/* * Copyright (c) 1998, 2025, 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. * */ #ifndef SHARE_RUNTIME_OBJECTMONITOR_HPP #define SHARE_RUNTIME_OBJECTMONITOR_HPP #include "memory/allocation.hpp" #include "memory/padded.hpp" #include "oops/markWord.hpp" #include "oops/oopHandle.hpp" #include "oops/weakHandle.hpp" #include "runtime/javaThread.hpp" #include "utilities/checkedCast.hpp" class ObjectMonitor; class ObjectMonitorContentionMark; class ParkEvent; class BasicLock; class ContinuationWrapper; class ObjectWaiter : public CHeapObj { public: enum TStates : uint8_t { TS_UNDEF, TS_READY, TS_RUN, TS_WAIT, TS_ENTER }; ObjectWaiter* volatile _next; ObjectWaiter* volatile _prev; JavaThread* _thread; OopHandle _vthread; ObjectMonitor* _monitor; uint64_t _notifier_tid; int _recursions; volatile TStates TState; volatile bool _notified; bool _is_wait; bool _at_reenter; bool _interrupted; bool _active; // Contention monitoring is enabled public: ObjectWaiter(JavaThread* current); ObjectWaiter(oop vthread, ObjectMonitor* mon); ~ObjectWaiter(); JavaThread* thread() const { return _thread; } bool is_vthread() const { return _thread == nullptr; } uint8_t state() const { return TState; } ObjectMonitor* monitor() const { return _monitor; } bool is_wait() const { return _is_wait; } bool notified() const { return _notified; } bool at_reenter() const { return _at_reenter; } bool at_monitorenter() const { return !_is_wait || _at_reenter || _notified; } oop vthread() const; void wait_reenter_begin(ObjectMonitor *mon); void wait_reenter_end(ObjectMonitor *mon); ObjectWaiter* const badObjectWaiterPtr = (ObjectWaiter*) 0xBAD; void set_bad_pointers() { #ifdef ASSERT this->_prev = badObjectWaiterPtr; this->_next = badObjectWaiterPtr; this->TState = ObjectWaiter::TS_RUN; #endif } ObjectWaiter* next() { assert (_next != badObjectWaiterPtr, "corrupted list!"); return _next; } ObjectWaiter* prev() { assert (_prev != badObjectWaiterPtr, "corrupted list!"); return _prev; } }; // The ObjectMonitor class implements the heavyweight version of a // JavaMonitor. The lightweight BasicLock/stack lock version has been // inflated into an ObjectMonitor. This inflation is typically due to // contention or use of Object.wait(). // // WARNING: This is a very sensitive and fragile class. DO NOT make any // changes unless you are fully aware of the underlying semantics. // // ObjectMonitor Layout Overview/Highlights/Restrictions: // // - For performance reasons we ensure the _metadata field is located at offset 0, // which in turn means that ObjectMonitor can't inherit from any other class nor use // any virtual member functions. // - The _metadata and _owner fields should be separated by enough space // to avoid false sharing due to parallel access by different threads. // This is an advisory recommendation. // - The general layout of the fields in ObjectMonitor is: // _metadata // // // _owner // // // - The VM assumes write ordering and machine word alignment with // respect to the _owner field and the that can // be read in parallel by other threads. // - Generally fields that are accessed closely together in time should // be placed proximally in space to promote data cache locality. That // is, temporal locality should condition spatial locality. // - We have to balance avoiding false sharing with excessive invalidation // from coherence traffic. As such, we try to cluster fields that tend // to be _written_ at approximately the same time onto the same data // cache line. // - We also have to balance the natural tension between minimizing // single threaded capacity misses with excessive multi-threaded // coherency misses. There is no single optimal layout for both // single-threaded and multi-threaded environments. // // - See TEST_VM(ObjectMonitor, sanity) gtest for how critical restrictions are // enforced. // // - Separating _owner from the by enough space to // avoid false sharing might be profitable. Given that the CAS in // monitorenter will invalidate the line underlying _owner. We want // to avoid an L1 data cache miss on that same line for monitorexit. // Putting these : // _recursions, _entry_list and _succ, all of which may be // fetched in the inflated unlock path, on a different cache line // would make them immune to CAS-based invalidation from the _owner // field. // // - TODO: The _recursions field should be of type int, or int32_t but not // intptr_t. There's no reason to use a 64-bit type for this field // in a 64-bit JVM. #define OM_CACHE_LINE_SIZE DEFAULT_CACHE_LINE_SIZE class ObjectMonitor : public CHeapObj { friend class VMStructs; JVMCI_ONLY(friend class JVMCIVMStructs;) static OopStorage* _oop_storage; // List of j.l.VirtualThread waiting to be unblocked by unblocker thread. static OopHandle _vthread_list_head; // ParkEvent of unblocker thread. static ParkEvent* _vthread_unparker_ParkEvent; // Because of frequent access, the metadata field is at offset zero (0). // Enforced by the assert() in metadata_addr(). // * LM_LIGHTWEIGHT with UseObjectMonitorTable: // Contains the _object's hashCode. // * LM_LEGACY, LM_MONITOR, LM_LIGHTWEIGHT without UseObjectMonitorTable: // Contains the displaced object header word - mark volatile uintptr_t _metadata; // metadata WeakHandle _object; // backward object pointer // Separate _metadata and _owner on different cache lines since both can // have busy multi-threaded access. _metadata and _object are set at initial // inflation. The _object does not change, so it is a good choice to share // its cache line with _metadata. DEFINE_PAD_MINUS_SIZE(0, OM_CACHE_LINE_SIZE, sizeof(_metadata) + sizeof(WeakHandle)); static const int64_t NO_OWNER = 0; static const int64_t ANONYMOUS_OWNER = 1; static const int64_t DEFLATER_MARKER = 2; int64_t volatile _owner; // Either owner_id of owner, NO_OWNER, ANONYMOUS_OWNER or DEFLATER_MARKER. volatile uint64_t _previous_owner_tid; // thread id of the previous owner of the monitor // Separate _owner and _next_om on different cache lines since // both can have busy multi-threaded access. _previous_owner_tid is only // changed by ObjectMonitor::exit() so it is a good choice to share the // cache line with _owner. DEFINE_PAD_MINUS_SIZE(1, OM_CACHE_LINE_SIZE, sizeof(void* volatile) + sizeof(volatile uint64_t)); ObjectMonitor* _next_om; // Next ObjectMonitor* linkage volatile intx _recursions; // recursion count, 0 for first entry ObjectWaiter* volatile _entry_list; // Threads blocked on entry or reentry. // The list is actually composed of wait-nodes, // acting as proxies for Threads. ObjectWaiter* volatile _entry_list_tail; // _entry_list is the head, this is the tail. int64_t volatile _succ; // Heir presumptive thread - used for futile wakeup throttling volatile int _SpinDuration; int _contentions; // Number of active contentions in enter(). It is used by is_busy() // along with other fields to determine if an ObjectMonitor can be // deflated. It is also used by the async deflation protocol. See // ObjectMonitor::deflate_monitor(). ObjectWaiter* volatile _wait_set; // LL of threads waiting on the monitor - wait() volatile int _waiters; // number of waiting threads volatile int _wait_set_lock; // protects wait set queue - simple spinlock // Used in LM_LEGACY mode to store BasicLock* in case of inflation by contending thread. BasicLock* volatile _stack_locker; public: static void Initialize(); static void Initialize2(); static OopHandle& vthread_list_head() { return _vthread_list_head; } static ParkEvent* vthread_unparker_ParkEvent() { return _vthread_unparker_ParkEvent; } static int Knob_SpinLimit; static ByteSize metadata_offset() { return byte_offset_of(ObjectMonitor, _metadata); } static ByteSize owner_offset() { return byte_offset_of(ObjectMonitor, _owner); } static ByteSize recursions_offset() { return byte_offset_of(ObjectMonitor, _recursions); } static ByteSize succ_offset() { return byte_offset_of(ObjectMonitor, _succ); } static ByteSize entry_list_offset() { return byte_offset_of(ObjectMonitor, _entry_list); } // ObjectMonitor references can be ORed with markWord::monitor_value // as part of the ObjectMonitor tagging mechanism. When we combine an // ObjectMonitor reference with an offset, we need to remove the tag // value in order to generate the proper address. // // We can either adjust the ObjectMonitor reference and then add the // offset or we can adjust the offset that is added to the ObjectMonitor // reference. The latter avoids an AGI (Address Generation Interlock) // stall so the helper macro adjusts the offset value that is returned // to the ObjectMonitor reference manipulation code: // #define OM_OFFSET_NO_MONITOR_VALUE_TAG(f) \ ((in_bytes(ObjectMonitor::f ## _offset())) - checked_cast(markWord::monitor_value)) uintptr_t metadata() const; void set_metadata(uintptr_t value); volatile uintptr_t* metadata_addr(); markWord header() const; void set_header(markWord hdr); intptr_t hash() const; void set_hash(intptr_t hash); bool is_busy() const { // TODO-FIXME: assert _owner == NO_OWNER implies _recursions = 0 intptr_t ret_code = intptr_t(_waiters) | intptr_t(_entry_list); int cnts = contentions(); // read once if (cnts > 0) { ret_code |= intptr_t(cnts); } if (!owner_is_DEFLATER_MARKER()) { ret_code |= intptr_t(owner_raw()); } return ret_code != 0; } const char* is_busy_to_string(stringStream* ss); bool is_entered(JavaThread* current) const; // Returns true if this OM has an owner, false otherwise. bool has_owner() const; int64_t owner() const; // Returns NO_OWNER if DEFLATER_MARKER is observed. int64_t owner_raw() const; // These methods return the value we set in _owner when acquiring // the monitor with the given thread/vthread, AKA owner_id. static int64_t owner_id_from(JavaThread* thread); static int64_t owner_id_from(oop vthread); // Returns true if owner field == DEFLATER_MARKER and false otherwise. bool owner_is_DEFLATER_MARKER() const; // Returns true if 'this' is being async deflated and false otherwise. bool is_being_async_deflated(); // Clear _owner field; current value must match thread's owner_id. void release_clear_owner(JavaThread* thread); // Simply set _owner field to new_value; current value must match old_value. void set_owner_from_raw(int64_t old_value, int64_t new_value); // Same as above but uses owner_id of current as new value. void set_owner_from(int64_t old_value, JavaThread* current); // Try to set _owner field to new_value if the current value matches // old_value, using Atomic::cmpxchg(). Otherwise, does not change the // _owner field. Returns the prior value of the _owner field. int64_t try_set_owner_from_raw(int64_t old_value, int64_t new_value); // Same as above but uses owner_id of current as new_value. int64_t try_set_owner_from(int64_t old_value, JavaThread* current); // Methods to check and set _succ. The successor is the thread selected // from _entry_list by the current owner when releasing the monitor, // to run again and re-try acquiring the monitor. It is used to avoid // unnecessary wake-ups if there is already a successor set. bool has_successor() const; bool has_successor(JavaThread* thread) const; void set_successor(JavaThread* thread); void set_successor(oop vthread); void clear_successor(); int64_t successor() const; // Returns true if _owner field == owner_id of thread, false otherwise. bool has_owner(JavaThread* thread) const { return owner() == owner_id_from(thread); } // Set _owner field to owner_id of thread; current value must be NO_OWNER. void set_owner(JavaThread* thread) { set_owner_from(NO_OWNER, thread); } // Try to set _owner field from NO_OWNER to owner_id of thread. bool try_set_owner(JavaThread* thread) { return try_set_owner_from(NO_OWNER, thread) == NO_OWNER; } bool has_anonymous_owner() const { return owner_raw() == ANONYMOUS_OWNER; } void set_anonymous_owner() { set_owner_from_raw(NO_OWNER, ANONYMOUS_OWNER); } void set_owner_from_anonymous(JavaThread* owner) { set_owner_from(ANONYMOUS_OWNER, owner); } // Get and set _stack_locker. BasicLock* stack_locker() const; void set_stack_locker(BasicLock* locker); // Simply get _next_om field. ObjectMonitor* next_om() const; // Simply set _next_om field to new_value. void set_next_om(ObjectMonitor* new_value); int contentions() const; void add_to_contentions(int value); intx recursions() const { return _recursions; } void set_recursions(size_t recursions); void increment_recursions(JavaThread* current); // JVM/TI GetObjectMonitorUsage() needs this: int waiters() const; ObjectWaiter* first_waiter() { return _wait_set; } ObjectWaiter* next_waiter(ObjectWaiter* o) { return o->_next; } JavaThread* thread_of_waiter(ObjectWaiter* o) { return o->_thread; } ObjectMonitor(oop object); ~ObjectMonitor(); oop object() const; oop object_peek() const; bool object_is_dead() const; bool object_refers_to(oop obj) const; // Returns true if the specified thread owns the ObjectMonitor. Otherwise // returns false and throws IllegalMonitorStateException (IMSE). bool check_owner(TRAPS); private: class ExitOnSuspend { protected: ObjectMonitor* _om; bool _om_exited; public: ExitOnSuspend(ObjectMonitor* om) : _om(om), _om_exited(false) {} void operator()(JavaThread* current); bool exited() { return _om_exited; } }; class ClearSuccOnSuspend { protected: ObjectMonitor* _om; public: ClearSuccOnSuspend(ObjectMonitor* om) : _om(om) {} void operator()(JavaThread* current); }; bool enter_is_async_deflating(); void notify_contended_enter(JavaThread *current); public: void enter_for_with_contention_mark(JavaThread* locking_thread, ObjectMonitorContentionMark& contention_mark); bool enter_for(JavaThread* locking_thread); bool enter(JavaThread* current); bool try_enter(JavaThread* current, bool check_for_recursion = true); bool spin_enter(JavaThread* current); void enter_with_contention_mark(JavaThread* current, ObjectMonitorContentionMark& contention_mark); void exit(JavaThread* current, bool not_suspended = true); bool resume_operation(JavaThread* current, ObjectWaiter* node, ContinuationWrapper& cont); void wait(jlong millis, bool interruptible, TRAPS); void notify(TRAPS); void notifyAll(TRAPS); void quick_notify(JavaThread* current); void quick_notifyAll(JavaThread* current); void print() const; #ifdef ASSERT void print_debug_style_on(outputStream* st) const; #endif void print_on(outputStream* st) const; // Use the following at your own risk intx complete_exit(JavaThread* current); private: void add_to_entry_list(JavaThread* current, ObjectWaiter* node); void add_waiter(ObjectWaiter* waiter); bool notify_internal(JavaThread* current); ObjectWaiter* dequeue_waiter(); void dequeue_specific_waiter(ObjectWaiter* waiter); void enter_internal(JavaThread* current); void reenter_internal(JavaThread* current, ObjectWaiter* current_node); void entry_list_build_dll(JavaThread* current); void unlink_after_acquire(JavaThread* current, ObjectWaiter* current_node); ObjectWaiter* entry_list_tail(JavaThread* current); bool vthread_monitor_enter(JavaThread* current, ObjectWaiter* node = nullptr); void vthread_wait(JavaThread* current, jlong millis); bool vthread_wait_reenter(JavaThread* current, ObjectWaiter* node, ContinuationWrapper& cont); void vthread_epilog(JavaThread* current, ObjectWaiter* node); enum class TryLockResult { Interference = -1, HasOwner = 0, Success = 1 }; bool try_lock_with_contention_mark(JavaThread* locking_thread, ObjectMonitorContentionMark& contention_mark); bool try_lock_or_add_to_entry_list(JavaThread* current, ObjectWaiter* node); TryLockResult try_lock(JavaThread* current); bool try_spin(JavaThread* current); bool short_fixed_spin(JavaThread* current, int spin_count, bool adapt); void exit_epilog(JavaThread* current, ObjectWaiter* Wakee); public: // Deflation support bool deflate_monitor(Thread* current); void install_displaced_markword_in_object(const oop obj); // JFR support static bool is_jfr_excluded(const Klass* monitor_klass); }; // RAII object to ensure that ObjectMonitor::is_being_async_deflated() is // stable within the context of this mark. class ObjectMonitorContentionMark : StackObj { DEBUG_ONLY(friend class ObjectMonitor;) ObjectMonitor* _monitor; bool _extended; NONCOPYABLE(ObjectMonitorContentionMark); public: explicit ObjectMonitorContentionMark(ObjectMonitor* monitor); ~ObjectMonitorContentionMark(); // Extends the contention scope beyond this objects lifetime. // Requires manual decrement of the contentions counter. void extend(); }; #endif // SHARE_RUNTIME_OBJECTMONITOR_HPP