1 /*
2 * Copyright (c) 2018, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 *
23 */
24
25 #include "precompiled.hpp"
26 #include "logging/logStream.hpp"
27 #include "memory/allocation.inline.hpp"
28 #include "runtime/jniHandles.inline.hpp"
29 #include "runtime/thread.inline.hpp"
30 #include "runtime/threadSMR.inline.hpp"
31 #include "services/threadService.hpp"
32 #include "utilities/copy.hpp"
33 #include "utilities/globalDefinitions.hpp"
34 #include "utilities/resourceHash.hpp"
35
36 Monitor* ThreadsSMRSupport::_delete_lock =
37 new Monitor(Monitor::special, "Thread_SMR_delete_lock",
38 false /* allow_vm_block */,
39 Monitor::_safepoint_check_never);
40 // The '_cnt', '_max' and '_times" fields are enabled via
41 // -XX:+EnableThreadSMRStatistics:
42
43 // # of parallel threads in _delete_lock->wait().
44 // Impl note: Hard to imagine > 64K waiting threads so this could be 16-bit,
45 // but there is no nice 16-bit _FORMAT support.
46 uint ThreadsSMRSupport::_delete_lock_wait_cnt = 0;
47
48 // Max # of parallel threads in _delete_lock->wait().
49 // Impl note: See _delete_lock_wait_cnt note.
50 uint ThreadsSMRSupport::_delete_lock_wait_max = 0;
51
52 // Flag to indicate when an _delete_lock->notify() is needed.
53 // Impl note: See _delete_lock_wait_cnt note.
54 volatile uint ThreadsSMRSupport::_delete_notify = 0;
55
56 // # of threads deleted over VM lifetime.
57 // Impl note: Atomically incremented over VM lifetime so use unsigned for more
58 // range. Unsigned 64-bit would be more future proof, but 64-bit atomic inc
59 // isn't available everywhere (or is it?).
60 volatile uint ThreadsSMRSupport::_deleted_thread_cnt = 0;
61
62 // Max time in millis to delete a thread.
63 // Impl note: 16-bit might be too small on an overloaded machine. Use
64 // unsigned since this is a time value. Set via Atomic::cmpxchg() in a
65 // loop for correctness.
66 volatile uint ThreadsSMRSupport::_deleted_thread_time_max = 0;
67
68 // Cumulative time in millis to delete threads.
69 // Impl note: Atomically added to over VM lifetime so use unsigned for more
70 // range. Unsigned 64-bit would be more future proof, but 64-bit atomic inc
71 // isn't available everywhere (or is it?).
72 volatile uint ThreadsSMRSupport::_deleted_thread_times = 0;
73
74 ThreadsList* volatile ThreadsSMRSupport::_java_thread_list = new ThreadsList(0);
75
76 // # of ThreadsLists allocated over VM lifetime.
77 // Impl note: We allocate a new ThreadsList for every thread create and
78 // every thread delete so we need a bigger type than the
79 // _deleted_thread_cnt field.
80 uint64_t ThreadsSMRSupport::_java_thread_list_alloc_cnt = 1;
81
82 // # of ThreadsLists freed over VM lifetime.
83 // Impl note: See _java_thread_list_alloc_cnt note.
84 uint64_t ThreadsSMRSupport::_java_thread_list_free_cnt = 0;
85
86 // Max size ThreadsList allocated.
87 // Impl note: Max # of threads alive at one time should fit in unsigned 32-bit.
88 uint ThreadsSMRSupport::_java_thread_list_max = 0;
89
90 // Max # of nested ThreadsLists for a thread.
91 // Impl note: Hard to imagine > 64K nested ThreadsLists so this could be
92 // 16-bit, but there is no nice 16-bit _FORMAT support.
93 uint ThreadsSMRSupport::_nested_thread_list_max = 0;
94
95 // # of ThreadsListHandles deleted over VM lifetime.
96 // Impl note: Atomically incremented over VM lifetime so use unsigned for
97 // more range. There will be fewer ThreadsListHandles than threads so
98 // unsigned 32-bit should be fine.
99 volatile uint ThreadsSMRSupport::_tlh_cnt = 0;
100
101 // Max time in millis to delete a ThreadsListHandle.
102 // Impl note: 16-bit might be too small on an overloaded machine. Use
103 // unsigned since this is a time value. Set via Atomic::cmpxchg() in a
104 // loop for correctness.
105 volatile uint ThreadsSMRSupport::_tlh_time_max = 0;
106
107 // Cumulative time in millis to delete ThreadsListHandles.
108 // Impl note: Atomically added to over VM lifetime so use unsigned for more
109 // range. Unsigned 64-bit would be more future proof, but 64-bit atomic inc
110 // isn't available everywhere (or is it?).
111 volatile uint ThreadsSMRSupport::_tlh_times = 0;
112
113 ThreadsList* ThreadsSMRSupport::_to_delete_list = NULL;
114
115 // # of parallel ThreadsLists on the to-delete list.
116 // Impl note: Hard to imagine > 64K ThreadsLists needing to be deleted so
117 // this could be 16-bit, but there is no nice 16-bit _FORMAT support.
118 uint ThreadsSMRSupport::_to_delete_list_cnt = 0;
119
120 // Max # of parallel ThreadsLists on the to-delete list.
121 // Impl note: See _to_delete_list_cnt note.
122 uint ThreadsSMRSupport::_to_delete_list_max = 0;
123
124
125 // 'inline' functions first so the definitions are before first use:
126
127 inline void ThreadsSMRSupport::add_deleted_thread_times(uint add_value) {
128 Atomic::add(add_value, &_deleted_thread_times);
129 }
130
131 inline void ThreadsSMRSupport::inc_deleted_thread_cnt() {
132 Atomic::inc(&_deleted_thread_cnt);
133 }
134
135 inline void ThreadsSMRSupport::inc_java_thread_list_alloc_cnt() {
136 _java_thread_list_alloc_cnt++;
137 }
138
139 inline void ThreadsSMRSupport::update_deleted_thread_time_max(uint new_value) {
140 while (true) {
141 uint cur_value = _deleted_thread_time_max;
142 if (new_value <= cur_value) {
143 // No need to update max value so we're done.
144 break;
145 }
146 if (Atomic::cmpxchg(new_value, &_deleted_thread_time_max, cur_value) == cur_value) {
147 // Updated max value so we're done. Otherwise try it all again.
148 break;
149 }
150 }
151 }
152
153 inline void ThreadsSMRSupport::update_java_thread_list_max(uint new_value) {
154 if (new_value > _java_thread_list_max) {
155 _java_thread_list_max = new_value;
156 }
157 }
158
159 inline ThreadsList* ThreadsSMRSupport::xchg_java_thread_list(ThreadsList* new_list) {
160 return (ThreadsList*)Atomic::xchg(new_list, &_java_thread_list);
161 }
162
163
164 // Hash table of pointers found by a scan. Used for collecting hazard
165 // pointers (ThreadsList references). Also used for collecting JavaThreads
166 // that are indirectly referenced by hazard ptrs. An instance of this
167 // class only contains one type of pointer.
168 //
169 class ThreadScanHashtable : public CHeapObj<mtThread> {
170 private:
171 static bool ptr_equals(void * const& s1, void * const& s2) {
172 return s1 == s2;
173 }
174
175 static unsigned int ptr_hash(void * const& s1) {
176 // 2654435761 = 2^32 * Phi (golden ratio)
177 return (unsigned int)(((uint32_t)(uintptr_t)s1) * 2654435761u);
178 }
179
180 int _table_size;
181 // ResourceHashtable SIZE is specified at compile time so our
182 // dynamic _table_size is unused for now; 1031 is the first prime
183 // after 1024.
184 typedef ResourceHashtable<void *, int, &ThreadScanHashtable::ptr_hash,
185 &ThreadScanHashtable::ptr_equals, 1031,
186 ResourceObj::C_HEAP, mtThread> PtrTable;
187 PtrTable * _ptrs;
188
189 public:
190 // ResourceHashtable is passed to various functions and populated in
191 // different places so we allocate it using C_HEAP to make it immune
192 // from any ResourceMarks that happen to be in the code paths.
193 ThreadScanHashtable(int table_size) : _table_size(table_size), _ptrs(new (ResourceObj::C_HEAP, mtThread) PtrTable()) {}
194
195 ~ThreadScanHashtable() { delete _ptrs; }
196
197 bool has_entry(void *pointer) {
198 int *val_ptr = _ptrs->get(pointer);
199 return val_ptr != NULL && *val_ptr == 1;
200 }
201
202 void add_entry(void *pointer) {
203 _ptrs->put(pointer, 1);
204 }
205 };
206
207 // Closure to gather JavaThreads indirectly referenced by hazard ptrs
208 // (ThreadsList references) into a hash table. This closure handles part 2
209 // of the dance - adding all the JavaThreads referenced by the hazard
210 // pointer (ThreadsList reference) to the hash table.
211 //
212 class AddThreadHazardPointerThreadClosure : public ThreadClosure {
213 private:
214 ThreadScanHashtable *_table;
215
216 public:
217 AddThreadHazardPointerThreadClosure(ThreadScanHashtable *table) : _table(table) {}
218
219 virtual void do_thread(Thread *thread) {
220 if (!_table->has_entry((void*)thread)) {
221 // The same JavaThread might be on more than one ThreadsList or
222 // more than one thread might be using the same ThreadsList. In
223 // either case, we only need a single entry for a JavaThread.
224 _table->add_entry((void*)thread);
225 }
226 }
227 };
228
229 // Closure to gather JavaThreads indirectly referenced by hazard ptrs
230 // (ThreadsList references) into a hash table. This closure handles part 1
231 // of the dance - hazard ptr chain walking and dispatch to another
232 // closure.
233 //
234 class ScanHazardPtrGatherProtectedThreadsClosure : public ThreadClosure {
235 private:
236 ThreadScanHashtable *_table;
237 public:
238 ScanHazardPtrGatherProtectedThreadsClosure(ThreadScanHashtable *table) : _table(table) {}
239
240 virtual void do_thread(Thread *thread) {
241 assert_locked_or_safepoint(Threads_lock);
242
243 if (thread == NULL) return;
244
245 // This code races with ThreadsSMRSupport::acquire_stable_list() which
246 // is lock-free so we have to handle some special situations.
247 //
248 ThreadsList *current_list = NULL;
249 while (true) {
250 current_list = thread->get_threads_hazard_ptr();
251 // No hazard ptr so nothing more to do.
252 if (current_list == NULL) {
253 assert(thread->get_nested_threads_hazard_ptr() == NULL,
254 "cannot have a nested hazard ptr with a NULL regular hazard ptr");
255 return;
256 }
257
258 // If the hazard ptr is verified as stable (since it is not tagged),
259 // then it is safe to use.
260 if (!Thread::is_hazard_ptr_tagged(current_list)) break;
261
262 // The hazard ptr is tagged as not yet verified as being stable
263 // so we are racing with acquire_stable_list(). This exchange
264 // attempts to invalidate the hazard ptr. If we win the race,
265 // then we can ignore this unstable hazard ptr and the other
266 // thread will retry the attempt to publish a stable hazard ptr.
267 // If we lose the race, then we retry our attempt to look at the
268 // hazard ptr.
269 if (thread->cmpxchg_threads_hazard_ptr(NULL, current_list) == current_list) return;
270 }
271
272 // The current JavaThread has a hazard ptr (ThreadsList reference)
273 // which might be _java_thread_list or it might be an older
274 // ThreadsList that has been removed but not freed. In either case,
275 // the hazard ptr is protecting all the JavaThreads on that
276 // ThreadsList.
277 AddThreadHazardPointerThreadClosure add_cl(_table);
278 current_list->threads_do(&add_cl);
279
280 // Any NestedThreadsLists are also protecting JavaThreads so
281 // gather those also; the ThreadsLists may be different.
282 for (NestedThreadsList* node = thread->get_nested_threads_hazard_ptr();
283 node != NULL; node = node->next()) {
284 node->t_list()->threads_do(&add_cl);
285 }
286 }
287 };
288
289 // Closure to gather hazard ptrs (ThreadsList references) into a hash table.
290 //
291 class ScanHazardPtrGatherThreadsListClosure : public ThreadClosure {
292 private:
293 ThreadScanHashtable *_table;
294 public:
295 ScanHazardPtrGatherThreadsListClosure(ThreadScanHashtable *table) : _table(table) {}
296
297 virtual void do_thread(Thread* thread) {
298 assert_locked_or_safepoint(Threads_lock);
299
300 if (thread == NULL) return;
301 ThreadsList *threads = thread->get_threads_hazard_ptr();
302 if (threads == NULL) {
303 assert(thread->get_nested_threads_hazard_ptr() == NULL,
304 "cannot have a nested hazard ptr with a NULL regular hazard ptr");
305 return;
306 }
307 // In this closure we always ignore the tag that might mark this
308 // hazard ptr as not yet verified. If we happen to catch an
309 // unverified hazard ptr that is subsequently discarded (not
310 // published), then the only side effect is that we might keep a
311 // to-be-deleted ThreadsList alive a little longer.
312 threads = Thread::untag_hazard_ptr(threads);
313 if (!_table->has_entry((void*)threads)) {
314 _table->add_entry((void*)threads);
315 }
316
317 // Any NestedThreadsLists are also protecting JavaThreads so
318 // gather those also; the ThreadsLists may be different.
319 for (NestedThreadsList* node = thread->get_nested_threads_hazard_ptr();
320 node != NULL; node = node->next()) {
321 threads = node->t_list();
322 if (!_table->has_entry((void*)threads)) {
323 _table->add_entry((void*)threads);
324 }
325 }
326 }
327 };
328
329 // Closure to print JavaThreads that have a hazard ptr (ThreadsList
330 // reference) that contains an indirect reference to a specific JavaThread.
331 //
332 class ScanHazardPtrPrintMatchingThreadsClosure : public ThreadClosure {
333 private:
334 JavaThread *_thread;
335 public:
336 ScanHazardPtrPrintMatchingThreadsClosure(JavaThread *thread) : _thread(thread) {}
337
338 virtual void do_thread(Thread *thread) {
339 assert_locked_or_safepoint(Threads_lock);
340
341 if (thread == NULL) return;
342 ThreadsList *current_list = thread->get_threads_hazard_ptr();
343 if (current_list == NULL) {
344 assert(thread->get_nested_threads_hazard_ptr() == NULL,
345 "cannot have a nested hazard ptr with a NULL regular hazard ptr");
346 return;
347 }
348 // If the hazard ptr is unverified, then ignore it.
349 if (Thread::is_hazard_ptr_tagged(current_list)) return;
350
351 // The current JavaThread has a hazard ptr (ThreadsList reference)
352 // which might be _java_thread_list or it might be an older
353 // ThreadsList that has been removed but not freed. In either case,
354 // the hazard ptr is protecting all the JavaThreads on that
355 // ThreadsList, but we only care about matching a specific JavaThread.
356 JavaThreadIterator jti(current_list);
357 for (JavaThread *p = jti.first(); p != NULL; p = jti.next()) {
358 if (p == _thread) {
359 log_debug(thread, smr)("tid=" UINTX_FORMAT ": ThreadsSMRSupport::smr_delete: thread1=" INTPTR_FORMAT " has a hazard pointer for thread2=" INTPTR_FORMAT, os::current_thread_id(), p2i(thread), p2i(_thread));
360 break;
361 }
362 }
363
364 // Any NestedThreadsLists are also protecting JavaThreads so
365 // check those also; the ThreadsLists may be different.
366 for (NestedThreadsList* node = thread->get_nested_threads_hazard_ptr();
367 node != NULL; node = node->next()) {
368 JavaThreadIterator jti(node->t_list());
369 for (JavaThread *p = jti.first(); p != NULL; p = jti.next()) {
370 if (p == _thread) {
371 log_debug(thread, smr)("tid=" UINTX_FORMAT ": ThreadsSMRSupport::smr_delete: thread1=" INTPTR_FORMAT " has a nested hazard pointer for thread2=" INTPTR_FORMAT, os::current_thread_id(), p2i(thread), p2i(_thread));
372 return;
373 }
374 }
375 }
376 }
377 };
378
379
380 // 'entries + 1' so we always have at least one entry.
381 ThreadsList::ThreadsList(int entries) : _length(entries), _threads(NEW_C_HEAP_ARRAY(JavaThread*, entries + 1, mtThread)), _next_list(NULL) {
382 *(JavaThread**)(_threads + entries) = NULL; // Make sure the extra entry is NULL.
383 }
384
385 ThreadsList::~ThreadsList() {
386 FREE_C_HEAP_ARRAY(JavaThread*, _threads);
387 }
388
389 // Add a JavaThread to a ThreadsList. The returned ThreadsList is a
390 // new copy of the specified ThreadsList with the specified JavaThread
391 // appended to the end.
392 ThreadsList *ThreadsList::add_thread(ThreadsList *list, JavaThread *java_thread) {
393 const uint index = list->_length;
394 const uint new_length = index + 1;
395 const uint head_length = index;
396 ThreadsList *const new_list = new ThreadsList(new_length);
397
398 if (head_length > 0) {
399 Copy::disjoint_words((HeapWord*)list->_threads, (HeapWord*)new_list->_threads, head_length);
400 }
401 *(JavaThread**)(new_list->_threads + index) = java_thread;
402
403 return new_list;
404 }
405
406 int ThreadsList::find_index_of_JavaThread(JavaThread *target) {
407 if (target == NULL) {
408 return -1;
409 }
410 for (uint i = 0; i < length(); i++) {
411 if (target == thread_at(i)) {
412 return (int)i;
413 }
414 }
415 return -1;
416 }
417
418 JavaThread* ThreadsList::find_JavaThread_from_java_tid(jlong java_tid) const {
419 for (uint i = 0; i < length(); i++) {
420 JavaThread* thread = thread_at(i);
421 oop tobj = thread->threadObj();
422 // Ignore the thread if it hasn't run yet, has exited
423 // or is starting to exit.
424 if (tobj != NULL && !thread->is_exiting() &&
425 java_tid == java_lang_Thread::thread_id(tobj)) {
426 // found a match
427 return thread;
428 }
429 }
430 return NULL;
431 }
432
433 bool ThreadsList::includes(const JavaThread * const p) const {
434 if (p == NULL) {
435 return false;
436 }
437 for (uint i = 0; i < length(); i++) {
438 if (thread_at(i) == p) {
439 return true;
440 }
441 }
442 return false;
443 }
444
445 // Remove a JavaThread from a ThreadsList. The returned ThreadsList is a
446 // new copy of the specified ThreadsList with the specified JavaThread
447 // removed.
448 ThreadsList *ThreadsList::remove_thread(ThreadsList* list, JavaThread* java_thread) {
449 assert(list->_length > 0, "sanity");
450
451 uint i = (uint)list->find_index_of_JavaThread(java_thread);
452 assert(i < list->_length, "did not find JavaThread on the list");
453 const uint index = i;
454 const uint new_length = list->_length - 1;
455 const uint head_length = index;
456 const uint tail_length = (new_length >= index) ? (new_length - index) : 0;
457 ThreadsList *const new_list = new ThreadsList(new_length);
458
459 if (head_length > 0) {
460 Copy::disjoint_words((HeapWord*)list->_threads, (HeapWord*)new_list->_threads, head_length);
461 }
462 if (tail_length > 0) {
463 Copy::disjoint_words((HeapWord*)list->_threads + index + 1, (HeapWord*)new_list->_threads + index, tail_length);
464 }
465
466 return new_list;
467 }
468
469 ThreadsListHandle::ThreadsListHandle(Thread *self) : _list(ThreadsSMRSupport::acquire_stable_list(self, /* is_ThreadsListSetter */ false)), _self(self) {
470 assert(self == Thread::current(), "sanity check");
471 if (EnableThreadSMRStatistics) {
472 _timer.start();
473 }
474 }
475
476 ThreadsListHandle::~ThreadsListHandle() {
477 ThreadsSMRSupport::release_stable_list(_self);
478 if (EnableThreadSMRStatistics) {
479 _timer.stop();
480 uint millis = (uint)_timer.milliseconds();
481 ThreadsSMRSupport::update_tlh_stats(millis);
482 }
483 }
484
485 // Convert an internal thread reference to a JavaThread found on the
486 // associated ThreadsList. This ThreadsListHandle "protects" the
487 // returned JavaThread *.
488 //
489 // If thread_oop_p is not NULL, then the caller wants to use the oop
490 // after this call so the oop is returned. On success, *jt_pp is set
491 // to the converted JavaThread * and true is returned. On error,
492 // returns false.
493 //
494 bool ThreadsListHandle::cv_internal_thread_to_JavaThread(jobject jthread,
495 JavaThread ** jt_pp,
496 oop * thread_oop_p) {
497 assert(this->list() != NULL, "must have a ThreadsList");
498 assert(jt_pp != NULL, "must have a return JavaThread pointer");
499 // thread_oop_p is optional so no assert()
500
501 // The JVM_* interfaces don't allow a NULL thread parameter; JVM/TI
502 // allows a NULL thread parameter to signify "current thread" which
503 // allows us to avoid calling cv_external_thread_to_JavaThread().
504 // The JVM_* interfaces have no such leeway.
505
506 oop thread_oop = JNIHandles::resolve_non_null(jthread);
507 // Looks like an oop at this point.
508 if (thread_oop_p != NULL) {
509 // Return the oop to the caller; the caller may still want
510 // the oop even if this function returns false.
511 *thread_oop_p = thread_oop;
512 }
513
514 JavaThread *java_thread = java_lang_Thread::thread(thread_oop);
515 if (java_thread == NULL) {
516 // The java.lang.Thread does not contain a JavaThread * so it has
517 // not yet run or it has died.
518 return false;
519 }
520 // Looks like a live JavaThread at this point.
521
522 if (java_thread != JavaThread::current()) {
523 // jthread is not for the current JavaThread so have to verify
524 // the JavaThread * against the ThreadsList.
525 if (EnableThreadSMRExtraValidityChecks && !includes(java_thread)) {
526 // Not on the JavaThreads list so it is not alive.
527 return false;
528 }
529 }
530
531 // Return a live JavaThread that is "protected" by the
532 // ThreadsListHandle in the caller.
533 *jt_pp = java_thread;
534 return true;
535 }
536
537 ThreadsListSetter::~ThreadsListSetter() {
538 if (_target_needs_release) {
539 // The hazard ptr in the target needs to be released.
540 ThreadsSMRSupport::release_stable_list(_target);
541 }
542 }
543
544 void ThreadsListSetter::set() {
545 assert(_target->get_threads_hazard_ptr() == NULL, "hazard ptr should not already be set");
546 (void) ThreadsSMRSupport::acquire_stable_list(_target, /* is_ThreadsListSetter */ true);
547 _target_needs_release = true;
548 }
549
550 // Acquire a stable ThreadsList.
551 //
552 ThreadsList *ThreadsSMRSupport::acquire_stable_list(Thread *self, bool is_ThreadsListSetter) {
553 assert(self != NULL, "sanity check");
554 // acquire_stable_list_nested_path() will grab the Threads_lock
555 // so let's make sure the ThreadsListHandle is in a safe place.
556 // ThreadsListSetter cannot make this check on this code path.
557 debug_only(if (!is_ThreadsListSetter && StrictSafepointChecks) self->check_for_valid_safepoint_state(/* potential_vm_operation */ false);)
558
559 if (self->get_threads_hazard_ptr() == NULL) {
560 // The typical case is first.
561 return acquire_stable_list_fast_path(self);
562 }
563
564 // The nested case is rare.
565 return acquire_stable_list_nested_path(self);
566 }
567
568 // Fast path (and lock free) way to acquire a stable ThreadsList.
569 //
570 ThreadsList *ThreadsSMRSupport::acquire_stable_list_fast_path(Thread *self) {
571 assert(self != NULL, "sanity check");
572 assert(self->get_threads_hazard_ptr() == NULL, "sanity check");
573 assert(self->get_nested_threads_hazard_ptr() == NULL,
574 "cannot have a nested hazard ptr with a NULL regular hazard ptr");
575
576 ThreadsList* threads;
577
578 // Stable recording of a hazard ptr for SMR. This code does not use
579 // locks so its use of the _java_thread_list & _threads_hazard_ptr
580 // fields is racy relative to code that uses those fields with locks.
581 // OrderAccess and Atomic functions are used to deal with those races.
582 //
583 while (true) {
584 threads = get_java_thread_list();
585
586 // Publish a tagged hazard ptr to denote that the hazard ptr is not
587 // yet verified as being stable. Due to the fence after the hazard
588 // ptr write, it will be sequentially consistent w.r.t. the
589 // sequentially consistent writes of the ThreadsList, even on
590 // non-multiple copy atomic machines where stores can be observed
591 // in different order from different observer threads.
592 ThreadsList* unverified_threads = Thread::tag_hazard_ptr(threads);
593 self->set_threads_hazard_ptr(unverified_threads);
594
595 // If _java_thread_list has changed, we have lost a race with
596 // Threads::add() or Threads::remove() and have to try again.
597 if (get_java_thread_list() != threads) {
598 continue;
599 }
600
601 // We try to remove the tag which will verify the hazard ptr as
602 // being stable. This exchange can race with a scanning thread
603 // which might invalidate the tagged hazard ptr to keep it from
604 // being followed to access JavaThread ptrs. If we lose the race,
605 // we simply retry. If we win the race, then the stable hazard
606 // ptr is officially published.
607 if (self->cmpxchg_threads_hazard_ptr(threads, unverified_threads) == unverified_threads) {
608 break;
609 }
610 }
611
612 // A stable hazard ptr has been published letting other threads know
613 // that the ThreadsList and the JavaThreads reachable from this list
614 // are protected and hence they should not be deleted until everyone
615 // agrees it is safe to do so.
616
617 return threads;
618 }
619
620 // Acquire a nested stable ThreadsList; this is rare so it uses
621 // Threads_lock.
622 //
623 ThreadsList *ThreadsSMRSupport::acquire_stable_list_nested_path(Thread *self) {
624 assert(self != NULL, "sanity check");
625 assert(self->get_threads_hazard_ptr() != NULL,
626 "cannot have a NULL regular hazard ptr when acquiring a nested hazard ptr");
627
628 // The thread already has a hazard ptr (ThreadsList ref) so we need
629 // to create a nested ThreadsListHandle with the current ThreadsList
630 // since it might be different than our current hazard ptr. The need
631 // for a nested ThreadsListHandle is rare so we do this while holding
632 // the Threads_lock so we don't race with the scanning code; the code
633 // is so much simpler this way.
634
635 NestedThreadsList* node;
636 {
637 // Only grab the Threads_lock if we don't already own it.
638 MutexLockerEx ml(Threads_lock->owned_by_self() ? NULL : Threads_lock);
639 node = new NestedThreadsList(get_java_thread_list());
640 // We insert at the front of the list to match up with the delete
641 // in release_stable_list().
642 node->set_next(self->get_nested_threads_hazard_ptr());
643 self->set_nested_threads_hazard_ptr(node);
644 if (EnableThreadSMRStatistics) {
645 self->inc_nested_threads_hazard_ptr_cnt();
646 if (self->nested_threads_hazard_ptr_cnt() > _nested_thread_list_max) {
647 _nested_thread_list_max = self->nested_threads_hazard_ptr_cnt();
648 }
649 }
650 }
651 log_debug(thread, smr)("tid=" UINTX_FORMAT ": ThreadsSMRSupport::acquire_stable_list: add NestedThreadsList node containing ThreadsList=" INTPTR_FORMAT, os::current_thread_id(), p2i(node->t_list()));
652
653 return node->t_list();
654 }
655
656 void ThreadsSMRSupport::add_thread(JavaThread *thread){
657 ThreadsList *new_list = ThreadsList::add_thread(ThreadsSMRSupport::get_java_thread_list(), thread);
658 if (EnableThreadSMRStatistics) {
659 ThreadsSMRSupport::inc_java_thread_list_alloc_cnt();
660 ThreadsSMRSupport::update_java_thread_list_max(new_list->length());
661 }
662 // Initial _java_thread_list will not generate a "Threads::add" mesg.
663 log_debug(thread, smr)("tid=" UINTX_FORMAT ": Threads::add: new ThreadsList=" INTPTR_FORMAT, os::current_thread_id(), p2i(new_list));
664
665 ThreadsList *old_list = ThreadsSMRSupport::xchg_java_thread_list(new_list);
666 ThreadsSMRSupport::free_list(old_list);
667 }
668
669 // set_delete_notify() and clear_delete_notify() are called
670 // under the protection of the delete_lock, but we also use an
671 // Atomic operation to ensure the memory update is seen earlier than
672 // when the delete_lock is dropped.
673 //
674 void ThreadsSMRSupport::clear_delete_notify() {
675 Atomic::dec(&_delete_notify);
676 }
677
678 bool ThreadsSMRSupport::delete_notify() {
679 // Use load_acquire() in order to see any updates to _delete_notify
680 // earlier than when delete_lock is grabbed.
681 return (OrderAccess::load_acquire(&_delete_notify) != 0);
682 }
683
684 // Safely free a ThreadsList after a Threads::add() or Threads::remove().
685 // The specified ThreadsList may not get deleted during this call if it
686 // is still in-use (referenced by a hazard ptr). Other ThreadsLists
687 // in the chain may get deleted by this call if they are no longer in-use.
688 void ThreadsSMRSupport::free_list(ThreadsList* threads) {
689 assert_locked_or_safepoint(Threads_lock);
690
691 threads->set_next_list(_to_delete_list);
692 _to_delete_list = threads;
693 if (EnableThreadSMRStatistics) {
694 _to_delete_list_cnt++;
695 if (_to_delete_list_cnt > _to_delete_list_max) {
696 _to_delete_list_max = _to_delete_list_cnt;
697 }
698 }
699
700 // Hash table size should be first power of two higher than twice the length of the ThreadsList
701 int hash_table_size = MIN2((int)get_java_thread_list()->length(), 32) << 1;
702 hash_table_size--;
703 hash_table_size |= hash_table_size >> 1;
704 hash_table_size |= hash_table_size >> 2;
705 hash_table_size |= hash_table_size >> 4;
706 hash_table_size |= hash_table_size >> 8;
707 hash_table_size |= hash_table_size >> 16;
708 hash_table_size++;
709
710 // Gather a hash table of the current hazard ptrs:
711 ThreadScanHashtable *scan_table = new ThreadScanHashtable(hash_table_size);
712 ScanHazardPtrGatherThreadsListClosure scan_cl(scan_table);
713 Threads::threads_do(&scan_cl);
714
715 // Walk through the linked list of pending freeable ThreadsLists
716 // and free the ones that are not referenced from hazard ptrs.
717 ThreadsList* current = _to_delete_list;
718 ThreadsList* prev = NULL;
719 ThreadsList* next = NULL;
720 bool threads_is_freed = false;
721 while (current != NULL) {
722 next = current->next_list();
723 if (!scan_table->has_entry((void*)current)) {
724 // This ThreadsList is not referenced by a hazard ptr.
725 if (prev != NULL) {
726 prev->set_next_list(next);
727 }
728 if (_to_delete_list == current) {
729 _to_delete_list = next;
730 }
731
732 log_debug(thread, smr)("tid=" UINTX_FORMAT ": ThreadsSMRSupport::free_list: threads=" INTPTR_FORMAT " is freed.", os::current_thread_id(), p2i(current));
733 if (current == threads) threads_is_freed = true;
734 delete current;
735 if (EnableThreadSMRStatistics) {
736 _java_thread_list_free_cnt++;
737 _to_delete_list_cnt--;
738 }
739 } else {
740 prev = current;
741 }
742 current = next;
743 }
744
745 if (!threads_is_freed) {
746 // Only report "is not freed" on the original call to
747 // free_list() for this ThreadsList.
748 log_debug(thread, smr)("tid=" UINTX_FORMAT ": ThreadsSMRSupport::free_list: threads=" INTPTR_FORMAT " is not freed.", os::current_thread_id(), p2i(threads));
749 }
750
751 delete scan_table;
752 }
753
754 // Return true if the specified JavaThread is protected by a hazard
755 // pointer (ThreadsList reference). Otherwise, returns false.
756 //
757 bool ThreadsSMRSupport::is_a_protected_JavaThread(JavaThread *thread) {
758 assert_locked_or_safepoint(Threads_lock);
759
760 // Hash table size should be first power of two higher than twice
761 // the length of the Threads list.
762 int hash_table_size = MIN2((int)get_java_thread_list()->length(), 32) << 1;
763 hash_table_size--;
764 hash_table_size |= hash_table_size >> 1;
765 hash_table_size |= hash_table_size >> 2;
766 hash_table_size |= hash_table_size >> 4;
767 hash_table_size |= hash_table_size >> 8;
768 hash_table_size |= hash_table_size >> 16;
769 hash_table_size++;
770
771 // Gather a hash table of the JavaThreads indirectly referenced by
772 // hazard ptrs.
773 ThreadScanHashtable *scan_table = new ThreadScanHashtable(hash_table_size);
774 ScanHazardPtrGatherProtectedThreadsClosure scan_cl(scan_table);
775 Threads::threads_do(&scan_cl);
776
777 bool thread_is_protected = false;
778 if (scan_table->has_entry((void*)thread)) {
779 thread_is_protected = true;
780 }
781 delete scan_table;
782 return thread_is_protected;
783 }
784
785 // Release a stable ThreadsList.
786 //
787 void ThreadsSMRSupport::release_stable_list(Thread *self) {
788 assert(self != NULL, "sanity check");
789 // release_stable_list_nested_path() will grab the Threads_lock
790 // so let's make sure the ThreadsListHandle is in a safe place.
791 debug_only(if (StrictSafepointChecks) self->check_for_valid_safepoint_state(/* potential_vm_operation */ false);)
792
793 if (self->get_nested_threads_hazard_ptr() == NULL) {
794 // The typical case is first.
795 release_stable_list_fast_path(self);
796 return;
797 }
798
799 // The nested case is rare.
800 release_stable_list_nested_path(self);
801 }
802
803 // Fast path way to release a stable ThreadsList. The release portion
804 // is lock-free, but the wake up portion is not.
805 //
806 void ThreadsSMRSupport::release_stable_list_fast_path(Thread *self) {
807 assert(self != NULL, "sanity check");
808 assert(self->get_threads_hazard_ptr() != NULL, "sanity check");
809 assert(self->get_nested_threads_hazard_ptr() == NULL,
810 "cannot have a nested hazard ptr when releasing a regular hazard ptr");
811
812 // After releasing the hazard ptr, other threads may go ahead and
813 // free up some memory temporarily used by a ThreadsList snapshot.
814 self->set_threads_hazard_ptr(NULL);
815
816 // We use double-check locking to reduce traffic on the system
817 // wide Thread-SMR delete_lock.
818 if (ThreadsSMRSupport::delete_notify()) {
819 // An exiting thread might be waiting in smr_delete(); we need to
820 // check with delete_lock to be sure.
821 release_stable_list_wake_up((char *) "regular hazard ptr");
822 }
823 }
824
825 // Release a nested stable ThreadsList; this is rare so it uses
826 // Threads_lock.
827 //
828 void ThreadsSMRSupport::release_stable_list_nested_path(Thread *self) {
829 assert(self != NULL, "sanity check");
830 assert(self->get_nested_threads_hazard_ptr() != NULL, "sanity check");
831 assert(self->get_threads_hazard_ptr() != NULL,
832 "must have a regular hazard ptr to have nested hazard ptrs");
833
834 // We have a nested ThreadsListHandle so we have to release it first.
835 // The need for a nested ThreadsListHandle is rare so we do this while
836 // holding the Threads_lock so we don't race with the scanning code;
837 // the code is so much simpler this way.
838
839 NestedThreadsList *node;
840 {
841 // Only grab the Threads_lock if we don't already own it.
842 MutexLockerEx ml(Threads_lock->owned_by_self() ? NULL : Threads_lock);
843 // We remove from the front of the list to match up with the insert
844 // in acquire_stable_list().
845 node = self->get_nested_threads_hazard_ptr();
846 self->set_nested_threads_hazard_ptr(node->next());
847 if (EnableThreadSMRStatistics) {
848 self->dec_nested_threads_hazard_ptr_cnt();
849 }
850 }
851
852 // An exiting thread might be waiting in smr_delete(); we need to
853 // check with delete_lock to be sure.
854 release_stable_list_wake_up((char *) "nested hazard ptr");
855
856 log_debug(thread, smr)("tid=" UINTX_FORMAT ": ThreadsSMRSupport::release_stable_list: delete NestedThreadsList node containing ThreadsList=" INTPTR_FORMAT, os::current_thread_id(), p2i(node->t_list()));
857
858 delete node;
859 }
860
861 // Wake up portion of the release stable ThreadsList protocol;
862 // uses the delete_lock().
863 //
864 void ThreadsSMRSupport::release_stable_list_wake_up(char *log_str) {
865 assert(log_str != NULL, "sanity check");
866
867 // Note: delete_lock is held in smr_delete() for the entire
868 // hazard ptr search so that we do not lose this notify() if
869 // the exiting thread has to wait. That code path also holds
870 // Threads_lock (which was grabbed before delete_lock) so that
871 // threads_do() can be called. This means the system can't start a
872 // safepoint which means this thread can't take too long to get to
873 // a safepoint because of being blocked on delete_lock.
874 //
875 MonitorLockerEx ml(ThreadsSMRSupport::delete_lock(), Monitor::_no_safepoint_check_flag);
876 if (ThreadsSMRSupport::delete_notify()) {
877 // Notify any exiting JavaThreads that are waiting in smr_delete()
878 // that we've released a ThreadsList.
879 ml.notify_all();
880 log_debug(thread, smr)("tid=" UINTX_FORMAT ": ThreadsSMRSupport::release_stable_list notified %s", os::current_thread_id(), log_str);
881 }
882 }
883
884 void ThreadsSMRSupport::remove_thread(JavaThread *thread) {
885 ThreadsList *new_list = ThreadsList::remove_thread(ThreadsSMRSupport::get_java_thread_list(), thread);
886 if (EnableThreadSMRStatistics) {
887 ThreadsSMRSupport::inc_java_thread_list_alloc_cnt();
888 // This list is smaller so no need to check for a "longest" update.
889 }
890
891 // Final _java_thread_list will not generate a "Threads::remove" mesg.
892 log_debug(thread, smr)("tid=" UINTX_FORMAT ": Threads::remove: new ThreadsList=" INTPTR_FORMAT, os::current_thread_id(), p2i(new_list));
893
894 ThreadsList *old_list = ThreadsSMRSupport::xchg_java_thread_list(new_list);
895 ThreadsSMRSupport::free_list(old_list);
896 }
897
898 // See note for clear_delete_notify().
899 //
900 void ThreadsSMRSupport::set_delete_notify() {
901 Atomic::inc(&_delete_notify);
902 }
903
904 // Safely delete a JavaThread when it is no longer in use by a
905 // ThreadsListHandle.
906 //
907 void ThreadsSMRSupport::smr_delete(JavaThread *thread) {
908 assert(!Threads_lock->owned_by_self(), "sanity");
909
910 bool has_logged_once = false;
911 elapsedTimer timer;
912 if (EnableThreadSMRStatistics) {
913 timer.start();
914 }
915
916 while (true) {
917 {
918 // No safepoint check because this JavaThread is not on the
919 // Threads list.
920 MutexLockerEx ml(Threads_lock, Mutex::_no_safepoint_check_flag);
921 // Cannot use a MonitorLockerEx helper here because we have
922 // to drop the Threads_lock first if we wait.
923 ThreadsSMRSupport::delete_lock()->lock_without_safepoint_check();
924 // Set the delete_notify flag after we grab delete_lock
925 // and before we scan hazard ptrs because we're doing
926 // double-check locking in release_stable_list().
927 ThreadsSMRSupport::set_delete_notify();
928
929 if (!is_a_protected_JavaThread(thread)) {
930 // This is the common case.
931 ThreadsSMRSupport::clear_delete_notify();
932 ThreadsSMRSupport::delete_lock()->unlock();
933 break;
934 }
935 if (!has_logged_once) {
936 has_logged_once = true;
937 log_debug(thread, smr)("tid=" UINTX_FORMAT ": ThreadsSMRSupport::smr_delete: thread=" INTPTR_FORMAT " is not deleted.", os::current_thread_id(), p2i(thread));
938 if (log_is_enabled(Debug, os, thread)) {
939 ScanHazardPtrPrintMatchingThreadsClosure scan_cl(thread);
940 Threads::threads_do(&scan_cl);
941 }
942 }
943 } // We have to drop the Threads_lock to wait or delete the thread
944
945 if (EnableThreadSMRStatistics) {
946 _delete_lock_wait_cnt++;
947 if (_delete_lock_wait_cnt > _delete_lock_wait_max) {
948 _delete_lock_wait_max = _delete_lock_wait_cnt;
949 }
950 }
951 // Wait for a release_stable_list() call before we check again. No
952 // safepoint check, no timeout, and not as suspend equivalent flag
953 // because this JavaThread is not on the Threads list.
954 ThreadsSMRSupport::delete_lock()->wait(Mutex::_no_safepoint_check_flag, 0,
955 !Mutex::_as_suspend_equivalent_flag);
956 if (EnableThreadSMRStatistics) {
957 _delete_lock_wait_cnt--;
958 }
959
960 ThreadsSMRSupport::clear_delete_notify();
961 ThreadsSMRSupport::delete_lock()->unlock();
962 // Retry the whole scenario.
963 }
964
965 if (ThreadLocalHandshakes) {
966 // The thread is about to be deleted so cancel any handshake.
967 thread->cancel_handshake();
968 }
969
970 delete thread;
971 if (EnableThreadSMRStatistics) {
972 timer.stop();
973 uint millis = (uint)timer.milliseconds();
974 ThreadsSMRSupport::inc_deleted_thread_cnt();
975 ThreadsSMRSupport::add_deleted_thread_times(millis);
976 ThreadsSMRSupport::update_deleted_thread_time_max(millis);
977 }
978
979 log_debug(thread, smr)("tid=" UINTX_FORMAT ": ThreadsSMRSupport::smr_delete: thread=" INTPTR_FORMAT " is deleted.", os::current_thread_id(), p2i(thread));
980 }
981
982
983 // Debug, logging, and printing stuff at the end:
984
985 // Log Threads class SMR info.
986 void ThreadsSMRSupport::log_statistics() {
987 LogTarget(Info, thread, smr) log;
988 if (log.is_enabled()) {
989 LogStream out(log);
990 print_info_on(&out);
991 }
992 }
993
994 // Print Threads class SMR info.
995 void ThreadsSMRSupport::print_info_on(outputStream* st) {
996 // Only grab the Threads_lock if we don't already own it
997 // and if we are not reporting an error.
998 MutexLockerEx ml((Threads_lock->owned_by_self() || VMError::is_error_reported()) ? NULL : Threads_lock);
999
1000 st->print_cr("Threads class SMR info:");
1001 st->print_cr("_java_thread_list=" INTPTR_FORMAT ", length=%u, "
1002 "elements={", p2i(_java_thread_list),
1003 _java_thread_list->length());
1004 print_info_elements_on(st, _java_thread_list);
1005 st->print_cr("}");
1006 if (_to_delete_list != NULL) {
1007 st->print_cr("_to_delete_list=" INTPTR_FORMAT ", length=%u, "
1008 "elements={", p2i(_to_delete_list),
1009 _to_delete_list->length());
1010 print_info_elements_on(st, _to_delete_list);
1011 st->print_cr("}");
1012 for (ThreadsList *t_list = _to_delete_list->next_list();
1013 t_list != NULL; t_list = t_list->next_list()) {
1014 st->print("next-> " INTPTR_FORMAT ", length=%u, "
1015 "elements={", p2i(t_list), t_list->length());
1016 print_info_elements_on(st, t_list);
1017 st->print_cr("}");
1018 }
1019 }
1020 if (!EnableThreadSMRStatistics) {
1021 return;
1022 }
1023 st->print_cr("_java_thread_list_alloc_cnt=" UINT64_FORMAT ","
1024 "_java_thread_list_free_cnt=" UINT64_FORMAT ","
1025 "_java_thread_list_max=%u, "
1026 "_nested_thread_list_max=%u",
1027 _java_thread_list_alloc_cnt,
1028 _java_thread_list_free_cnt,
1029 _java_thread_list_max,
1030 _nested_thread_list_max);
1031 if (_tlh_cnt > 0) {
1032 st->print_cr("_tlh_cnt=%u"
1033 ", _tlh_times=%u"
1034 ", avg_tlh_time=%0.2f"
1035 ", _tlh_time_max=%u",
1036 _tlh_cnt, _tlh_times,
1037 ((double) _tlh_times / _tlh_cnt),
1038 _tlh_time_max);
1039 }
1040 if (_deleted_thread_cnt > 0) {
1041 st->print_cr("_deleted_thread_cnt=%u"
1042 ", _deleted_thread_times=%u"
1043 ", avg_deleted_thread_time=%0.2f"
1044 ", _deleted_thread_time_max=%u",
1045 _deleted_thread_cnt, _deleted_thread_times,
1046 ((double) _deleted_thread_times / _deleted_thread_cnt),
1047 _deleted_thread_time_max);
1048 }
1049 st->print_cr("_delete_lock_wait_cnt=%u, _delete_lock_wait_max=%u",
1050 _delete_lock_wait_cnt, _delete_lock_wait_max);
1051 st->print_cr("_to_delete_list_cnt=%u, _to_delete_list_max=%u",
1052 _to_delete_list_cnt, _to_delete_list_max);
1053 }
1054
1055 // Print ThreadsList elements (4 per line).
1056 void ThreadsSMRSupport::print_info_elements_on(outputStream* st, ThreadsList* t_list) {
1057 uint cnt = 0;
1058 JavaThreadIterator jti(t_list);
1059 for (JavaThread *jt = jti.first(); jt != NULL; jt = jti.next()) {
1060 st->print(INTPTR_FORMAT, p2i(jt));
1061 if (cnt < t_list->length() - 1) {
1062 // Separate with comma or comma-space except for the last one.
1063 if (((cnt + 1) % 4) == 0) {
1064 // Four INTPTR_FORMAT fit on an 80 column line so end the
1065 // current line with just a comma.
1066 st->print_cr(",");
1067 } else {
1068 // Not the last one on the current line so use comma-space:
1069 st->print(", ");
1070 }
1071 } else {
1072 // Last one so just end the current line.
1073 st->cr();
1074 }
1075 cnt++;
1076 }
1077 }