1 /*
2 * Copyright (c) 2017, 2019, 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 "runtime/vmOperations.hpp"
32 #include "services/threadService.hpp"
33 #include "utilities/copy.hpp"
34 #include "utilities/globalDefinitions.hpp"
35 #include "utilities/ostream.hpp"
36 #include "utilities/resourceHash.hpp"
37 #include "utilities/vmError.hpp"
38
39 // The '_cnt', '_max' and '_times" fields are enabled via
40 // -XX:+EnableThreadSMRStatistics:
41
42 // # of parallel threads in _delete_lock->wait().
43 // Impl note: Hard to imagine > 64K waiting threads so this could be 16-bit,
44 // but there is no nice 16-bit _FORMAT support.
45 uint ThreadsSMRSupport::_delete_lock_wait_cnt = 0;
46
47 // Max # of parallel threads in _delete_lock->wait().
48 // Impl note: See _delete_lock_wait_cnt note.
49 uint ThreadsSMRSupport::_delete_lock_wait_max = 0;
50
51 // Flag to indicate when an _delete_lock->notify() is needed.
52 // Impl note: See _delete_lock_wait_cnt note.
53 volatile uint ThreadsSMRSupport::_delete_notify = 0;
54
55 // # of threads deleted over VM lifetime.
56 // Impl note: Atomically incremented over VM lifetime so use unsigned for more
57 // range. Unsigned 64-bit would be more future proof, but 64-bit atomic inc
58 // isn't available everywhere (or is it?).
59 volatile uint ThreadsSMRSupport::_deleted_thread_cnt = 0;
60
61 // Max time in millis to delete a thread.
62 // Impl note: 16-bit might be too small on an overloaded machine. Use
63 // unsigned since this is a time value. Set via Atomic::cmpxchg() in a
64 // loop for correctness.
65 volatile uint ThreadsSMRSupport::_deleted_thread_time_max = 0;
66
67 // Cumulative time in millis to delete threads.
68 // Impl note: Atomically added to over VM lifetime so use unsigned for more
69 // range. Unsigned 64-bit would be more future proof, but 64-bit atomic inc
70 // isn't available everywhere (or is it?).
71 volatile uint ThreadsSMRSupport::_deleted_thread_times = 0;
72
73 // The bootstrap list is empty and cannot be freed.
74 ThreadsList ThreadsSMRSupport::_bootstrap_list = ThreadsList(0);
75
76 // This is the VM's current "threads list" and it contains all of
77 // the JavaThreads the VM considers to be alive at this moment in
78 // time. The other ThreadsList objects in the VM contain past
79 // snapshots of the "threads list". _java_thread_list is initially
80 // set to _bootstrap_list so that we can detect when we have a very
81 // early use of a ThreadsListHandle.
82 ThreadsList* volatile ThreadsSMRSupport::_java_thread_list = &_bootstrap_list;
83
84 // # of ThreadsLists allocated over VM lifetime.
85 // Impl note: We allocate a new ThreadsList for every thread create and
86 // every thread delete so we need a bigger type than the
87 // _deleted_thread_cnt field.
88 uint64_t ThreadsSMRSupport::_java_thread_list_alloc_cnt = 1;
89
90 // # of ThreadsLists freed over VM lifetime.
91 // Impl note: See _java_thread_list_alloc_cnt note.
92 uint64_t ThreadsSMRSupport::_java_thread_list_free_cnt = 0;
93
94 // Max size ThreadsList allocated.
95 // Impl note: Max # of threads alive at one time should fit in unsigned 32-bit.
96 uint ThreadsSMRSupport::_java_thread_list_max = 0;
97
98 // Max # of nested ThreadsLists for a thread.
99 // Impl note: Hard to imagine > 64K nested ThreadsLists so this could be
100 // 16-bit, but there is no nice 16-bit _FORMAT support.
101 uint ThreadsSMRSupport::_nested_thread_list_max = 0;
102
103 // # of ThreadsListHandles deleted over VM lifetime.
104 // Impl note: Atomically incremented over VM lifetime so use unsigned for
105 // more range. There will be fewer ThreadsListHandles than threads so
106 // unsigned 32-bit should be fine.
107 volatile uint ThreadsSMRSupport::_tlh_cnt = 0;
108
109 // Max time in millis to delete a ThreadsListHandle.
110 // Impl note: 16-bit might be too small on an overloaded machine. Use
111 // unsigned since this is a time value. Set via Atomic::cmpxchg() in a
112 // loop for correctness.
113 volatile uint ThreadsSMRSupport::_tlh_time_max = 0;
114
115 // Cumulative time in millis to delete ThreadsListHandles.
116 // Impl note: Atomically added to over VM lifetime so use unsigned for more
117 // range. Unsigned 64-bit would be more future proof, but 64-bit atomic inc
118 // isn't available everywhere (or is it?).
119 volatile uint ThreadsSMRSupport::_tlh_times = 0;
120
121 ThreadsList* ThreadsSMRSupport::_to_delete_list = NULL;
122
123 // # of parallel ThreadsLists on the to-delete list.
124 // Impl note: Hard to imagine > 64K ThreadsLists needing to be deleted so
125 // this could be 16-bit, but there is no nice 16-bit _FORMAT support.
126 uint ThreadsSMRSupport::_to_delete_list_cnt = 0;
127
128 // Max # of parallel ThreadsLists on the to-delete list.
129 // Impl note: See _to_delete_list_cnt note.
130 uint ThreadsSMRSupport::_to_delete_list_max = 0;
131
132
133 // 'inline' functions first so the definitions are before first use:
134
135 inline void ThreadsSMRSupport::add_deleted_thread_times(uint add_value) {
136 Atomic::add(add_value, &_deleted_thread_times);
137 }
138
139 inline void ThreadsSMRSupport::inc_deleted_thread_cnt() {
140 Atomic::inc(&_deleted_thread_cnt);
141 }
142
143 inline void ThreadsSMRSupport::inc_java_thread_list_alloc_cnt() {
144 _java_thread_list_alloc_cnt++;
145 }
146
147 inline bool ThreadsSMRSupport::is_bootstrap_list(ThreadsList* list) {
148 return list == &_bootstrap_list;
149 }
150
151 inline void ThreadsSMRSupport::update_deleted_thread_time_max(uint new_value) {
152 while (true) {
153 uint cur_value = _deleted_thread_time_max;
154 if (new_value <= cur_value) {
155 // No need to update max value so we're done.
156 break;
157 }
158 if (Atomic::cmpxchg(new_value, &_deleted_thread_time_max, cur_value) == cur_value) {
159 // Updated max value so we're done. Otherwise try it all again.
160 break;
161 }
162 }
163 }
164
165 inline void ThreadsSMRSupport::update_java_thread_list_max(uint new_value) {
166 if (new_value > _java_thread_list_max) {
167 _java_thread_list_max = new_value;
168 }
169 }
170
171 inline ThreadsList* ThreadsSMRSupport::xchg_java_thread_list(ThreadsList* new_list) {
172 return (ThreadsList*)Atomic::xchg(new_list, &_java_thread_list);
173 }
174
175 // Hash table of pointers found by a scan. Used for collecting hazard
176 // pointers (ThreadsList references). Also used for collecting JavaThreads
177 // that are indirectly referenced by hazard ptrs. An instance of this
178 // class only contains one type of pointer.
179 //
180 class ThreadScanHashtable : public CHeapObj<mtThread> {
181 private:
182 static bool ptr_equals(void * const& s1, void * const& s2) {
183 return s1 == s2;
184 }
185
186 static unsigned int ptr_hash(void * const& s1) {
187 // 2654435761 = 2^32 * Phi (golden ratio)
188 return (unsigned int)(((uint32_t)(uintptr_t)s1) * 2654435761u);
189 }
190
191 int _table_size;
192 // ResourceHashtable SIZE is specified at compile time so our
193 // dynamic _table_size is unused for now; 1031 is the first prime
194 // after 1024.
195 typedef ResourceHashtable<void *, int, &ThreadScanHashtable::ptr_hash,
196 &ThreadScanHashtable::ptr_equals, 1031,
197 ResourceObj::C_HEAP, mtThread> PtrTable;
198 PtrTable * _ptrs;
199
200 public:
201 // ResourceHashtable is passed to various functions and populated in
202 // different places so we allocate it using C_HEAP to make it immune
203 // from any ResourceMarks that happen to be in the code paths.
204 ThreadScanHashtable(int table_size) : _table_size(table_size), _ptrs(new (ResourceObj::C_HEAP, mtThread) PtrTable()) {}
205
206 ~ThreadScanHashtable() { delete _ptrs; }
207
208 bool has_entry(void *pointer) {
209 int *val_ptr = _ptrs->get(pointer);
210 return val_ptr != NULL && *val_ptr == 1;
211 }
212
213 void add_entry(void *pointer) {
214 _ptrs->put(pointer, 1);
215 }
216 };
217
218 // Closure to gather JavaThreads indirectly referenced by hazard ptrs
219 // (ThreadsList references) into a hash table. This closure handles part 2
220 // of the dance - adding all the JavaThreads referenced by the hazard
221 // pointer (ThreadsList reference) to the hash table.
222 //
223 class AddThreadHazardPointerThreadClosure : public ThreadClosure {
224 private:
225 ThreadScanHashtable *_table;
226
227 public:
228 AddThreadHazardPointerThreadClosure(ThreadScanHashtable *table) : _table(table) {}
229
230 virtual void do_thread(Thread *thread) {
231 if (!_table->has_entry((void*)thread)) {
232 // The same JavaThread might be on more than one ThreadsList or
233 // more than one thread might be using the same ThreadsList. In
234 // either case, we only need a single entry for a JavaThread.
235 _table->add_entry((void*)thread);
236 }
237 }
238 };
239
240 // Closure to gather JavaThreads indirectly referenced by hazard ptrs
241 // (ThreadsList references) into a hash table. This closure handles part 1
242 // of the dance - hazard ptr chain walking and dispatch to another
243 // closure.
244 //
245 class ScanHazardPtrGatherProtectedThreadsClosure : public ThreadClosure {
246 private:
247 ThreadScanHashtable *_table;
248 public:
249 ScanHazardPtrGatherProtectedThreadsClosure(ThreadScanHashtable *table) : _table(table) {}
250
251 virtual void do_thread(Thread *thread) {
252 assert_locked_or_safepoint(Threads_lock);
253
254 if (thread == NULL) return;
255
256 // This code races with ThreadsSMRSupport::acquire_stable_list() which
257 // is lock-free so we have to handle some special situations.
258 //
259 ThreadsList *current_list = NULL;
260 while (true) {
261 current_list = thread->get_threads_hazard_ptr();
262 // No hazard ptr so nothing more to do.
263 if (current_list == NULL) {
264 return;
265 }
266
267 // If the hazard ptr is verified as stable (since it is not tagged),
268 // then it is safe to use.
269 if (!Thread::is_hazard_ptr_tagged(current_list)) break;
270
271 // The hazard ptr is tagged as not yet verified as being stable
272 // so we are racing with acquire_stable_list(). This exchange
273 // attempts to invalidate the hazard ptr. If we win the race,
274 // then we can ignore this unstable hazard ptr and the other
275 // thread will retry the attempt to publish a stable hazard ptr.
276 // If we lose the race, then we retry our attempt to look at the
277 // hazard ptr.
278 if (thread->cmpxchg_threads_hazard_ptr(NULL, current_list) == current_list) return;
279 }
280
281 // The current JavaThread has a hazard ptr (ThreadsList reference)
282 // which might be _java_thread_list or it might be an older
283 // ThreadsList that has been removed but not freed. In either case,
284 // the hazard ptr is protecting all the JavaThreads on that
285 // ThreadsList.
286 AddThreadHazardPointerThreadClosure add_cl(_table);
287 current_list->threads_do(&add_cl);
288 }
289 };
290
291 // Closure to gather hazard ptrs (ThreadsList references) into a hash table.
292 //
293 class ScanHazardPtrGatherThreadsListClosure : public ThreadClosure {
294 private:
295 ThreadScanHashtable *_table;
296 public:
297 ScanHazardPtrGatherThreadsListClosure(ThreadScanHashtable *table) : _table(table) {}
298
299 virtual void do_thread(Thread* thread) {
300 assert_locked_or_safepoint(Threads_lock);
301
302 if (thread == NULL) return;
303 ThreadsList *threads = thread->get_threads_hazard_ptr();
304 if (threads == NULL) {
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 };
318
319 // Closure to print JavaThreads that have a hazard ptr (ThreadsList
320 // reference) that contains an indirect reference to a specific JavaThread.
321 //
322 class ScanHazardPtrPrintMatchingThreadsClosure : public ThreadClosure {
323 private:
324 JavaThread *_thread;
325 public:
326 ScanHazardPtrPrintMatchingThreadsClosure(JavaThread *thread) : _thread(thread) {}
327
328 virtual void do_thread(Thread *thread) {
329 assert_locked_or_safepoint(Threads_lock);
330
331 if (thread == NULL) return;
332 ThreadsList *current_list = thread->get_threads_hazard_ptr();
333 if (current_list == NULL) {
334 return;
335 }
336 // If the hazard ptr is unverified, then ignore it.
337 if (Thread::is_hazard_ptr_tagged(current_list)) return;
338
339 // The current JavaThread has a hazard ptr (ThreadsList reference)
340 // which might be _java_thread_list or it might be an older
341 // ThreadsList that has been removed but not freed. In either case,
342 // the hazard ptr is protecting all the JavaThreads on that
343 // ThreadsList, but we only care about matching a specific JavaThread.
344 JavaThreadIterator jti(current_list);
345 for (JavaThread *p = jti.first(); p != NULL; p = jti.next()) {
346 if (p == _thread) {
347 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));
348 break;
349 }
350 }
351 }
352 };
353
354 // Closure to determine if the specified JavaThread is found by
355 // threads_do().
356 //
357 class VerifyHazardPtrThreadClosure : public ThreadClosure {
358 private:
359 bool _found;
360 Thread *_self;
361
362 public:
363 VerifyHazardPtrThreadClosure(Thread *self) : _found(false), _self(self) {}
364
365 bool found() const { return _found; }
366
367 virtual void do_thread(Thread *thread) {
368 if (thread == _self) {
369 _found = true;
370 }
371 }
372 };
373
374
375 // Acquire a stable ThreadsList.
376 //
377 void SafeThreadsListPtr::acquire_stable_list() {
378 assert(_thread != NULL, "sanity check");
379 _needs_release = true;
380 _previous = _thread->_threads_list_ptr;
381 _thread->_threads_list_ptr = this;
382
383 if (_thread->get_threads_hazard_ptr() == NULL) {
384 // The typical case is first.
385 acquire_stable_list_fast_path();
386 return;
387 }
388
389 // The nested case is rare.
390 acquire_stable_list_nested_path();
391 }
392
393 // Fast path way to acquire a stable ThreadsList.
394 //
395 void SafeThreadsListPtr::acquire_stable_list_fast_path() {
396 assert(_thread != NULL, "sanity check");
397 assert(_thread->get_threads_hazard_ptr() == NULL, "sanity check");
398
399 ThreadsList* threads;
400
401 // Stable recording of a hazard ptr for SMR. This code does not use
402 // locks so its use of the _smr_java_thread_list & _threads_hazard_ptr
403 // fields is racy relative to code that uses those fields with locks.
404 // OrderAccess and Atomic functions are used to deal with those races.
405 //
406 while (true) {
407 threads = ThreadsSMRSupport::get_java_thread_list();
408
409 // Publish a tagged hazard ptr to denote that the hazard ptr is not
410 // yet verified as being stable. Due to the fence after the hazard
411 // ptr write, it will be sequentially consistent w.r.t. the
412 // sequentially consistent writes of the ThreadsList, even on
413 // non-multiple copy atomic machines where stores can be observed
414 // in different order from different observer threads.
415 ThreadsList* unverified_threads = Thread::tag_hazard_ptr(threads);
416 _thread->set_threads_hazard_ptr(unverified_threads);
417
418 // If _smr_java_thread_list has changed, we have lost a race with
419 // Threads::add() or Threads::remove() and have to try again.
420 if (ThreadsSMRSupport::get_java_thread_list() != threads) {
421 continue;
422 }
423
424 // We try to remove the tag which will verify the hazard ptr as
425 // being stable. This exchange can race with a scanning thread
426 // which might invalidate the tagged hazard ptr to keep it from
427 // being followed to access JavaThread ptrs. If we lose the race,
428 // we simply retry. If we win the race, then the stable hazard
429 // ptr is officially published.
430 if (_thread->cmpxchg_threads_hazard_ptr(threads, unverified_threads) == unverified_threads) {
431 break;
432 }
433 }
434
435 // A stable hazard ptr has been published letting other threads know
436 // that the ThreadsList and the JavaThreads reachable from this list
437 // are protected and hence they should not be deleted until everyone
438 // agrees it is safe to do so.
439
440 _list = threads;
441
442 verify_hazard_ptr_scanned();
443 }
444
445 // Acquire a nested stable ThreadsList; this is rare so it uses
446 // reference counting.
447 //
448 void SafeThreadsListPtr::acquire_stable_list_nested_path() {
449 assert(_thread != NULL, "sanity check");
450 assert(_thread->get_threads_hazard_ptr() != NULL,
451 "cannot have a NULL regular hazard ptr when acquiring a nested hazard ptr");
452
453 // The thread already has a hazard ptr (ThreadsList ref) so we need
454 // to create a nested ThreadsListHandle with the current ThreadsList
455 // since it might be different than our current hazard ptr. To remedy
456 // the situation, the ThreadsList pointed to by the pre-existing
457 // stable hazard ptr is reference counted before the hazard ptr may
458 // be released and moved to a new ThreadsList. The old ThreadsList
459 // is remembered in the ThreadsListHandle.
460
461 ThreadsList* current_list = _previous->_list;
462 if (EnableThreadSMRStatistics) {
463 _thread->inc_nested_threads_hazard_ptr_cnt();
464 }
465 current_list->inc_nested_handle_cnt();
466 _previous->_has_ref_count = true; // promote SafeThreadsListPtr to be reference counted
467 _thread->_threads_hazard_ptr = NULL; // clear the hazard ptr so we can go through the fast path below
468
469 if (EnableThreadSMRStatistics && _thread->nested_threads_hazard_ptr_cnt() > ThreadsSMRSupport::_nested_thread_list_max) {
470 ThreadsSMRSupport::_nested_thread_list_max = _thread->nested_threads_hazard_ptr_cnt();
471 }
472
473 acquire_stable_list_fast_path();
474
475 verify_hazard_ptr_scanned();
476
477 log_debug(thread, smr)("tid=" UINTX_FORMAT ": SafeThreadsListPtr::acquire_stable_list: add nested list pointer to ThreadsList=" INTPTR_FORMAT, os::current_thread_id(), p2i(_list));
478 }
479
480 // Release a stable ThreadsList.
481 //
482 void SafeThreadsListPtr::release_stable_list() {
483 assert(_thread != NULL, "sanity check");
484 assert(_thread->_threads_list_ptr == this, "sanity check");
485 _thread->_threads_list_ptr = _previous;
486
487 if (_has_ref_count) {
488 // If a SafeThreadsListPtr has been promoted to use reference counting
489 // due to nesting of ThreadsListHandles, then the reference count must be
490 // decremented, at which point it may be freed. The forgotten value of
491 // the list no longer matters at this point and should already be NULL.
492 assert(_thread->get_threads_hazard_ptr() == NULL, "sanity check");
493 if (EnableThreadSMRStatistics) {
494 _thread->dec_nested_threads_hazard_ptr_cnt();
495 }
496 _list->dec_nested_handle_cnt();
497
498 log_debug(thread, smr)("tid=" UINTX_FORMAT ": SafeThreadsListPtr::release_stable_list: delete nested list pointer to ThreadsList=" INTPTR_FORMAT, os::current_thread_id(), p2i(_list));
499 } else {
500 // The normal case: a leaf ThreadsListHandle. This merely requires setting
501 // the thread hazard ptr back to NULL.
502 assert(_thread->get_threads_hazard_ptr() != NULL, "sanity check");
503 _thread->set_threads_hazard_ptr(NULL);
504 }
505
506 // After releasing the hazard ptr, other threads may go ahead and
507 // free up some memory temporarily used by a ThreadsList snapshot.
508
509 // We use double-check locking to reduce traffic on the system
510 // wide Thread-SMR delete_lock.
511 if (ThreadsSMRSupport::delete_notify()) {
512 // An exiting thread might be waiting in smr_delete(); we need to
513 // check with delete_lock to be sure.
514 ThreadsSMRSupport::release_stable_list_wake_up(_has_ref_count);
515 }
516 }
517
518 // Verify that the stable hazard ptr used to safely keep threads
519 // alive is scanned by threads_do() which is a key piece of honoring
520 // the Thread-SMR protocol.
521 void SafeThreadsListPtr::verify_hazard_ptr_scanned() {
522 #ifdef ASSERT
523 assert(_list != NULL, "_list must not be NULL");
524
525 if (ThreadsSMRSupport::is_bootstrap_list(_list)) {
526 // We are early in VM bootstrapping so nothing to do here.
527 return;
528 }
529
530 // The closure will attempt to verify that the calling thread can
531 // be found by threads_do() on the specified ThreadsList. If it
532 // is successful, then the specified ThreadsList was acquired as
533 // a stable hazard ptr by the calling thread in a way that honored
534 // the Thread-SMR protocol.
535 //
536 // If the calling thread cannot be found by threads_do() and if
537 // it is not the shutdown thread, then the calling thread is not
538 // honoring the Thread-SMR ptotocol. This means that the specified
539 // ThreadsList is not a stable hazard ptr and can be freed by
540 // another thread from the to-be-deleted list at any time.
541 //
542 // Note: The shutdown thread has removed itself from the Threads
543 // list and is safe to have a waiver from this check because
544 // VM_Exit::_shutdown_thread is not set until after the VMThread
545 // has started the final safepoint which holds the Threads_lock
546 // for the remainder of the VM's life.
547 //
548 VerifyHazardPtrThreadClosure cl(_thread);
549 ThreadsSMRSupport::threads_do(&cl, _list);
550
551 // If the calling thread is not honoring the Thread-SMR protocol,
552 // then we will either crash in threads_do() above because 'threads'
553 // was freed by another thread or we will fail the assert() below.
554 // In either case, we won't get past this point with a badly placed
555 // ThreadsListHandle.
556
557 assert(cl.found() || _thread == VM_Exit::shutdown_thread(), "Acquired a ThreadsList snapshot from a thread not recognized by the Thread-SMR protocol.");
558 #endif
559 }
560
561 // 'entries + 1' so we always have at least one entry.
562 ThreadsList::ThreadsList(int entries) :
563 _length(entries),
564 _next_list(NULL),
565 _threads(NEW_C_HEAP_ARRAY(JavaThread*, entries + 1, mtThread)),
566 _nested_handle_cnt(0)
567 {
568 *(JavaThread**)(_threads + entries) = NULL; // Make sure the extra entry is NULL.
569 }
570
571 ThreadsList::~ThreadsList() {
572 FREE_C_HEAP_ARRAY(JavaThread*, _threads);
573 }
574
575 // Add a JavaThread to a ThreadsList. The returned ThreadsList is a
576 // new copy of the specified ThreadsList with the specified JavaThread
577 // appended to the end.
578 ThreadsList *ThreadsList::add_thread(ThreadsList *list, JavaThread *java_thread) {
579 const uint index = list->_length;
580 const uint new_length = index + 1;
581 const uint head_length = index;
582 ThreadsList *const new_list = new ThreadsList(new_length);
583
584 if (head_length > 0) {
585 Copy::disjoint_words((HeapWord*)list->_threads, (HeapWord*)new_list->_threads, head_length);
586 }
587 *(JavaThread**)(new_list->_threads + index) = java_thread;
588
589 return new_list;
590 }
591
592 void ThreadsList::dec_nested_handle_cnt() {
593 // The decrement only needs to be MO_ACQ_REL since the reference
594 // counter is volatile (and the hazard ptr is already NULL).
595 Atomic::dec(&_nested_handle_cnt);
596 }
597
598 int ThreadsList::find_index_of_JavaThread(JavaThread *target) {
599 if (target == NULL) {
600 return -1;
601 }
602 for (uint i = 0; i < length(); i++) {
603 if (target == thread_at(i)) {
604 return (int)i;
605 }
606 }
607 return -1;
608 }
609
610 JavaThread* ThreadsList::find_JavaThread_from_java_tid(jlong java_tid) const {
611 for (uint i = 0; i < length(); i++) {
612 JavaThread* thread = thread_at(i);
613 oop tobj = thread->threadObj();
614 // Ignore the thread if it hasn't run yet, has exited
615 // or is starting to exit.
616 if (tobj != NULL && !thread->is_exiting() &&
617 java_tid == java_lang_Thread::thread_id(tobj)) {
618 // found a match
619 return thread;
620 }
621 }
622 return NULL;
623 }
624
625 void ThreadsList::inc_nested_handle_cnt() {
626 // The increment needs to be MO_SEQ_CST so that the reference counter
627 // update is seen before the subsequent hazard ptr update.
628 Atomic::inc(&_nested_handle_cnt);
629 }
630
631 bool ThreadsList::includes(const JavaThread * const p) const {
632 if (p == NULL) {
633 return false;
634 }
635 for (uint i = 0; i < length(); i++) {
636 if (thread_at(i) == p) {
637 return true;
638 }
639 }
640 return false;
641 }
642
643 // Remove a JavaThread from a ThreadsList. The returned ThreadsList is a
644 // new copy of the specified ThreadsList with the specified JavaThread
645 // removed.
646 ThreadsList *ThreadsList::remove_thread(ThreadsList* list, JavaThread* java_thread) {
647 assert(list->_length > 0, "sanity");
648
649 uint i = (uint)list->find_index_of_JavaThread(java_thread);
650 assert(i < list->_length, "did not find JavaThread on the list");
651 const uint index = i;
652 const uint new_length = list->_length - 1;
653 const uint head_length = index;
654 const uint tail_length = (new_length >= index) ? (new_length - index) : 0;
655 ThreadsList *const new_list = new ThreadsList(new_length);
656
657 if (head_length > 0) {
658 Copy::disjoint_words((HeapWord*)list->_threads, (HeapWord*)new_list->_threads, head_length);
659 }
660 if (tail_length > 0) {
661 Copy::disjoint_words((HeapWord*)list->_threads + index + 1, (HeapWord*)new_list->_threads + index, tail_length);
662 }
663
664 return new_list;
665 }
666
667 ThreadsListHandle::ThreadsListHandle(Thread *self) : _list_ptr(self, /* acquire */ true) {
668 assert(self == Thread::current(), "sanity check");
669 if (EnableThreadSMRStatistics) {
670 _timer.start();
671 }
672 }
673
674 ThreadsListHandle::~ThreadsListHandle() {
675 if (EnableThreadSMRStatistics) {
676 _timer.stop();
677 uint millis = (uint)_timer.milliseconds();
678 ThreadsSMRSupport::update_tlh_stats(millis);
679 }
680 }
681
682 // Convert an internal thread reference to a JavaThread found on the
683 // associated ThreadsList. This ThreadsListHandle "protects" the
684 // returned JavaThread *.
685 //
686 // If thread_oop_p is not NULL, then the caller wants to use the oop
687 // after this call so the oop is returned. On success, *jt_pp is set
688 // to the converted JavaThread * and true is returned. On error,
689 // returns false.
690 //
691 bool ThreadsListHandle::cv_internal_thread_to_JavaThread(jobject jthread,
692 JavaThread ** jt_pp,
693 oop * thread_oop_p) {
694 assert(this->list() != NULL, "must have a ThreadsList");
695 assert(jt_pp != NULL, "must have a return JavaThread pointer");
696 // thread_oop_p is optional so no assert()
697
698 // The JVM_* interfaces don't allow a NULL thread parameter; JVM/TI
699 // allows a NULL thread parameter to signify "current thread" which
700 // allows us to avoid calling cv_external_thread_to_JavaThread().
701 // The JVM_* interfaces have no such leeway.
702
703 oop thread_oop = JNIHandles::resolve_non_null(jthread);
704 // Looks like an oop at this point.
705 if (thread_oop_p != NULL) {
706 // Return the oop to the caller; the caller may still want
707 // the oop even if this function returns false.
708 *thread_oop_p = thread_oop;
709 }
710
711 JavaThread *java_thread = java_lang_Thread::thread(thread_oop);
712 if (java_thread == NULL) {
713 // The java.lang.Thread does not contain a JavaThread * so it has
714 // not yet run or it has died.
715 return false;
716 }
717 // Looks like a live JavaThread at this point.
718
719 if (java_thread != JavaThread::current()) {
720 // jthread is not for the current JavaThread so have to verify
721 // the JavaThread * against the ThreadsList.
722 if (EnableThreadSMRExtraValidityChecks && !includes(java_thread)) {
723 // Not on the JavaThreads list so it is not alive.
724 return false;
725 }
726 }
727
728 // Return a live JavaThread that is "protected" by the
729 // ThreadsListHandle in the caller.
730 *jt_pp = java_thread;
731 return true;
732 }
733
734 void ThreadsSMRSupport::add_thread(JavaThread *thread){
735 ThreadsList *new_list = ThreadsList::add_thread(get_java_thread_list(), thread);
736 if (EnableThreadSMRStatistics) {
737 inc_java_thread_list_alloc_cnt();
738 update_java_thread_list_max(new_list->length());
739 }
740 // Initial _java_thread_list will not generate a "Threads::add" mesg.
741 log_debug(thread, smr)("tid=" UINTX_FORMAT ": Threads::add: new ThreadsList=" INTPTR_FORMAT, os::current_thread_id(), p2i(new_list));
742
743 ThreadsList *old_list = xchg_java_thread_list(new_list);
744 free_list(old_list);
745 }
746
747 // set_delete_notify() and clear_delete_notify() are called
748 // under the protection of the delete_lock, but we also use an
749 // Atomic operation to ensure the memory update is seen earlier than
750 // when the delete_lock is dropped.
751 //
752 void ThreadsSMRSupport::clear_delete_notify() {
753 Atomic::dec(&_delete_notify);
754 }
755
756 bool ThreadsSMRSupport::delete_notify() {
757 // Use load_acquire() in order to see any updates to _delete_notify
758 // earlier than when delete_lock is grabbed.
759 return (OrderAccess::load_acquire(&_delete_notify) != 0);
760 }
761
762 // Safely free a ThreadsList after a Threads::add() or Threads::remove().
763 // The specified ThreadsList may not get deleted during this call if it
764 // is still in-use (referenced by a hazard ptr). Other ThreadsLists
765 // in the chain may get deleted by this call if they are no longer in-use.
766 void ThreadsSMRSupport::free_list(ThreadsList* threads) {
767 assert_locked_or_safepoint(Threads_lock);
768
769 if (is_bootstrap_list(threads)) {
770 // The bootstrap list cannot be freed and is empty so
771 // it does not need to be scanned. Nothing to do here.
772 log_debug(thread, smr)("tid=" UINTX_FORMAT ": ThreadsSMRSupport::free_list: bootstrap ThreadsList=" INTPTR_FORMAT " is no longer in use.", os::current_thread_id(), p2i(threads));
773 return;
774 }
775
776 threads->set_next_list(_to_delete_list);
777 _to_delete_list = threads;
778 if (EnableThreadSMRStatistics) {
779 _to_delete_list_cnt++;
780 if (_to_delete_list_cnt > _to_delete_list_max) {
781 _to_delete_list_max = _to_delete_list_cnt;
782 }
783 }
784
785 // Hash table size should be first power of two higher than twice the length of the ThreadsList
786 int hash_table_size = MIN2((int)get_java_thread_list()->length(), 32) << 1;
787 hash_table_size--;
788 hash_table_size |= hash_table_size >> 1;
789 hash_table_size |= hash_table_size >> 2;
790 hash_table_size |= hash_table_size >> 4;
791 hash_table_size |= hash_table_size >> 8;
792 hash_table_size |= hash_table_size >> 16;
793 hash_table_size++;
794
795 // Gather a hash table of the current hazard ptrs:
796 ThreadScanHashtable *scan_table = new ThreadScanHashtable(hash_table_size);
797 ScanHazardPtrGatherThreadsListClosure scan_cl(scan_table);
798 threads_do(&scan_cl);
799 OrderAccess::acquire(); // Must order reads of hazard ptr before reads of
800 // nested reference counters
801
802 // Walk through the linked list of pending freeable ThreadsLists
803 // and free the ones that are not referenced from hazard ptrs.
804 ThreadsList* current = _to_delete_list;
805 ThreadsList* prev = NULL;
806 ThreadsList* next = NULL;
807 bool threads_is_freed = false;
808 while (current != NULL) {
809 next = current->next_list();
810 if (!scan_table->has_entry((void*)current) && current->_nested_handle_cnt == 0) {
811 // This ThreadsList is not referenced by a hazard ptr.
812 if (prev != NULL) {
813 prev->set_next_list(next);
814 }
815 if (_to_delete_list == current) {
816 _to_delete_list = next;
817 }
818
819 log_debug(thread, smr)("tid=" UINTX_FORMAT ": ThreadsSMRSupport::free_list: threads=" INTPTR_FORMAT " is freed.", os::current_thread_id(), p2i(current));
820 if (current == threads) threads_is_freed = true;
821 delete current;
822 if (EnableThreadSMRStatistics) {
823 _java_thread_list_free_cnt++;
824 _to_delete_list_cnt--;
825 }
826 } else {
827 prev = current;
828 }
829 current = next;
830 }
831
832 if (!threads_is_freed) {
833 // Only report "is not freed" on the original call to
834 // free_list() for this ThreadsList.
835 log_debug(thread, smr)("tid=" UINTX_FORMAT ": ThreadsSMRSupport::free_list: threads=" INTPTR_FORMAT " is not freed.", os::current_thread_id(), p2i(threads));
836 }
837
838 delete scan_table;
839 }
840
841 // Return true if the specified JavaThread is protected by a hazard
842 // pointer (ThreadsList reference). Otherwise, returns false.
843 //
844 bool ThreadsSMRSupport::is_a_protected_JavaThread(JavaThread *thread) {
845 assert_locked_or_safepoint(Threads_lock);
846
847 // Hash table size should be first power of two higher than twice
848 // the length of the Threads list.
849 int hash_table_size = MIN2((int)get_java_thread_list()->length(), 32) << 1;
850 hash_table_size--;
851 hash_table_size |= hash_table_size >> 1;
852 hash_table_size |= hash_table_size >> 2;
853 hash_table_size |= hash_table_size >> 4;
854 hash_table_size |= hash_table_size >> 8;
855 hash_table_size |= hash_table_size >> 16;
856 hash_table_size++;
857
858 // Gather a hash table of the JavaThreads indirectly referenced by
859 // hazard ptrs.
860 ThreadScanHashtable *scan_table = new ThreadScanHashtable(hash_table_size);
861 ScanHazardPtrGatherProtectedThreadsClosure scan_cl(scan_table);
862 threads_do(&scan_cl);
863 OrderAccess::acquire(); // Must order reads of hazard ptr before reads of
864 // nested reference counters
865
866 // Walk through the linked list of pending freeable ThreadsLists
867 // and include the ones that are currently in use by a nested
868 // ThreadsListHandle in the search set.
869 ThreadsList* current = _to_delete_list;
870 while (current != NULL) {
871 if (current->_nested_handle_cnt != 0) {
872 // 'current' is in use by a nested ThreadsListHandle so the hazard
873 // ptr is protecting all the JavaThreads on that ThreadsList.
874 AddThreadHazardPointerThreadClosure add_cl(scan_table);
875 current->threads_do(&add_cl);
876 }
877 current = current->next_list();
878 }
879
880 bool thread_is_protected = false;
881 if (scan_table->has_entry((void*)thread)) {
882 thread_is_protected = true;
883 }
884 delete scan_table;
885 return thread_is_protected;
886 }
887
888 // Wake up portion of the release stable ThreadsList protocol;
889 // uses the delete_lock().
890 //
891 void ThreadsSMRSupport::release_stable_list_wake_up(bool is_nested) {
892 const char* log_str = is_nested ? "nested hazard ptr" : "regular hazard ptr";
893
894 // Note: delete_lock is held in smr_delete() for the entire
895 // hazard ptr search so that we do not lose this notify() if
896 // the exiting thread has to wait. That code path also holds
897 // Threads_lock (which was grabbed before delete_lock) so that
898 // threads_do() can be called. This means the system can't start a
899 // safepoint which means this thread can't take too long to get to
900 // a safepoint because of being blocked on delete_lock.
901 //
902 MonitorLocker ml(ThreadsSMRSupport::delete_lock(), Monitor::_no_safepoint_check_flag);
903 if (ThreadsSMRSupport::delete_notify()) {
904 // Notify any exiting JavaThreads that are waiting in smr_delete()
905 // that we've released a ThreadsList.
906 ml.notify_all();
907 log_debug(thread, smr)("tid=" UINTX_FORMAT ": ThreadsSMRSupport::release_stable_list notified %s", os::current_thread_id(), log_str);
908 }
909 }
910
911 void ThreadsSMRSupport::remove_thread(JavaThread *thread) {
912 ThreadsList *new_list = ThreadsList::remove_thread(ThreadsSMRSupport::get_java_thread_list(), thread);
913 if (EnableThreadSMRStatistics) {
914 ThreadsSMRSupport::inc_java_thread_list_alloc_cnt();
915 // This list is smaller so no need to check for a "longest" update.
916 }
917
918 // Final _java_thread_list will not generate a "Threads::remove" mesg.
919 log_debug(thread, smr)("tid=" UINTX_FORMAT ": Threads::remove: new ThreadsList=" INTPTR_FORMAT, os::current_thread_id(), p2i(new_list));
920
921 ThreadsList *old_list = ThreadsSMRSupport::xchg_java_thread_list(new_list);
922 ThreadsSMRSupport::free_list(old_list);
923 }
924
925 // See note for clear_delete_notify().
926 //
927 void ThreadsSMRSupport::set_delete_notify() {
928 Atomic::inc(&_delete_notify);
929 }
930
931 // Safely delete a JavaThread when it is no longer in use by a
932 // ThreadsListHandle.
933 //
934 void ThreadsSMRSupport::smr_delete(JavaThread *thread) {
935 assert(!Threads_lock->owned_by_self(), "sanity");
936
937 bool has_logged_once = false;
938 elapsedTimer timer;
939 if (EnableThreadSMRStatistics) {
940 timer.start();
941 }
942
943 while (true) {
944 {
945 // No safepoint check because this JavaThread is not on the
946 // Threads list.
947 MutexLocker ml(Threads_lock, Mutex::_no_safepoint_check_flag);
948 // Cannot use a MonitorLocker helper here because we have
949 // to drop the Threads_lock first if we wait.
950 ThreadsSMRSupport::delete_lock()->lock_without_safepoint_check();
951 // Set the delete_notify flag after we grab delete_lock
952 // and before we scan hazard ptrs because we're doing
953 // double-check locking in release_stable_list().
954 ThreadsSMRSupport::set_delete_notify();
955
956 if (!is_a_protected_JavaThread(thread)) {
957 // This is the common case.
958 ThreadsSMRSupport::clear_delete_notify();
959 ThreadsSMRSupport::delete_lock()->unlock();
960 break;
961 }
962 if (!has_logged_once) {
963 has_logged_once = true;
964 log_debug(thread, smr)("tid=" UINTX_FORMAT ": ThreadsSMRSupport::smr_delete: thread=" INTPTR_FORMAT " is not deleted.", os::current_thread_id(), p2i(thread));
965 if (log_is_enabled(Debug, os, thread)) {
966 ScanHazardPtrPrintMatchingThreadsClosure scan_cl(thread);
967 threads_do(&scan_cl);
968 ThreadsList* current = _to_delete_list;
969 while (current != NULL) {
970 if (current->_nested_handle_cnt != 0 && current->includes(thread)) {
971 log_debug(thread, smr)("tid=" UINTX_FORMAT ": ThreadsSMRSupport::smr_delete: found nested hazard pointer to thread=" INTPTR_FORMAT, os::current_thread_id(), p2i(thread));
972 }
973 current = current->next_list();
974 }
975 }
976 }
977 } // We have to drop the Threads_lock to wait or delete the thread
978
979 if (EnableThreadSMRStatistics) {
980 _delete_lock_wait_cnt++;
981 if (_delete_lock_wait_cnt > _delete_lock_wait_max) {
982 _delete_lock_wait_max = _delete_lock_wait_cnt;
983 }
984 }
985 // Wait for a release_stable_list() call before we check again. No
986 // safepoint check, no timeout, and not as suspend equivalent flag
987 // because this JavaThread is not on the Threads list.
988 ThreadsSMRSupport::delete_lock()->wait_without_safepoint_check();
989 if (EnableThreadSMRStatistics) {
990 _delete_lock_wait_cnt--;
991 }
992
993 ThreadsSMRSupport::clear_delete_notify();
994 ThreadsSMRSupport::delete_lock()->unlock();
995 // Retry the whole scenario.
996 }
997
998 delete thread;
999 if (EnableThreadSMRStatistics) {
1000 timer.stop();
1001 uint millis = (uint)timer.milliseconds();
1002 ThreadsSMRSupport::inc_deleted_thread_cnt();
1003 ThreadsSMRSupport::add_deleted_thread_times(millis);
1004 ThreadsSMRSupport::update_deleted_thread_time_max(millis);
1005 }
1006
1007 log_debug(thread, smr)("tid=" UINTX_FORMAT ": ThreadsSMRSupport::smr_delete: thread=" INTPTR_FORMAT " is deleted.", os::current_thread_id(), p2i(thread));
1008 }
1009
1010 // Apply the closure to all threads in the system, with a snapshot of
1011 // all JavaThreads provided by the list parameter.
1012 void ThreadsSMRSupport::threads_do(ThreadClosure *tc, ThreadsList *list) {
1013 list->threads_do(tc);
1014 Threads::non_java_threads_do(tc);
1015 }
1016
1017 // Apply the closure to all threads in the system.
1018 void ThreadsSMRSupport::threads_do(ThreadClosure *tc) {
1019 threads_do(tc, _java_thread_list);
1020 }
1021
1022
1023 // Debug, logging, and printing stuff at the end:
1024
1025 // Print SMR info for a SafeThreadsListPtr to a given output stream.
1026 void SafeThreadsListPtr::print_on(outputStream* st) {
1027 if (this == _thread->_threads_list_ptr) {
1028 // The top level hazard ptr.
1029 st->print(" _threads_hazard_ptr=" INTPTR_FORMAT, p2i(_list));
1030 } else {
1031 // Nested hazard ptrs.
1032 st->print(", _nested_threads_hazard_ptr=" INTPTR_FORMAT, p2i(_list));
1033 }
1034 }
1035
1036 // Log Threads class SMR info.
1037 void ThreadsSMRSupport::log_statistics() {
1038 LogTarget(Info, thread, smr) log;
1039 if (log.is_enabled()) {
1040 LogStream out(log);
1041 print_info_on(&out);
1042 }
1043 }
1044
1045 // Print SMR info for a thread to a given output stream.
1046 void ThreadsSMRSupport::print_info_on(const Thread* thread, outputStream* st) {
1047 if (thread->_threads_hazard_ptr != NULL) {
1048 st->print(" _threads_hazard_ptr=" INTPTR_FORMAT, p2i(thread->_threads_hazard_ptr));
1049 }
1050 if (EnableThreadSMRStatistics && thread->_threads_list_ptr != NULL) {
1051 // The count is only interesting if we have a _threads_list_ptr.
1052 st->print(", _nested_threads_hazard_ptr_cnt=%u", thread->_nested_threads_hazard_ptr_cnt);
1053 }
1054 if (SafepointSynchronize::is_at_safepoint() || Thread::current() == thread) {
1055 // It is only safe to walk the list if we're at a safepoint or the
1056 // calling thread is walking its own list.
1057 SafeThreadsListPtr* current = thread->_threads_list_ptr;
1058 if (current != NULL) {
1059 // Skip the top nesting level as it is always printed above.
1060 current = current->previous();
1061 }
1062 while (current != NULL) {
1063 current->print_on(st);
1064 current = current->previous();
1065 }
1066 }
1067 }
1068
1069 // Print Threads class SMR info.
1070 void ThreadsSMRSupport::print_info_on(outputStream* st) {
1071 // Only grab the Threads_lock if we don't already own it and if we
1072 // are not reporting an error.
1073 // Note: Not grabbing the Threads_lock during error reporting is
1074 // dangerous because the data structures we want to print can be
1075 // freed concurrently. However, grabbing the Threads_lock during
1076 // error reporting can be equally dangerous since this thread might
1077 // block during error reporting or a nested error could leave the
1078 // Threads_lock held. The classic no win scenario.
1079 //
1080 MutexLocker ml((Threads_lock->owned_by_self() || VMError::is_error_reported()) ? NULL : Threads_lock);
1081
1082 st->print_cr("Threads class SMR info:");
1083 st->print_cr("_java_thread_list=" INTPTR_FORMAT ", length=%u, "
1084 "elements={", p2i(_java_thread_list),
1085 _java_thread_list->length());
1086 print_info_elements_on(st, _java_thread_list);
1087 st->print_cr("}");
1088 if (_to_delete_list != NULL) {
1089 st->print_cr("_to_delete_list=" INTPTR_FORMAT ", length=%u, "
1090 "elements={", p2i(_to_delete_list),
1091 _to_delete_list->length());
1092 print_info_elements_on(st, _to_delete_list);
1093 st->print_cr("}");
1094 for (ThreadsList *t_list = _to_delete_list->next_list();
1095 t_list != NULL; t_list = t_list->next_list()) {
1096 st->print("next-> " INTPTR_FORMAT ", length=%u, "
1097 "elements={", p2i(t_list), t_list->length());
1098 print_info_elements_on(st, t_list);
1099 st->print_cr("}");
1100 }
1101 }
1102 if (!EnableThreadSMRStatistics) {
1103 return;
1104 }
1105 st->print_cr("_java_thread_list_alloc_cnt=" UINT64_FORMAT ", "
1106 "_java_thread_list_free_cnt=" UINT64_FORMAT ", "
1107 "_java_thread_list_max=%u, "
1108 "_nested_thread_list_max=%u",
1109 _java_thread_list_alloc_cnt,
1110 _java_thread_list_free_cnt,
1111 _java_thread_list_max,
1112 _nested_thread_list_max);
1113 if (_tlh_cnt > 0) {
1114 st->print_cr("_tlh_cnt=%u"
1115 ", _tlh_times=%u"
1116 ", avg_tlh_time=%0.2f"
1117 ", _tlh_time_max=%u",
1118 _tlh_cnt, _tlh_times,
1119 ((double) _tlh_times / _tlh_cnt),
1120 _tlh_time_max);
1121 }
1122 if (_deleted_thread_cnt > 0) {
1123 st->print_cr("_deleted_thread_cnt=%u"
1124 ", _deleted_thread_times=%u"
1125 ", avg_deleted_thread_time=%0.2f"
1126 ", _deleted_thread_time_max=%u",
1127 _deleted_thread_cnt, _deleted_thread_times,
1128 ((double) _deleted_thread_times / _deleted_thread_cnt),
1129 _deleted_thread_time_max);
1130 }
1131 st->print_cr("_delete_lock_wait_cnt=%u, _delete_lock_wait_max=%u",
1132 _delete_lock_wait_cnt, _delete_lock_wait_max);
1133 st->print_cr("_to_delete_list_cnt=%u, _to_delete_list_max=%u",
1134 _to_delete_list_cnt, _to_delete_list_max);
1135 }
1136
1137 // Print ThreadsList elements (4 per line).
1138 void ThreadsSMRSupport::print_info_elements_on(outputStream* st, ThreadsList* t_list) {
1139 uint cnt = 0;
1140 JavaThreadIterator jti(t_list);
1141 for (JavaThread *jt = jti.first(); jt != NULL; jt = jti.next()) {
1142 st->print(INTPTR_FORMAT, p2i(jt));
1143 if (cnt < t_list->length() - 1) {
1144 // Separate with comma or comma-space except for the last one.
1145 if (((cnt + 1) % 4) == 0) {
1146 // Four INTPTR_FORMAT fit on an 80 column line so end the
1147 // current line with just a comma.
1148 st->print_cr(",");
1149 } else {
1150 // Not the last one on the current line so use comma-space:
1151 st->print(", ");
1152 }
1153 } else {
1154 // Last one so just end the current line.
1155 st->cr();
1156 }
1157 cnt++;
1158 }
1159 }