thread's omFreeList.
// Use bulk transfers to reduce the allocation rate and heat
// on various locks.
Thread::muxAcquire(&gListLock, "omAlloc");
for (int i = Self->omFreeProvision; --i >= 0 && gFreeList != NULL;) {
gMonitorFreeCount--;
ObjectMonitor * take = gFreeList;
gFreeList = take->FreeNext;
guarantee(take->object() == NULL, "invariant");
guarantee(!take->is_busy(), "invariant");
take->Recycle();
omRelease(Self, take, false);
}
Thread::muxRelease(&gListLock);
Self->omFreeProvision += 1 + (Self->omFreeProvision/2);
if (Self->omFreeProvision > MAXPRIVATE) Self->omFreeProvision = MAXPRIVATE;
const int mx = MonitorBound;
if (mx > 0 && (gMonitorPopulation-gMonitorFreeCount) > mx) {
// We can't safely induce a STW safepoint from omAlloc() as our thread
// state may not be appropriate for such activities and callers may hold
// naked oops, so instead we defer the action.
InduceScavenge(Self, "omAlloc");
}
continue;
}
// 3: allocate a block of new ObjectMonitors
// Both the local and global free lists are empty -- resort to malloc().
// In the current implementation objectMonitors are TSM - immortal.
// Ideally, we'd write "new ObjectMonitor[_BLOCKSIZE], but we want
// each ObjectMonitor to start at the beginning of a cache line,
// so we use align_up().
// A better solution would be to use C++ placement-new.
// BEWARE: As it stands currently, we don't run the ctors!
assert(_BLOCKSIZE > 1, "invariant");
size_t neededsize = sizeof(PaddedEnd<ObjectMonitor>) * _BLOCKSIZE;
PaddedEnd<ObjectMonitor> * temp;
size_t aligned_size = neededsize + (DEFAULT_CACHE_LINE_SIZE - 1);
void* real_malloc_addr = (void *)NEW_C_HEAP_ARRAY(char, aligned_size,
mtInternal);
temp = (PaddedEnd<ObjectMonitor> *)
align_up(real_malloc_addr, DEFAULT_CACHE_LINE_SIZE);
// NOTE: (almost) no way to recover if allocation failed.
// We might be able to induce a STW safepoint and scavenge enough
// objectMonitors to permit progress.
if (temp == NULL) {
vm_exit_out_of_memory(neededsize, OOM_MALLOC_ERROR,
"Allocate ObjectMonitors");
}
(void)memset((void *) temp, 0, neededsize);
// Format the block.
// initialize the linked list, each monitor points to its next
// forming the single linked free list, the very first monitor
// will points to next block, which forms the block list.
// The trick of using the 1st element in the block as gBlockList
// linkage should be reconsidered. A better implementation would
// look like: class Block { Block * next; int N; ObjectMonitor Body [N] ; }
for (int i = 1; i < _BLOCKSIZE; i++) {
temp[i].FreeNext = (ObjectMonitor *)&temp[i+1];
}
// terminate the last monitor as the end of list
temp[_BLOCKSIZE - 1].FreeNext = NULL;
// Element [0] is reserved for global list linkage
temp[0].set_object(CHAINMARKER);
// Consider carving out this thread's current request from the
// block in hand. This avoids some lock traffic and redundant
// list activity.
// Acquire the gListLock to manipulate gBlockList and gFreeList.
// An Oyama-Taura-Yonezawa scheme might be more efficient.
Thread::muxAcquire(&gListLock, "omAlloc [2]");
gMonitorPopulation += _BLOCKSIZE-1;
gMonitorFreeCount += _BLOCKSIZE-1;
// Add the new block to the list of extant blocks (gBlockList).
// The very first objectMonitor in a block is reserved and dedicated.
// It serves as blocklist "next" linkage.
temp[0].FreeNext = gBlockList;
// There are lock-free uses of gBlockList so make sure that
// the previous stores happen before we update gBlockList.
OrderAccess::release_store(&gBlockList, temp);
// Add the new string of objectMonitors to the global free list
temp[_BLOCKSIZE - 1].FreeNext = gFreeList;
gFreeList = temp + 1;
Thread::muxRelease(&gListLock);
}
}
// Place "m" on the caller's private per-thread omFreeList.
// In practice there's no need to clamp or limit the number of
// monitors on a thread's omFreeList as the only time we'll call
// omRelease is to return a monitor to the free list after a CAS
// attempt failed. This doesn't allow unbounded #s of monitors to
// accumulate on a thread's free list.
//
// Key constraint: all ObjectMonitors on a thread's free list and the global
// free list must have their object field set to null. This prevents the
// scavenger -- deflate_idle_monitors -- from reclaiming them.
void ObjectSynchronizer::omRelease(Thread * Self, ObjectMonitor * m,
bool fromPerThreadAlloc) {
guarantee(m->object() == NULL, "invariant");
guarantee(((m->is_busy()|m->_recursions) == 0), "freeing in-use monitor");
// Remove from omInUseList
if (fromPerThreadAlloc) {
ObjectMonitor* cur_mid_in_use = NULL;
bool extracted = false;
for (ObjectMonitor* mid = Self->omInUseList; mid != NULL; cur_mid_in_use = mid, mid = mid->FreeNext) {
if (m == mid) {
// extract from per-thread in-use list
if (mid == Self->omInUseList) {
Self->omInUseList = mid->FreeNext;
} else if (cur_mid_in_use != NULL) {
cur_mid_in_use->FreeNext = mid->FreeNext; // maintain the current thread in-use list
}
extracted = true;
Self->omInUseCount--;
break;
}
}
assert(extracted, "Should have extracted from in-use list");
}
// FreeNext is used for both omInUseList and omFreeList, so clear old before setting new
m->FreeNext = Self->omFreeList;
Self->omFreeList = m;
Self->omFreeCount++;
}
// Return the monitors of a moribund thread's local free list to
// the global free list. Typically a thread calls omFlush() when
// it's dying. We could also consider having the VM thread steal
// monitors from threads that have not run java code over a few
// consecutive STW safepoints. Relatedly, we might decay
// omFreeProvision at STW safepoints.
//
// Also return the monitors of a moribund thread's omInUseList to
// a global gOmInUseList under the global list lock so these
// will continue to be scanned.
//
// We currently call omFlush() from Threads::remove() _before the thread
// has been excised from the thread list and is no longer a mutator.
// This means that omFlush() can not run concurrently with a safepoint and
// interleave with the scavenge operator. In particular, this ensures that
// the thread's monitors are scanned by a GC safepoint, either via
// Thread::oops_do() (if safepoint happens before omFlush()) or via
// ObjectSynchronizer::oops_do() (if it happens after omFlush() and the thread's
// monitors have been transferred to the global in-use list).
void ObjectSynchronizer::omFlush(Thread * Self) {
ObjectMonitor * list = Self->omFreeList; // Null-terminated SLL
Self->omFreeList = NULL;
ObjectMonitor * tail = NULL;
int tally = 0;
if (list != NULL) {
ObjectMonitor * s;
// The thread is going away, the per-thread free monitors
// are freed via set_owner(NULL)
// Link them to tail, which will be linked into the global free list
// gFreeList below, under the gListLock
for (s = list; s != NULL; s = s->FreeNext) {
tally++;
tail = s;
guarantee(s->object() == NULL, "invariant");
guarantee(!s->is_busy(), "invariant");
s->set_owner(NULL); // redundant but good hygiene
}
guarantee(tail != NULL && list != NULL, "invariant");
}
ObjectMonitor * inUseList = Self->omInUseList;
ObjectMonitor * inUseTail = NULL;
int inUseTally = 0;
if (inUseList != NULL) {
Self->omInUseList = NULL;
ObjectMonitor *cur_om;
// The thread is going away, however the omInUseList inflated
// monitors may still be in-use by other threads.
// Link them to inUseTail, which will be linked into the global in-use list
// gOmInUseList below, under the gListLock
for (cur_om = inUseList; cur_om != NULL; cur_om = cur_om->FreeNext) {
inUseTail = cur_om;
inUseTally++;
}
assert(Self->omInUseCount == inUseTally, "in-use count off");
Self->omInUseCount = 0;
guarantee(inUseTail != NULL && inUseList != NULL, "invariant");
}
Thread::muxAcquire(&gListLock, "omFlush");
if (tail != NULL) {
tail->FreeNext = gFreeList;
gFreeList = list;
gMonitorFreeCount += tally;
assert(Self->omFreeCount == tally, "free-count off");
Self->omFreeCount = 0;
}
if (inUseTail != NULL) {
inUseTail->FreeNext = gOmInUseList;
gOmInUseList = inUseList;
gOmInUseCount += inUseTally;
}
Thread::muxRelease(&gListLock);
}
static void post_monitor_inflate_event(EventJavaMonitorInflate* event,
const oop obj,
ObjectSynchronizer::InflateCause cause) {
assert(event != NULL, "invariant");
assert(event->should_commit(), "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
ObjectMonitor* ObjectSynchronizer::inflate_helper(oop obj) {
markOop mark = obj->mark();
if (mark->has_monitor()) {
assert(ObjectSynchronizer::verify_objmon_isinpool(mark->monitor()), "monitor is invalid");
assert(mark->monitor()->header()->is_neutral(), "monitor must record a good object header");
return mark->monitor();
}
return ObjectSynchronizer::inflate(Thread::current(),
obj,
inflate_cause_vm_internal);
}
ObjectMonitor* ObjectSynchronizer::inflate(Thread * Self,
oop object,
const InflateCause cause) {
// Inflate mutates the heap ...
// Relaxing assertion for bug 6320749.
assert(Universe::verify_in_progress() ||
!SafepointSynchronize::is_at_safepoint(), "invariant");
EventJavaMonitorInflate event;
for (;;) {
const markOop mark = object->mark();
assert(!mark->has_bias_pattern(), "invariant");
// 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.
// * BIASED - Illegal. We should never see this
// CASE: inflated
if (mark->has_monitor()) {
ObjectMonitor * inf = mark->monitor();
assert(inf->header()->is_neutral(), "invariant");
assert(oopDesc::equals((oop) inf->object(), object), "invariant");
assert(ObjectSynchronizer::verify_objmon_isinpool(inf), "monitor is invalid");
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 == markOopDesc::INFLATING()) {
ReadStableMark(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 INFLATED appeared in the mark, thus increasing
// the odds of inflation contention.
//
// We now use per-thread private objectmonitor free lists.
// These list are reprovisioned from the global free list outside the
// critical INFLATING...ST interval. A thread can transfer
// multiple objectmonitors en-mass from the global free list to its local free list.
// This reduces coherency traffic and lock contention on the global free list.
// Using such local free lists, it doesn't matter if the omAlloc() call appears
// before or after the CAS(INFLATING) operation.
// See the comments in omAlloc().
if (mark->has_locker()) {
ObjectMonitor * m = omAlloc(Self);
// 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.
m->Recycle();
m->_Responsible = NULL;
m->_recursions = 0;
m->_SpinDuration = ObjectMonitor::Knob_SpinLimit; // Consider: maintain by type/class
markOop cmp = object->cas_set_mark(markOopDesc::INFLATING(), mark);
if (cmp != mark) {
omRelease(Self, m, true);
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 (hashcode, 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.
markOop dmw = mark->displaced_mark_helper();
assert(dmw->is_neutral(), "invariant");
// 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 = Self.
// 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(mark->locker());
m->set_object(object);
// 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() == markOopDesc::INFLATING(), "invariant");
object->release_set_mark(markOopDesc::encode(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(Debug, monitorinflation)) {
if (object->is_instance()) {
ResourceMark rm;
log_debug(monitorinflation)("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
p2i(object), p2i(object->mark()),
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 Self.
// An inflateTry() method that we could call from fast_enter() and slow_enter()
// would be useful.
assert(mark->is_neutral(), "invariant");
ObjectMonitor * m = omAlloc(Self);
// prepare m for installation - set monitor to initial state
m->Recycle();
m->set_header(mark);
m->set_owner(NULL);
m->set_object(object);
m->_recursions = 0;
m->_Responsible = NULL;
m->_SpinDuration = ObjectMonitor::Knob_SpinLimit; // consider: keep metastats by type/class
if (object->cas_set_mark(markOopDesc::encode(m), mark) != mark) {
m->set_object(NULL);
m->set_owner(NULL);
m->Recycle();
omRelease(Self, m, true);
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.
}
// 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(Debug, monitorinflation)) {
if (object->is_instance()) {
ResourceMark rm;
log_debug(monitorinflation)("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
p2i(object), p2i(object->mark()),
object->klass()->external_name());
}
}
if (event.should_commit()) {
post_monitor_inflate_event(&event, object, cause);
}
return m;
}
}
// We create a list of in-use monitors for each thread.
//
// deflate_thread_local_monitors() scans a single thread's in-use list, while
// deflate_idle_monitors() scans only a global list of in-use monitors which
// is populated only as a thread dies (see omFlush()).
//
// These operations are called at all safepoints, immediately after mutators
// are stopped, but before any objects have moved. Collectively they traverse
// the population of in-use monitors, deflating where possible. The scavenged
// monitors are returned to the monitor free list.
//
// Beware that we scavenge at *every* stop-the-world point. Having a large
// number of monitors in-use could negatively impact performance. We also want
// to minimize the total # of monitors in circulation, as they incur a small
// footprint penalty.
//
// Perversely, the heap size -- and thus the STW safepoint rate --
// typically drives the scavenge rate. Large heaps can mean infrequent GC,
// which in turn can mean large(r) numbers of objectmonitors in circulation.
// This is an unfortunate aspect of this design.
// Deflate a single monitor if not in-use
// Return true if deflated, false if in-use
bool ObjectSynchronizer::deflate_monitor(ObjectMonitor* mid, oop obj,
ObjectMonitor** freeHeadp,
ObjectMonitor** freeTailp) {
bool deflated;
// Normal case ... The monitor is associated with obj.
guarantee(obj->mark() == markOopDesc::encode(mid), "invariant");
guarantee(mid == obj->mark()->monitor(), "invariant");
guarantee(mid->header()->is_neutral(), "invariant");
if (mid->is_busy()) {
deflated = false;
} else {
// Deflate the monitor if it is no longer being used
// It's idle - scavenge and return to the global free list
// plain old deflation ...
if (log_is_enabled(Debug, monitorinflation)) {
if (obj->is_instance()) {
ResourceMark rm;
log_debug(monitorinflation)("Deflating object " INTPTR_FORMAT " , "
"mark " INTPTR_FORMAT " , type %s",
p2i(obj), p2i(obj->mark()),
obj->klass()->external_name());
}
}
// Restore the header back to obj
obj->release_set_mark(mid->header());
mid->clear();
assert(mid->object() == NULL, "invariant");
// Move the object to the working free list defined by freeHeadp, freeTailp
if (*freeHeadp == NULL) *freeHeadp = mid;
if (*freeTailp != NULL) {
ObjectMonitor * prevtail = *freeTailp;
assert(prevtail->FreeNext == NULL, "cleaned up deflated?");
prevtail->FreeNext = mid;
}
*freeTailp = mid;
deflated = true;
}
return deflated;
}
// Walk a given monitor list, and deflate idle monitors
// The given list could be a per-thread list or a global list
// Caller acquires gListLock.
//
// In the case of parallel processing of thread local monitor lists,
// work is done by Threads::parallel_threads_do() which ensures that
// each Java thread is processed by exactly one worker thread, and
// thus avoid conflicts that would arise when worker threads would
// process the same monitor lists concurrently.
//
// See also ParallelSPCleanupTask and
// SafepointSynchronize::do_cleanup_tasks() in safepoint.cpp and
// Threads::parallel_java_threads_do() in thread.cpp.
int ObjectSynchronizer::deflate_monitor_list(ObjectMonitor** listHeadp,
ObjectMonitor** freeHeadp,
ObjectMonitor** freeTailp) {
ObjectMonitor* mid;
ObjectMonitor* next;
ObjectMonitor* cur_mid_in_use = NULL;
int deflated_count = 0;
for (mid = *listHeadp; mid != NULL;) {
oop obj = (oop) mid->object();
if (obj != NULL && deflate_monitor(mid, obj, freeHeadp, freeTailp)) {
// if deflate_monitor succeeded,
// extract from per-thread in-use list
if (mid == *listHeadp) {
*listHeadp = mid->FreeNext;
} else if (cur_mid_in_use != NULL) {
cur_mid_in_use->FreeNext = mid->FreeNext; // maintain the current thread in-use list
}
next = mid->FreeNext;
mid->FreeNext = NULL; // This mid is current tail in the freeHeadp list
mid = next;
deflated_count++;
} else {
cur_mid_in_use = mid;
mid = mid->FreeNext;
}
}
return deflated_count;
}
void ObjectSynchronizer::prepare_deflate_idle_monitors(DeflateMonitorCounters* counters) {
counters->nInuse = 0; // currently associated with objects
counters->nInCirculation = 0; // extant
counters->nScavenged = 0; // reclaimed
}
void ObjectSynchronizer::deflate_idle_monitors(DeflateMonitorCounters* counters) {
assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
bool deflated = false;
ObjectMonitor * freeHeadp = NULL; // Local SLL of scavenged monitors
ObjectMonitor * freeTailp = NULL;
// Prevent omFlush from changing mids in Thread dtor's during deflation
// And in case the vm thread is acquiring a lock during a safepoint
// See e.g. 6320749
Thread::muxAcquire(&gListLock, "scavenge - return");
// Note: the thread-local monitors lists get deflated in
// a separate pass. See deflate_thread_local_monitors().
// For moribund threads, scan gOmInUseList
if (gOmInUseList) {
counters->nInCirculation += gOmInUseCount;
int deflated_count = deflate_monitor_list((ObjectMonitor **)&gOmInUseList, &freeHeadp, &freeTailp);
gOmInUseCount -= deflated_count;
counters->nScavenged += deflated_count;
counters->nInuse += gOmInUseCount;
}
// Move the scavenged monitors back to the global free list.
if (freeHeadp != NULL) {
guarantee(freeTailp != NULL && counters->nScavenged > 0, "invariant");
assert(freeTailp->FreeNext == NULL, "invariant");
// constant-time list splice - prepend scavenged segment to gFreeList
freeTailp->FreeNext = gFreeList;
gFreeList = freeHeadp;
}
Thread::muxRelease(&gListLock);
}
void ObjectSynchronizer::finish_deflate_idle_monitors(DeflateMonitorCounters* counters) {
gMonitorFreeCount += counters->nScavenged;
// Consider: audit gFreeList to ensure that gMonitorFreeCount and list agree.
ForceMonitorScavenge = 0; // Reset
OM_PERFDATA_OP(Deflations, inc(counters->nScavenged));
OM_PERFDATA_OP(MonExtant, set_value(counters->nInCirculation));
// TODO: Add objectMonitor leak detection.
// Audit/inventory the objectMonitors -- make sure they're all accounted for.
GVars.stwRandom = os::random();
GVars.stwCycle++;
}
void ObjectSynchronizer::deflate_thread_local_monitors(Thread* thread, DeflateMonitorCounters* counters) {
assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
ObjectMonitor * freeHeadp = NULL; // Local SLL of scavenged monitors
ObjectMonitor * freeTailp = NULL;
int deflated_count = deflate_monitor_list(thread->omInUseList_addr(), &freeHeadp, &freeTailp);
Thread::muxAcquire(&gListLock, "scavenge - return");
// Adjust counters
counters->nInCirculation += thread->omInUseCount;
thread->omInUseCount -= deflated_count;
counters->nScavenged += deflated_count;
counters->nInuse += thread->omInUseCount;
// Move the scavenged monitors back to the global free list.
if (freeHeadp != NULL) {
guarantee(freeTailp != NULL && deflated_count > 0, "invariant");
assert(freeTailp->FreeNext == NULL, "invariant");
// constant-time list splice - prepend scavenged segment to gFreeList
freeTailp->FreeNext = gFreeList;
gFreeList = freeHeadp;
}
Thread::muxRelease(&gListLock);
}
// Monitor cleanup on JavaThread::exit
// Iterate through monitor cache and attempt to release thread's monitors
// Gives up on a particular monitor if an exception occurs, but continues
// the overall iteration, swallowing the exception.
class ReleaseJavaMonitorsClosure: public MonitorClosure {
private:
TRAPS;
public:
ReleaseJavaMonitorsClosure(Thread* thread) : THREAD(thread) {}
void do_monitor(ObjectMonitor* mid) {
if (mid->owner() == THREAD) {
(void)mid->complete_exit(CHECK);
}
}
};
// Release all inflated monitors owned by 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 No_Savepoint_Verifier 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(TRAPS) {
assert(THREAD == JavaThread::current(), "must be current Java thread");
NoSafepointVerifier nsv;
ReleaseJavaMonitorsClosure rjmc(THREAD);
Thread::muxAcquire(&gListLock, "release_monitors_owned_by_thread");
ObjectSynchronizer::monitors_iterate(&rjmc);
Thread::muxRelease(&gListLock);
THREAD->clear_pending_exception();
}
const char* ObjectSynchronizer::inflate_cause_name(const InflateCause cause) {
switch (cause) {
case inflate_cause_vm_internal: return "VM Internal";
case inflate_cause_monitor_enter: return "Monitor Enter";
case inflate_cause_wait: return "Monitor Wait";
case inflate_cause_notify: return "Monitor Notify";
case inflate_cause_hash_code: return "Monitor Hash Code";
case inflate_cause_jni_enter: return "JNI Monitor Enter";
case inflate_cause_jni_exit: return "JNI Monitor Exit";
default:
ShouldNotReachHere();
}
return "Unknown";
}
//------------------------------------------------------------------------------
// Debugging code
u_char* ObjectSynchronizer::get_gvars_addr() {
return (u_char*)&GVars;
}
u_char* ObjectSynchronizer::get_gvars_hcSequence_addr() {
return (u_char*)&GVars.hcSequence;
}
size_t ObjectSynchronizer::get_gvars_size() {
return sizeof(SharedGlobals);
}
u_char* ObjectSynchronizer::get_gvars_stwRandom_addr() {
return (u_char*)&GVars.stwRandom;
}
#ifndef PRODUCT
// Check if monitor belongs to the monitor cache
// The list is grow-only so it's *relatively* safe to traverse
// the list of extant blocks without taking a lock.
int ObjectSynchronizer::verify_objmon_isinpool(ObjectMonitor *monitor) {
PaddedEnd<ObjectMonitor> * block = OrderAccess::load_acquire(&gBlockList);
while (block != NULL) {
assert(block->object() == CHAINMARKER, "must be a block header");
if (monitor > &block[0] && monitor < &block[_BLOCKSIZE]) {
address mon = (address)monitor;
address blk = (address)block;
size_t diff = mon - blk;
assert((diff % sizeof(PaddedEnd<ObjectMonitor>)) == 0, "must be aligned");
return 1;
}
block = (PaddedEnd<ObjectMonitor> *)block->FreeNext;
}
return 0;
}
#endif