Java example source code file (synchronizer.cpp)
The synchronizer.cpp Java example source code
/*
* Copyright (c) 1998, 2013, 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 "memory/resourceArea.hpp"
#include "oops/markOop.hpp"
#include "oops/oop.inline.hpp"
#include "runtime/biasedLocking.hpp"
#include "runtime/handles.inline.hpp"
#include "runtime/interfaceSupport.hpp"
#include "runtime/mutexLocker.hpp"
#include "runtime/objectMonitor.hpp"
#include "runtime/objectMonitor.inline.hpp"
#include "runtime/osThread.hpp"
#include "runtime/stubRoutines.hpp"
#include "runtime/synchronizer.hpp"
#include "runtime/thread.inline.hpp"
#include "utilities/dtrace.hpp"
#include "utilities/events.hpp"
#include "utilities/preserveException.hpp"
#ifdef TARGET_OS_FAMILY_linux
# include "os_linux.inline.hpp"
#endif
#ifdef TARGET_OS_FAMILY_solaris
# include "os_solaris.inline.hpp"
#endif
#ifdef TARGET_OS_FAMILY_windows
# include "os_windows.inline.hpp"
#endif
#ifdef TARGET_OS_FAMILY_bsd
# include "os_bsd.inline.hpp"
#endif
#if defined(__GNUC__)
// Need to inhibit inlining for older versions of GCC to avoid build-time failures
#define ATTR __attribute__((noinline))
#else
#define ATTR
#endif
// 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(); \
}
#ifndef USDT2
HS_DTRACE_PROBE_DECL5(hotspot, monitor__wait,
jlong, uintptr_t, char*, int, long);
HS_DTRACE_PROBE_DECL4(hotspot, monitor__waited,
jlong, uintptr_t, char*, int);
#define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
{ \
if (DTraceMonitorProbes) { \
DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
HS_DTRACE_PROBE5(hotspot, monitor__wait, jtid, \
(monitor), bytes, len, (millis)); \
} \
}
#define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
{ \
if (DTraceMonitorProbes) { \
DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
HS_DTRACE_PROBE4(hotspot, monitor__##probe, jtid, \
(uintptr_t)(monitor), bytes, len); \
} \
}
#else /* USDT2 */
#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_waited HOTSPOT_MONITOR_PROBE_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); \
} \
}
#endif /* USDT2 */
#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 InflationLocks [NINFLATIONLOCKS] ;
ObjectMonitor * ObjectSynchronizer::gBlockList = NULL ;
ObjectMonitor * volatile ObjectSynchronizer::gFreeList = NULL ;
ObjectMonitor * volatile ObjectSynchronizer::gOmInUseList = NULL ;
int ObjectSynchronizer::gOmInUseCount = 0;
static volatile intptr_t ListLock = 0 ; // protects global monitor free-list cache
static volatile int MonitorFreeCount = 0 ; // # on gFreeList
static volatile int MonitorPopulation = 0 ; // # Extant -- in circulation
#define CHAINMARKER (cast_to_oop<intptr_t>(-1))
// -----------------------------------------------------------------------------
// 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) {
assert(!object->mark()->has_bias_pattern(), "should not see bias pattern here");
// if displaced header is null, the previous enter is recursive enter, no-op
markOop dhw = lock->displaced_header();
markOop mark ;
if (dhw == NULL) {
// Recursive stack-lock.
// Diagnostics -- Could be: stack-locked, inflating, inflated.
mark = object->mark() ;
assert (!mark->is_neutral(), "invariant") ;
if (mark->has_locker() && mark != markOopDesc::INFLATING()) {
assert(THREAD->is_lock_owned((address)mark->locker()), "invariant") ;
}
if (mark->has_monitor()) {
ObjectMonitor * m = mark->monitor() ;
assert(((oop)(m->object()))->mark() == mark, "invariant") ;
assert(m->is_entered(THREAD), "invariant") ;
}
return ;
}
mark = object->mark() ;
// If the object is stack-locked by the current thread, try to
// swing the displaced header from the box back to the mark.
if (mark == (markOop) lock) {
assert (dhw->is_neutral(), "invariant") ;
if ((markOop) Atomic::cmpxchg_ptr (dhw, object->mark_addr(), mark) == mark) {
TEVENT (fast_exit: release stacklock) ;
return;
}
}
ObjectSynchronizer::inflate(THREAD, object)->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 == (markOop) Atomic::cmpxchg_ptr(lock, obj()->mark_addr(), mark)) {
TEVENT (slow_enter: release stacklock) ;
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;
}
#if 0
// The following optimization isn't particularly useful.
if (mark->has_monitor() && mark->monitor()->is_entered(THREAD)) {
lock->set_displaced_header (NULL) ;
return ;
}
#endif
// 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())->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) {
TEVENT (complete_exit) ;
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());
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) {
TEVENT (reenter) ;
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());
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) { // possible entry from jni enter
// the current locking is from JNI instead of Java code
TEVENT (jni_enter) ;
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())->enter(THREAD);
THREAD->set_current_pending_monitor_is_from_java(true);
}
// NOTE: must use heavy weight monitor to handle jni monitor enter
bool ObjectSynchronizer::jni_try_enter(Handle obj, Thread* THREAD) {
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_helper(obj());
return monitor->try_enter(THREAD);
}
// NOTE: must use heavy weight monitor to handle jni monitor exit
void ObjectSynchronizer::jni_exit(oop obj, Thread* THREAD) {
TEVENT (jni_exit) ;
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);
// 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) {
TEVENT (ObjectLocker) ;
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()
void 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) {
TEVENT (wait - throw IAX) ;
THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
}
ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj());
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 and remove the call */
// DTRACE_MONITOR_PROBE(waited, monitor, obj(), THREAD);
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) {
TEVENT (wait - throw IAX) ;
THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
}
ObjectSynchronizer::inflate(THREAD, obj()) -> 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())->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())->notifyAll(THREAD);
}
// -----------------------------------------------------------------------------
// Hash Code handling
//
// Performance concern:
// OrderAccess::storestore() calls release() which STs 0 into the global volatile
// OrderAccess::Dummy variable. This store is unnecessary for correctness.
// Many threads STing into a common location causes considerable cache migration
// or "sloshing" on large SMP system. As such, I avoid using OrderAccess::storestore()
// until it's repaired. 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.sun.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 {
// These are highly shared mostly-read variables.
// To avoid false-sharing they need to be the sole occupants of a $ line.
double padPrefix [8];
volatile int stwRandom ;
volatile int stwCycle ;
// Hot RW variables -- Sequester to avoid false-sharing
double padSuffix [16];
volatile int hcSequence ;
double padFinal [8] ;
} ;
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::NakedYield() ;
TEVENT (Inflate: INFLATING - 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 (InflationLocks + ix, "InflationLock") ;
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::NakedYield() ;
}
}
Thread::muxRelease (InflationLocks + ix ) ;
TEVENT (Inflate: INFLATING - yield/park) ;
}
} 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 an unguarded global Park-Miller RNG,
// so it's possible for two threads to race and generate the same 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") ;
TEVENT (hashCode: GENERATE) ;
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()) {
// Box and unbox the raw reference just in case we cause a 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() ||
!SafepointSynchronize::is_at_safepoint(), "invariant") ;
assert (Universe::verify_in_progress() ||
Self->is_Java_thread() , "invariant") ;
assert (Universe::verify_in_progress() ||
((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 = (markOop) Atomic::cmpxchg_ptr(temp, obj->mark_addr(), 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);
// 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 = (markOop) Atomic::cmpxchg_ptr(temp, monitor, 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 querys 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
// ower_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(Handle h_obj, bool doLock) {
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(owner, doLock);
}
// 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) {
ObjectMonitor* block = gBlockList;
ObjectMonitor* mid;
while (block) {
assert(block->object() == CHAINMARKER, "must be a block header");
for (int i = _BLOCKSIZE - 1; i > 0; i--) {
mid = block + i;
oop object = (oop) mid->object();
if (object != NULL) {
closure->do_monitor(mid);
}
}
block = (ObjectMonitor*) block->FreeNext;
}
}
// Get the next block in the block list.
static inline ObjectMonitor* next(ObjectMonitor* block) {
assert(block->object() == CHAINMARKER, "must be a block header");
block = block->FreeNext ;
assert(block == NULL || block->object() == CHAINMARKER, "must be a block header");
return block;
}
void ObjectSynchronizer::oops_do(OopClosure* f) {
assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
for (ObjectMonitor* block = gBlockList; block != NULL; block = next(block)) {
assert(block->object() == CHAINMARKER, "must be a block header");
for (int i = 1; i < _BLOCKSIZE; i++) {
ObjectMonitor* mid = &block[i];
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 ListLock. 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) {
if (ObjectMonitor::Knob_Verbose) {
::printf ("Monitor scavenge - Induced STW @%s (%d)\n", Whence, ForceMonitorScavenge) ;
::fflush(stdout) ;
}
// 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_ForceAsyncSafepoint()) ;
if (ObjectMonitor::Knob_Verbose) {
::printf ("Monitor scavenge - STW posted @%s (%d)\n", Whence, ForceMonitorScavenge) ;
::fflush(stdout) ;
}
}
}
/* Too slow for general assert or debug
void ObjectSynchronizer::verifyInUse (Thread *Self) {
ObjectMonitor* mid;
int inusetally = 0;
for (mid = Self->omInUseList; mid != NULL; mid = mid->FreeNext) {
inusetally ++;
}
assert(inusetally == Self->omInUseCount, "inuse count off");
int freetally = 0;
for (mid = Self->omFreeList; mid != NULL; mid = mid->FreeNext) {
freetally ++;
}
assert(freetally == Self->omFreeCount, "free count off");
}
*/
ObjectMonitor * ATTR 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 ListLock 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 -- ;
// CONSIDER: set m->FreeNext = BAD -- diagnostic hygiene
guarantee (m->object() == NULL, "invariant") ;
if (MonitorInUseLists) {
m->FreeNext = Self->omInUseList;
Self->omInUseList = m;
Self->omInUseCount ++;
// verifyInUse(Self);
} else {
m->FreeNext = NULL;
}
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 (&ListLock, "omAlloc") ;
for (int i = Self->omFreeProvision; --i >= 0 && gFreeList != NULL; ) {
MonitorFreeCount --;
ObjectMonitor * take = gFreeList ;
gFreeList = take->FreeNext ;
guarantee (take->object() == NULL, "invariant") ;
guarantee (!take->is_busy(), "invariant") ;
take->Recycle() ;
omRelease (Self, take, false) ;
}
Thread::muxRelease (&ListLock) ;
Self->omFreeProvision += 1 + (Self->omFreeProvision/2) ;
if (Self->omFreeProvision > MAXPRIVATE ) Self->omFreeProvision = MAXPRIVATE ;
TEVENT (omFirst - reprovision) ;
const int mx = MonitorBound ;
if (mx > 0 && (MonitorPopulation-MonitorFreeCount) > 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.
assert (_BLOCKSIZE > 1, "invariant") ;
ObjectMonitor * temp = new ObjectMonitor[_BLOCKSIZE];
// 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 (sizeof (ObjectMonitor[_BLOCKSIZE]), OOM_MALLOC_ERROR,
"Allocate ObjectMonitors");
}
// 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 = &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 ListLock to manipulate BlockList and FreeList.
// An Oyama-Taura-Yonezawa scheme might be more efficient.
Thread::muxAcquire (&ListLock, "omAlloc [2]") ;
MonitorPopulation += _BLOCKSIZE-1;
MonitorFreeCount += _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;
gBlockList = temp;
// Add the new string of objectMonitors to the global free list
temp[_BLOCKSIZE - 1].FreeNext = gFreeList ;
gFreeList = temp + 1;
Thread::muxRelease (&ListLock) ;
TEVENT (Allocate block of monitors) ;
}
}
// 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.
//
void ObjectSynchronizer::omRelease (Thread * Self, ObjectMonitor * m, bool fromPerThreadAlloc) {
guarantee (m->object() == NULL, "invariant") ;
// Remove from omInUseList
if (MonitorInUseLists && fromPerThreadAlloc) {
ObjectMonitor* curmidinuse = NULL;
for (ObjectMonitor* mid = Self->omInUseList; mid != NULL; ) {
if (m == mid) {
// extract from per-thread in-use-list
if (mid == Self->omInUseList) {
Self->omInUseList = mid->FreeNext;
} else if (curmidinuse != NULL) {
curmidinuse->FreeNext = mid->FreeNext; // maintain the current thread inuselist
}
Self->omInUseCount --;
// verifyInUse(Self);
break;
} else {
curmidinuse = mid;
mid = mid->FreeNext;
}
}
}
// FreeNext is used for both onInUseList 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 the Thread:: dtor _after the thread
// has been excised from the thread list and is no longer a mutator.
// That means that omFlush() can run concurrently with a safepoint and
// the scavenge operator. Calling omFlush() from JavaThread::exit() might
// be a better choice as we could safely reason that that the JVM is
// not at a safepoint at the time of the call, and thus there could
// be not inopportune interleavings between omFlush() and the scavenge
// operator.
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 ;
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
TEVENT (omFlush - Move one) ;
}
guarantee (Tail != NULL && List != NULL, "invariant") ;
}
ObjectMonitor * InUseList = Self->omInUseList;
ObjectMonitor * InUseTail = NULL ;
int InUseTally = 0;
if (InUseList != NULL) {
Self->omInUseList = NULL;
ObjectMonitor *curom;
for (curom = InUseList; curom != NULL; curom = curom->FreeNext) {
InUseTail = curom;
InUseTally++;
}
// TODO debug
assert(Self->omInUseCount == InUseTally, "inuse count off");
Self->omInUseCount = 0;
guarantee (InUseTail != NULL && InUseList != NULL, "invariant");
}
Thread::muxAcquire (&ListLock, "omFlush") ;
if (Tail != NULL) {
Tail->FreeNext = gFreeList ;
gFreeList = List ;
MonitorFreeCount += Tally;
}
if (InUseTail != NULL) {
InUseTail->FreeNext = gOmInUseList;
gOmInUseList = InUseList;
gOmInUseCount += InUseTally;
}
Thread::muxRelease (&ListLock) ;
TEVENT (omFlush) ;
}
// 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);
}
// Note that we could encounter some performance loss through false-sharing as
// multiple locks occupy the same $ line. Padding might be appropriate.
ObjectMonitor * ATTR ObjectSynchronizer::inflate (Thread * Self, oop object) {
// Inflate mutates the heap ...
// Relaxing assertion for bug 6320749.
assert (Universe::verify_in_progress() ||
!SafepointSynchronize::is_at_safepoint(), "invariant") ;
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 (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()) {
TEVENT (Inflate: spin while 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->OwnerIsThread = 0 ;
m->_recursions = 0 ;
m->_SpinDuration = ObjectMonitor::Knob_SpinLimit ; // Consider: maintain by type/class
markOop cmp = (markOop) Atomic::cmpxchg_ptr (markOopDesc::INFLATING(), object->mark_addr(), 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-work.
// 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 and
// m->OwnerIsThread = 1. 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 ...
if (ObjectMonitor::_sync_Inflations != NULL) ObjectMonitor::_sync_Inflations->inc() ;
TEVENT(Inflate: overwrite stacklock) ;
if (TraceMonitorInflation) {
if (object->is_instance()) {
ResourceMark rm;
tty->print_cr("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
(void *) object, (intptr_t) object->mark(),
object->klass()->external_name());
}
}
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->OwnerIsThread = 1 ;
m->_recursions = 0 ;
m->_Responsible = NULL ;
m->_SpinDuration = ObjectMonitor::Knob_SpinLimit ; // consider: keep metastats by type/class
if (Atomic::cmpxchg_ptr (markOopDesc::encode(m), object->mark_addr(), mark) != mark) {
m->set_object (NULL) ;
m->set_owner (NULL) ;
m->OwnerIsThread = 0 ;
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 ...
if (ObjectMonitor::_sync_Inflations != NULL) ObjectMonitor::_sync_Inflations->inc() ;
TEVENT(Inflate: overwrite neutral) ;
if (TraceMonitorInflation) {
if (object->is_instance()) {
ResourceMark rm;
tty->print_cr("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
(void *) object, (intptr_t) object->mark(),
object->klass()->external_name());
}
}
return m ;
}
}
// Note that we could encounter some performance loss through false-sharing as
// multiple locks occupy the same $ line. Padding might be appropriate.
// Deflate_idle_monitors() is called at all safepoints, immediately
// after all mutators are stopped, but before any objects have moved.
// It traverses the list of known monitors, deflating where possible.
// The scavenged monitor are returned to the monitor free list.
//
// Beware that we scavenge at *every* stop-the-world point.
// Having a large number of monitors in-circulation negatively
// impacts the performance of some applications (e.g., PointBase).
// Broadly, we want to minimize the # of monitors in circulation.
//
// We have added a flag, MonitorInUseLists, which creates a list
// of active monitors for each thread. deflate_idle_monitors()
// only scans the per-thread inuse lists. omAlloc() puts all
// assigned monitors on the per-thread list. deflate_idle_monitors()
// returns the non-busy monitors to the global free list.
// When a thread dies, omFlush() adds the list of active monitors for
// that thread to a global gOmInUseList acquiring the
// global list lock. deflate_idle_monitors() acquires the global
// list lock to scan for non-busy monitors to the global free list.
// An alternative could have used a single global inuse list. The
// downside would have been the additional cost of acquiring the global list lock
// for every omAlloc().
//
// 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.
//
enum ManifestConstants {
ClearResponsibleAtSTW = 0,
MaximumRecheckInterval = 1000
} ;
// 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()) {
if (ClearResponsibleAtSTW) mid->_Responsible = NULL ;
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 ...
TEVENT (deflate_idle_monitors - scavenge1) ;
if (TraceMonitorInflation) {
if (obj->is_instance()) {
ResourceMark rm;
tty->print_cr("Deflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
(void *) obj, (intptr_t) 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 FreeHead,FreeTail.
if (*FreeHeadp == NULL) *FreeHeadp = mid;
if (*FreeTailp != NULL) {
ObjectMonitor * prevtail = *FreeTailp;
assert(prevtail->FreeNext == NULL, "cleaned up deflated?"); // TODO KK
prevtail->FreeNext = mid;
}
*FreeTailp = mid;
deflated = true;
}
return deflated;
}
// Caller acquires ListLock
int ObjectSynchronizer::walk_monitor_list(ObjectMonitor** listheadp,
ObjectMonitor** FreeHeadp, ObjectMonitor** FreeTailp) {
ObjectMonitor* mid;
ObjectMonitor* next;
ObjectMonitor* curmidinuse = NULL;
int deflatedcount = 0;
for (mid = *listheadp; mid != NULL; ) {
oop obj = (oop) mid->object();
bool deflated = false;
if (obj != NULL) {
deflated = deflate_monitor(mid, obj, FreeHeadp, FreeTailp);
}
if (deflated) {
// extract from per-thread in-use-list
if (mid == *listheadp) {
*listheadp = mid->FreeNext;
} else if (curmidinuse != NULL) {
curmidinuse->FreeNext = mid->FreeNext; // maintain the current thread inuselist
}
next = mid->FreeNext;
mid->FreeNext = NULL; // This mid is current tail in the FreeHead list
mid = next;
deflatedcount++;
} else {
curmidinuse = mid;
mid = mid->FreeNext;
}
}
return deflatedcount;
}
void ObjectSynchronizer::deflate_idle_monitors() {
assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
int nInuse = 0 ; // currently associated with objects
int nInCirculation = 0 ; // extant
int nScavenged = 0 ; // reclaimed
bool deflated = false;
ObjectMonitor * FreeHead = NULL ; // Local SLL of scavenged monitors
ObjectMonitor * FreeTail = NULL ;
TEVENT (deflate_idle_monitors) ;
// 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 (&ListLock, "scavenge - return") ;
if (MonitorInUseLists) {
int inUse = 0;
for (JavaThread* cur = Threads::first(); cur != NULL; cur = cur->next()) {
nInCirculation+= cur->omInUseCount;
int deflatedcount = walk_monitor_list(cur->omInUseList_addr(), &FreeHead, &FreeTail);
cur->omInUseCount-= deflatedcount;
// verifyInUse(cur);
nScavenged += deflatedcount;
nInuse += cur->omInUseCount;
}
// For moribund threads, scan gOmInUseList
if (gOmInUseList) {
nInCirculation += gOmInUseCount;
int deflatedcount = walk_monitor_list((ObjectMonitor **)&gOmInUseList, &FreeHead, &FreeTail);
gOmInUseCount-= deflatedcount;
nScavenged += deflatedcount;
nInuse += gOmInUseCount;
}
} else for (ObjectMonitor* block = gBlockList; block != NULL; block = next(block)) {
// Iterate over all extant monitors - Scavenge all idle monitors.
assert(block->object() == CHAINMARKER, "must be a block header");
nInCirculation += _BLOCKSIZE ;
for (int i = 1 ; i < _BLOCKSIZE; i++) {
ObjectMonitor* mid = &block[i];
oop obj = (oop) mid->object();
if (obj == NULL) {
// The monitor is not associated with an object.
// The monitor should either be a thread-specific private
// free list or the global free list.
// obj == NULL IMPLIES mid->is_busy() == 0
guarantee (!mid->is_busy(), "invariant") ;
continue ;
}
deflated = deflate_monitor(mid, obj, &FreeHead, &FreeTail);
if (deflated) {
mid->FreeNext = NULL ;
nScavenged ++ ;
} else {
nInuse ++;
}
}
}
MonitorFreeCount += nScavenged;
// Consider: audit gFreeList to ensure that MonitorFreeCount and list agree.
if (ObjectMonitor::Knob_Verbose) {
::printf ("Deflate: InCirc=%d InUse=%d Scavenged=%d ForceMonitorScavenge=%d : pop=%d free=%d\n",
nInCirculation, nInuse, nScavenged, ForceMonitorScavenge,
MonitorPopulation, MonitorFreeCount) ;
::fflush(stdout) ;
}
ForceMonitorScavenge = 0; // Reset
// Move the scavenged monitors back to the global free list.
if (FreeHead != NULL) {
guarantee (FreeTail != NULL && nScavenged > 0, "invariant") ;
assert (FreeTail->FreeNext == NULL, "invariant") ;
// constant-time list splice - prepend scavenged segment to gFreeList
FreeTail->FreeNext = gFreeList ;
gFreeList = FreeHead ;
}
Thread::muxRelease (&ListLock) ;
if (ObjectMonitor::_sync_Deflations != NULL) ObjectMonitor::_sync_Deflations->inc(nScavenged) ;
if (ObjectMonitor::_sync_MonExtant != NULL) ObjectMonitor::_sync_MonExtant ->set_value(nInCirculation);
// TODO: Add objectMonitor leak detection.
// Audit/inventory the objectMonitors -- make sure they're all accounted for.
GVars.stwRandom = os::random() ;
GVars.stwCycle ++ ;
}
// 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");
No_Safepoint_Verifier nsv ;
ReleaseJavaMonitorsClosure rjmc(THREAD);
Thread::muxAcquire(&ListLock, "release_monitors_owned_by_thread");
ObjectSynchronizer::monitors_iterate(&rjmc);
Thread::muxRelease(&ListLock);
THREAD->clear_pending_exception();
}
//------------------------------------------------------------------------------
// Non-product code
#ifndef PRODUCT
// Verify all monitors in the monitor cache, the verification is weak.
void ObjectSynchronizer::verify() {
ObjectMonitor* block = gBlockList;
ObjectMonitor* mid;
while (block) {
assert(block->object() == CHAINMARKER, "must be a block header");
for (int i = 1; i < _BLOCKSIZE; i++) {
mid = block + i;
oop object = (oop) mid->object();
if (object != NULL) {
mid->verify();
}
}
block = (ObjectMonitor*) block->FreeNext;
}
}
// 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) {
ObjectMonitor* block = gBlockList;
while (block) {
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(ObjectMonitor)) == 0, "check");
return 1;
}
block = (ObjectMonitor*) block->FreeNext;
}
return 0;
}
#endif
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