1 /*
2 * Copyright (c) 1997, 2025, Oracle and/or its affiliates. All rights reserved.
3 * Copyright (c) 2021, Azul Systems, Inc. All rights reserved.
4 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
5 *
6 * This code is free software; you can redistribute it and/or modify it
7 * under the terms of the GNU General Public License version 2 only, as
8 * published by the Free Software Foundation.
9 *
10 * This code is distributed in the hope that it will be useful, but WITHOUT
11 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
12 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
13 * version 2 for more details (a copy is included in the LICENSE file that
14 * accompanied this code).
15 *
16 * You should have received a copy of the GNU General Public License version
17 * 2 along with this work; if not, write to the Free Software Foundation,
18 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
19 *
20 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
21 * or visit www.oracle.com if you need additional information or have any
22 * questions.
23 *
24 */
25
26 #include "precompiled.hpp"
27 #include "cds/cdsConfig.hpp"
28 #include "classfile/javaClasses.hpp"
29 #include "classfile/javaThreadStatus.hpp"
30 #include "gc/shared/barrierSet.hpp"
31 #include "jfr/jfrEvents.hpp"
32 #include "jvm.h"
33 #include "jvmtifiles/jvmtiEnv.hpp"
34 #include "logging/log.hpp"
35 #include "memory/allocation.inline.hpp"
36 #include "memory/iterator.hpp"
37 #include "memory/resourceArea.hpp"
38 #include "nmt/memTracker.hpp"
39 #include "oops/oop.inline.hpp"
40 #include "runtime/atomic.hpp"
41 #include "runtime/handles.inline.hpp"
42 #include "runtime/javaThread.inline.hpp"
43 #include "runtime/nonJavaThread.hpp"
44 #include "runtime/orderAccess.hpp"
45 #include "runtime/osThread.hpp"
46 #include "runtime/safepoint.hpp"
47 #include "runtime/safepointMechanism.inline.hpp"
48 #include "runtime/thread.inline.hpp"
49 #include "runtime/threadSMR.inline.hpp"
50 #include "utilities/macros.hpp"
51 #include "utilities/spinYield.hpp"
52 #if INCLUDE_JFR
53 #include "jfr/jfr.hpp"
54 #endif
55
56 #ifndef USE_LIBRARY_BASED_TLS_ONLY
57 // Current thread is maintained as a thread-local variable
58 THREAD_LOCAL Thread* Thread::_thr_current = nullptr;
59 #endif
60
61 // ======= Thread ========
62 // Base class for all threads: VMThread, WatcherThread, ConcurrentMarkSweepThread,
63 // JavaThread
64
65 Thread::Thread(MemTag mem_tag) {
66
67 DEBUG_ONLY(_run_state = PRE_CALL_RUN;)
68
69 // stack and get_thread
70 set_stack_base(nullptr);
71 set_stack_size(0);
72 set_lgrp_id(-1);
73 DEBUG_ONLY(clear_suspendible_thread();)
74 DEBUG_ONLY(clear_indirectly_suspendible_thread();)
75 DEBUG_ONLY(clear_indirectly_safepoint_thread();)
76
77 // allocated data structures
78 set_osthread(nullptr);
79 set_resource_area(new (mem_tag) ResourceArea(mem_tag));
80 DEBUG_ONLY(_current_resource_mark = nullptr;)
81 set_handle_area(new (mem_tag) HandleArea(mem_tag, nullptr));
82 set_metadata_handles(new (mtClass) GrowableArray<Metadata*>(30, mtClass));
83 set_last_handle_mark(nullptr);
84
85 // Initial value of zero ==> never claimed.
86 _threads_do_token = 0;
87 _threads_hazard_ptr = nullptr;
88 _threads_list_ptr = nullptr;
89 _nested_threads_hazard_ptr_cnt = 0;
90 _rcu_counter = 0;
91
92 // the handle mark links itself to last_handle_mark
93 new HandleMark(this);
94
95 // plain initialization
96 debug_only(_owned_locks = nullptr;)
97 NOT_PRODUCT(_skip_gcalot = false;)
98 _jvmti_env_iteration_count = 0;
99 set_allocated_bytes(0);
100 _current_pending_raw_monitor = nullptr;
101 _vm_error_callbacks = nullptr;
102
103 // thread-specific hashCode stream generator state - Marsaglia shift-xor form
104 // If we are dumping, keep ihashes constant. Note that during dumping we only
105 // ever run one java thread, and no other thread should generate ihashes either,
106 // so using a constant seed should work fine.
107 _hashStateX = CDSConfig::is_dumping_static_archive() ? 0x12345678 : os::random();
108 _hashStateY = 842502087;
109 _hashStateZ = 0x8767; // (int)(3579807591LL & 0xffff) ;
110 _hashStateW = 273326509;
111
112 // Many of the following fields are effectively final - immutable
113 // Note that nascent threads can't use the Native Monitor-Mutex
114 // construct until the _MutexEvent is initialized ...
115 // CONSIDER: instead of using a fixed set of purpose-dedicated ParkEvents
116 // we might instead use a stack of ParkEvents that we could provision on-demand.
117 // The stack would act as a cache to avoid calls to ParkEvent::Allocate()
118 // and ::Release()
119 _ParkEvent = ParkEvent::Allocate(this);
120
121 #ifdef CHECK_UNHANDLED_OOPS
122 if (CheckUnhandledOops) {
123 _unhandled_oops = new UnhandledOops(this);
124 }
125 #endif // CHECK_UNHANDLED_OOPS
126
127 // Notify the barrier set that a thread is being created. The initial
128 // thread is created before the barrier set is available. The call to
129 // BarrierSet::on_thread_create() for this thread is therefore deferred
130 // to BarrierSet::set_barrier_set().
131 BarrierSet* const barrier_set = BarrierSet::barrier_set();
132 if (barrier_set != nullptr) {
133 barrier_set->on_thread_create(this);
134 } else {
135 // Only the main thread should be created before the barrier set
136 // and that happens just before Thread::current is set. No other thread
137 // can attach as the VM is not created yet, so they can't execute this code.
138 // If the main thread creates other threads before the barrier set that is an error.
139 assert(Thread::current_or_null() == nullptr, "creating thread before barrier set");
140 }
141
142 MACOS_AARCH64_ONLY(DEBUG_ONLY(_wx_init = false));
143 }
144
145 #ifdef ASSERT
146 address Thread::stack_base() const {
147 // Note: can't report Thread::name() here as that can require a ResourceMark which we
148 // can't use because this gets called too early in the thread initialization.
149 assert(_stack_base != nullptr, "Stack base not yet set for thread id:%d (0 if not set)",
150 osthread() != nullptr ? osthread()->thread_id() : 0);
151 return _stack_base;
152 }
153 #endif
154
155 void Thread::initialize_tlab() {
156 if (UseTLAB) {
157 tlab().initialize();
158 }
159 }
160
161 void Thread::initialize_thread_current() {
162 #ifndef USE_LIBRARY_BASED_TLS_ONLY
163 assert(_thr_current == nullptr, "Thread::current already initialized");
164 _thr_current = this;
165 #endif
166 assert(ThreadLocalStorage::thread() == nullptr, "ThreadLocalStorage::thread already initialized");
167 ThreadLocalStorage::set_thread(this);
168 assert(Thread::current() == ThreadLocalStorage::thread(), "TLS mismatch!");
169 }
170
171 void Thread::clear_thread_current() {
172 assert(Thread::current() == ThreadLocalStorage::thread(), "TLS mismatch!");
173 #ifndef USE_LIBRARY_BASED_TLS_ONLY
174 _thr_current = nullptr;
175 #endif
176 ThreadLocalStorage::set_thread(nullptr);
177 }
178
179 void Thread::record_stack_base_and_size() {
180 // Note: at this point, Thread object is not yet initialized. Do not rely on
181 // any members being initialized. Do not rely on Thread::current() being set.
182 // If possible, refrain from doing anything which may crash or assert since
183 // quite probably those crash dumps will be useless.
184 address base;
185 size_t size;
186 os::current_stack_base_and_size(&base, &size);
187 set_stack_base(base);
188 set_stack_size(size);
189
190 // Set stack limits after thread is initialized.
191 if (is_Java_thread()) {
192 JavaThread::cast(this)->stack_overflow_state()->initialize(stack_base(), stack_end());
193 }
194 }
195
196 void Thread::register_thread_stack_with_NMT() {
197 MemTracker::record_thread_stack(stack_end(), stack_size());
198 }
199
200 void Thread::unregister_thread_stack_with_NMT() {
201 MemTracker::release_thread_stack(stack_end(), stack_size());
202 }
203
204 void Thread::call_run() {
205 DEBUG_ONLY(_run_state = CALL_RUN;)
206
207 // At this point, Thread object should be fully initialized and
208 // Thread::current() should be set.
209
210 assert(Thread::current_or_null() != nullptr, "current thread is unset");
211 assert(Thread::current_or_null() == this, "current thread is wrong");
212
213 // Perform common initialization actions
214
215 MACOS_AARCH64_ONLY(this->init_wx());
216
217 register_thread_stack_with_NMT();
218
219 JFR_ONLY(Jfr::on_thread_start(this);)
220
221 log_debug(os, thread)("Thread %zu stack dimensions: "
222 PTR_FORMAT "-" PTR_FORMAT " (" SIZE_FORMAT "k).",
223 os::current_thread_id(), p2i(stack_end()),
224 p2i(stack_base()), stack_size()/1024);
225
226 // Perform <ChildClass> initialization actions
227 DEBUG_ONLY(_run_state = PRE_RUN;)
228 this->pre_run();
229
230 // Invoke <ChildClass>::run()
231 DEBUG_ONLY(_run_state = RUN;)
232 this->run();
233 // Returned from <ChildClass>::run(). Thread finished.
234
235 // Perform common tear-down actions
236
237 assert(Thread::current_or_null() != nullptr, "current thread is unset");
238 assert(Thread::current_or_null() == this, "current thread is wrong");
239
240 // Perform <ChildClass> tear-down actions
241 DEBUG_ONLY(_run_state = POST_RUN;)
242 this->post_run();
243
244 // Note: at this point the thread object may already have deleted itself,
245 // so from here on do not dereference *this*. Not all thread types currently
246 // delete themselves when they terminate. But no thread should ever be deleted
247 // asynchronously with respect to its termination - that is what _run_state can
248 // be used to check.
249
250 // Logically we should do this->unregister_thread_stack_with_NMT() here, but we
251 // had to move that into post_run() because of the `this` deletion issue.
252
253 assert(Thread::current_or_null() == nullptr, "current thread still present");
254 }
255
256 Thread::~Thread() {
257
258 // Attached threads will remain in PRE_CALL_RUN, as will threads that don't actually
259 // get started due to errors etc. Any active thread should at least reach post_run
260 // before it is deleted (usually in post_run()).
261 assert(_run_state == PRE_CALL_RUN ||
262 _run_state == POST_RUN, "Active Thread deleted before post_run(): "
263 "_run_state=%d", (int)_run_state);
264
265 // Notify the barrier set that a thread is being destroyed. Note that a barrier
266 // set might not be available if we encountered errors during bootstrapping.
267 BarrierSet* const barrier_set = BarrierSet::barrier_set();
268 if (barrier_set != nullptr) {
269 barrier_set->on_thread_destroy(this);
270 }
271
272 // deallocate data structures
273 delete resource_area();
274 // since the handle marks are using the handle area, we have to deallocated the root
275 // handle mark before deallocating the thread's handle area,
276 assert(last_handle_mark() != nullptr, "check we have an element");
277 delete last_handle_mark();
278 assert(last_handle_mark() == nullptr, "check we have reached the end");
279
280 ParkEvent::Release(_ParkEvent);
281 // Set to null as a termination indicator for has_terminated().
282 Atomic::store(&_ParkEvent, (ParkEvent*)nullptr);
283
284 delete handle_area();
285 delete metadata_handles();
286
287 // osthread() can be null, if creation of thread failed.
288 if (osthread() != nullptr) os::free_thread(osthread());
289
290 // Clear Thread::current if thread is deleting itself and it has not
291 // already been done. This must be done before the memory is deallocated.
292 // Needed to ensure JNI correctly detects non-attached threads.
293 if (this == Thread::current_or_null()) {
294 Thread::clear_thread_current();
295 }
296
297 CHECK_UNHANDLED_OOPS_ONLY(if (CheckUnhandledOops) delete unhandled_oops();)
298 }
299
300 #ifdef ASSERT
301 // A JavaThread is considered dangling if it not handshake-safe with respect to
302 // the current thread, it is not on a ThreadsList, or not at safepoint.
303 void Thread::check_for_dangling_thread_pointer(Thread *thread) {
304 assert(!thread->is_Java_thread() ||
305 JavaThread::cast(thread)->is_handshake_safe_for(Thread::current()) ||
306 !JavaThread::cast(thread)->on_thread_list() ||
307 SafepointSynchronize::is_at_safepoint() ||
308 ThreadsSMRSupport::is_a_protected_JavaThread_with_lock(JavaThread::cast(thread)),
309 "possibility of dangling Thread pointer");
310 }
311 #endif
312
313 // Is the target JavaThread protected by the calling Thread or by some other
314 // mechanism?
315 //
316 bool Thread::is_JavaThread_protected(const JavaThread* target) {
317 Thread* current_thread = Thread::current();
318
319 // Do the simplest check first:
320 if (SafepointSynchronize::is_at_safepoint()) {
321 // The target is protected since JavaThreads cannot exit
322 // while we're at a safepoint.
323 return true;
324 }
325
326 // If the target hasn't been started yet then it is trivially
327 // "protected". We assume the caller is the thread that will do
328 // the starting.
329 if (target->osthread() == nullptr || target->osthread()->get_state() <= INITIALIZED) {
330 return true;
331 }
332
333 // Now make the simple checks based on who the caller is:
334 if (current_thread == target || Threads_lock->owner() == current_thread) {
335 // Target JavaThread is self or calling thread owns the Threads_lock.
336 // Second check is the same as Threads_lock->owner_is_self(),
337 // but we already have the current thread so check directly.
338 return true;
339 }
340
341 // Check the ThreadsLists associated with the calling thread (if any)
342 // to see if one of them protects the target JavaThread:
343 if (is_JavaThread_protected_by_TLH(target)) {
344 return true;
345 }
346
347 // Use this debug code with -XX:+UseNewCode to diagnose locations that
348 // are missing a ThreadsListHandle or other protection mechanism:
349 // guarantee(!UseNewCode, "current_thread=" INTPTR_FORMAT " is not protecting target="
350 // INTPTR_FORMAT, p2i(current_thread), p2i(target));
351
352 // Note: Since 'target' isn't protected by a TLH, the call to
353 // target->is_handshake_safe_for() may crash, but we have debug bits so
354 // we'll be able to figure out what protection mechanism is missing.
355 assert(target->is_handshake_safe_for(current_thread), "JavaThread=" INTPTR_FORMAT
356 " is not protected and not handshake safe.", p2i(target));
357
358 // The target JavaThread is not protected so it is not safe to query:
359 return false;
360 }
361
362 // Is the target JavaThread protected by a ThreadsListHandle (TLH) associated
363 // with the calling Thread?
364 //
365 bool Thread::is_JavaThread_protected_by_TLH(const JavaThread* target) {
366 Thread* current_thread = Thread::current();
367
368 // Check the ThreadsLists associated with the calling thread (if any)
369 // to see if one of them protects the target JavaThread:
370 for (SafeThreadsListPtr* stlp = current_thread->_threads_list_ptr;
371 stlp != nullptr; stlp = stlp->previous()) {
372 if (stlp->list()->includes(target)) {
373 // The target JavaThread is protected by this ThreadsList:
374 return true;
375 }
376 }
377
378 // The target JavaThread is not protected by a TLH so it is not safe to query:
379 return false;
380 }
381
382 void Thread::set_priority(Thread* thread, ThreadPriority priority) {
383 debug_only(check_for_dangling_thread_pointer(thread);)
384 // Can return an error!
385 (void)os::set_priority(thread, priority);
386 }
387
388
389 void Thread::start(Thread* thread) {
390 // Start is different from resume in that its safety is guaranteed by context or
391 // being called from a Java method synchronized on the Thread object.
392 if (thread->is_Java_thread()) {
393 // Initialize the thread state to RUNNABLE before starting this thread.
394 // Can not set it after the thread started because we do not know the
395 // exact thread state at that time. It could be in MONITOR_WAIT or
396 // in SLEEPING or some other state.
397 java_lang_Thread::set_thread_status(JavaThread::cast(thread)->threadObj(),
398 JavaThreadStatus::RUNNABLE);
399 }
400 os::start_thread(thread);
401 }
402
403 // GC Support
404 bool Thread::claim_par_threads_do(uintx claim_token) {
405 uintx token = _threads_do_token;
406 if (token != claim_token) {
407 uintx res = Atomic::cmpxchg(&_threads_do_token, token, claim_token);
408 if (res == token) {
409 return true;
410 }
411 guarantee(res == claim_token, "invariant");
412 }
413 return false;
414 }
415
416 void Thread::oops_do_no_frames(OopClosure* f, NMethodClosure* cf) {
417 // Do oop for ThreadShadow
418 f->do_oop((oop*)&_pending_exception);
419 handle_area()->oops_do(f);
420 }
421
422 // If the caller is a NamedThread, then remember, in the current scope,
423 // the given JavaThread in its _processed_thread field.
424 class RememberProcessedThread: public StackObj {
425 NamedThread* _cur_thr;
426 public:
427 RememberProcessedThread(Thread* thread) {
428 Thread* self = Thread::current();
429 if (self->is_Named_thread()) {
430 _cur_thr = (NamedThread *)self;
431 assert(_cur_thr->processed_thread() == nullptr, "nesting not supported");
432 _cur_thr->set_processed_thread(thread);
433 } else {
434 _cur_thr = nullptr;
435 }
436 }
437
438 ~RememberProcessedThread() {
439 if (_cur_thr) {
440 assert(_cur_thr->processed_thread() != nullptr, "nesting not supported");
441 _cur_thr->set_processed_thread(nullptr);
442 }
443 }
444 };
445
446 void Thread::oops_do(OopClosure* f, NMethodClosure* cf) {
447 // Record JavaThread to GC thread
448 RememberProcessedThread rpt(this);
449 oops_do_no_frames(f, cf);
450 oops_do_frames(f, cf);
451 }
452
453 void Thread::metadata_handles_do(void f(Metadata*)) {
454 // Only walk the Handles in Thread.
455 if (metadata_handles() != nullptr) {
456 for (int i = 0; i< metadata_handles()->length(); i++) {
457 f(metadata_handles()->at(i));
458 }
459 }
460 }
461
462 void Thread::print_on(outputStream* st, bool print_extended_info) const {
463 // get_priority assumes osthread initialized
464 if (osthread() != nullptr) {
465 int os_prio;
466 if (os::get_native_priority(this, &os_prio) == OS_OK) {
467 st->print("os_prio=%d ", os_prio);
468 }
469
470 st->print("cpu=%.2fms ",
471 (double)os::thread_cpu_time(const_cast<Thread*>(this), true) / 1000000.0
472 );
473 st->print("elapsed=%.2fs ",
474 (double)_statistical_info.getElapsedTime() / 1000.0
475 );
476 if (is_Java_thread() && (PrintExtendedThreadInfo || print_extended_info)) {
477 size_t allocated_bytes = (size_t) const_cast<Thread*>(this)->cooked_allocated_bytes();
478 st->print("allocated=" SIZE_FORMAT "%s ",
479 byte_size_in_proper_unit(allocated_bytes),
480 proper_unit_for_byte_size(allocated_bytes)
481 );
482 st->print("defined_classes=" INT64_FORMAT " ", _statistical_info.getDefineClassCount());
483 }
484
485 st->print("tid=" INTPTR_FORMAT " ", p2i(this));
486 if (!is_Java_thread() || !JavaThread::cast(this)->is_vthread_mounted()) {
487 osthread()->print_on(st);
488 }
489 }
490 ThreadsSMRSupport::print_info_on(this, st);
491 st->print(" ");
492 debug_only(if (WizardMode) print_owned_locks_on(st);)
493 }
494
495 void Thread::print() const { print_on(tty); }
496
497 // Thread::print_on_error() is called by fatal error handler. Don't use
498 // any lock or allocate memory.
499 void Thread::print_on_error(outputStream* st, char* buf, int buflen) const {
500 assert(!(is_Compiler_thread() || is_Java_thread()), "Can't call name() here if it allocates");
501
502 st->print("%s \"%s\"", type_name(), name());
503
504 OSThread* os_thr = osthread();
505 if (os_thr != nullptr) {
506 st->fill_to(67);
507 if (os_thr->get_state() != ZOMBIE) {
508 // Use raw field members for stack base/size as this could be
509 // called before a thread has run enough to initialize them.
510 st->print(" [id=%d, stack(" PTR_FORMAT "," PTR_FORMAT ") (" PROPERFMT ")]",
511 osthread()->thread_id(), p2i(_stack_base - _stack_size), p2i(_stack_base),
512 PROPERFMTARGS(_stack_size));
513 } else {
514 st->print(" terminated");
515 }
516 } else {
517 st->print(" unknown state (no osThread)");
518 }
519 ThreadsSMRSupport::print_info_on(this, st);
520 }
521
522 void Thread::print_value_on(outputStream* st) const {
523 if (is_Named_thread()) {
524 st->print(" \"%s\" ", name());
525 }
526 st->print(INTPTR_FORMAT, p2i(this)); // print address
527 }
528
529 #ifdef ASSERT
530 void Thread::print_owned_locks_on(outputStream* st) const {
531 Mutex* cur = _owned_locks;
532 if (cur == nullptr) {
533 st->print(" (no locks) ");
534 } else {
535 st->print_cr(" Locks owned:");
536 while (cur) {
537 cur->print_on(st);
538 cur = cur->next();
539 }
540 }
541 }
542
543 Thread* Thread::_starting_thread = nullptr;
544
545 bool Thread::is_starting_thread(const Thread* t) {
546 assert(_starting_thread != nullptr, "invariant");
547 return t == _starting_thread;
548 }
549 #endif // ASSERT
550
551 bool Thread::set_as_starting_thread(JavaThread* jt) {
552 assert(jt != nullptr, "invariant");
553 assert(_starting_thread == nullptr, "already initialized: "
554 "_starting_thread=" INTPTR_FORMAT, p2i(_starting_thread));
555 // NOTE: this must be called from Threads::create_vm().
556 DEBUG_ONLY(_starting_thread = jt;)
557 return os::create_main_thread(jt);
558 }
559
560 // Ad-hoc mutual exclusion primitives: SpinLock
561 //
562 // We employ SpinLocks _only for low-contention, fixed-length
563 // short-duration critical sections where we're concerned
564 // about native mutex_t or HotSpot Mutex:: latency.
565 //
566 // TODO-FIXME: ListLock should be of type SpinLock.
567 // We should make this a 1st-class type, integrated into the lock
568 // hierarchy as leaf-locks. Critically, the SpinLock structure
569 // should have sufficient padding to avoid false-sharing and excessive
570 // cache-coherency traffic.
571
572
573 typedef volatile int SpinLockT;
574
575 void Thread::SpinAcquire(volatile int * adr, const char * LockName) {
576 if (Atomic::cmpxchg(adr, 0, 1) == 0) {
577 return; // normal fast-path return
578 }
579
580 // Slow-path : We've encountered contention -- Spin/Yield/Block strategy.
581 int ctr = 0;
582 int Yields = 0;
583 for (;;) {
584 while (*adr != 0) {
585 ++ctr;
586 if ((ctr & 0xFFF) == 0 || !os::is_MP()) {
587 if (Yields > 5) {
588 os::naked_short_sleep(1);
589 } else {
590 os::naked_yield();
591 ++Yields;
592 }
593 } else {
594 SpinPause();
595 }
596 }
597 if (Atomic::cmpxchg(adr, 0, 1) == 0) return;
598 }
599 }
600
601 void Thread::SpinRelease(volatile int * adr) {
602 assert(*adr != 0, "invariant");
603 OrderAccess::fence(); // guarantee at least release consistency.
604 // Roach-motel semantics.
605 // It's safe if subsequent LDs and STs float "up" into the critical section,
606 // but prior LDs and STs within the critical section can't be allowed
607 // to reorder or float past the ST that releases the lock.
608 // Loads and stores in the critical section - which appear in program
609 // order before the store that releases the lock - must also appear
610 // before the store that releases the lock in memory visibility order.
611 // Conceptually we need a #loadstore|#storestore "release" MEMBAR before
612 // the ST of 0 into the lock-word which releases the lock, so fence
613 // more than covers this on all platforms.
614 *adr = 0;
615 }