1 /*
  2  * Copyright (c) 2003, 2021, 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 "classfile/classLoaderData.inline.hpp"
 27 #include "classfile/placeholders.hpp"
 28 #include "logging/log.hpp"
 29 #include "logging/logTag.hpp"
 30 #include "logging/logStream.hpp"
 31 #include "memory/resourceArea.hpp"
 32 #include "runtime/mutexLocker.hpp"
 33 #include "runtime/thread.hpp"
 34 #include "utilities/hashtable.inline.hpp"
 35 
 36 // SeenThread objects represent list of threads that are
 37 // currently performing a load action on a class.
 38 // For class circularity, set before loading a superclass.
 39 // For bootclasssearchpath, set before calling load_instance_class.
 40 // Defining must be single threaded on a class/classloader basis
 41 // For DEFINE_CLASS, the head of the queue owns the
 42 // define token and the rest of the threads wait to return the
 43 // result the first thread gets.
 44 class SeenThread: public CHeapObj<mtInternal> {
 45 private:
 46    JavaThread* _thread;
 47    SeenThread* _stnext;
 48    SeenThread* _stprev;
 49 public:
 50    SeenThread(JavaThread* thread) {
 51        _thread = thread;
 52        _stnext = NULL;
 53        _stprev = NULL;
 54    }
 55    JavaThread* thread()          const { return _thread;}
 56    void set_thread(JavaThread* thread) { _thread = thread; }
 57 
 58    SeenThread* next()        const { return _stnext;}
 59    void set_next(SeenThread* seen) { _stnext = seen; }
 60    void set_prev(SeenThread* seen) { _stprev = seen; }
 61 
 62   void print_action_queue(outputStream* st) {
 63     SeenThread* seen = this;
 64     while (seen != NULL) {
 65       seen->thread()->print_value_on(st);
 66       st->print(", ");
 67       seen = seen->next();
 68     }
 69   }
 70 };
 71 
 72 SeenThread* PlaceholderEntry::actionToQueue(PlaceholderTable::classloadAction action) {
 73   SeenThread* queuehead = NULL;
 74   switch (action) {
 75     case PlaceholderTable::LOAD_INSTANCE:
 76        queuehead = _loadInstanceThreadQ;
 77        break;
 78     case PlaceholderTable::LOAD_SUPER:
 79        queuehead = _superThreadQ;
 80        break;
 81     case PlaceholderTable::DEFINE_CLASS:
 82        queuehead = _defineThreadQ;
 83        break;
 84     default: Unimplemented();
 85   }
 86   return queuehead;
 87 }
 88 
 89 void PlaceholderEntry::set_threadQ(SeenThread* seenthread, PlaceholderTable::classloadAction action) {
 90   switch (action) {
 91     case PlaceholderTable::LOAD_INSTANCE:
 92        _loadInstanceThreadQ = seenthread;
 93        break;
 94     case PlaceholderTable::LOAD_SUPER:
 95        _superThreadQ = seenthread;
 96        break;
 97     case PlaceholderTable::DEFINE_CLASS:
 98        _defineThreadQ = seenthread;
 99        break;
100     default: Unimplemented();
101   }
102   return;
103 }
104 
105 // Doubly-linked list of Threads per action for class/classloader pair
106 // Class circularity support: links in thread before loading superclass
107 // bootstrap loader support:  links in a thread before load_instance_class
108 // definers: use as queue of define requestors, including owner of
109 // define token. Appends for debugging of requestor order
110 void PlaceholderEntry::add_seen_thread(JavaThread* thread, PlaceholderTable::classloadAction action) {
111   assert_lock_strong(SystemDictionary_lock);
112   SeenThread* threadEntry = new SeenThread(thread);
113   SeenThread* seen = actionToQueue(action);
114 
115   assert(action != PlaceholderTable::LOAD_INSTANCE || seen == NULL,
116          "Only one LOAD_INSTANCE allowed at a time");
117 
118   if (seen == NULL) {
119     set_threadQ(threadEntry, action);
120     return;
121   }
122   SeenThread* next;
123   while ((next = seen->next()) != NULL) {
124     seen = next;
125   }
126   seen->set_next(threadEntry);
127   threadEntry->set_prev(seen);
128   return;
129 }
130 
131 bool PlaceholderEntry::check_seen_thread(JavaThread* thread, PlaceholderTable::classloadAction action) {
132   assert_lock_strong(SystemDictionary_lock);
133   SeenThread* threadQ = actionToQueue(action);
134   SeenThread* seen = threadQ;
135   while (seen) {
136     if (thread == seen->thread()) {
137       return true;
138     }
139     seen = seen->next();
140   }
141   return false;
142 }
143 
144 // returns true if seenthreadQ is now empty
145 // Note, caller must ensure probe still exists while holding
146 // SystemDictionary_lock
147 // ignores if cleanup has already been done
148 // if found, deletes SeenThread
149 bool PlaceholderEntry::remove_seen_thread(JavaThread* thread, PlaceholderTable::classloadAction action) {
150   assert_lock_strong(SystemDictionary_lock);
151   SeenThread* threadQ = actionToQueue(action);
152   SeenThread* seen = threadQ;
153   SeenThread* prev = NULL;
154   while (seen) {
155     if (thread == seen->thread()) {
156       if (prev) {
157         prev->set_next(seen->next());
158       } else {
159         set_threadQ(seen->next(), action);
160       }
161       if (seen->next()) {
162         seen->next()->set_prev(prev);
163       }
164       delete seen;
165       break;
166     }
167     prev = seen;
168     seen = seen->next();
169   }
170   return (actionToQueue(action) == NULL);
171 }
172 
173 
174 // Placeholder methods
175 
176 PlaceholderEntry* PlaceholderTable::new_entry(int hash, Symbol* name,
177                                               ClassLoaderData* loader_data,
178                                               Symbol* supername) {
179   PlaceholderEntry* entry = (PlaceholderEntry*)Hashtable<Symbol*, mtClass>::new_entry(hash, name);
180   // Hashtable with Symbol* literal must increment and decrement refcount.
181   name->increment_refcount();
182   entry->set_loader_data(loader_data);
183   entry->set_supername(supername);
184   entry->set_superThreadQ(NULL);
185   entry->set_loadInstanceThreadQ(NULL);
186   entry->set_defineThreadQ(NULL);
187   entry->set_definer(NULL);
188   entry->set_instance_klass(NULL);
189   return entry;
190 }
191 
192 void PlaceholderTable::free_entry(PlaceholderEntry* entry) {
193   // decrement Symbol refcount here because Hashtable doesn't.
194   entry->literal()->decrement_refcount();
195   if (entry->supername() != NULL) entry->supername()->decrement_refcount();
196   BasicHashtable<mtClass>::free_entry(entry);
197 }
198 
199 
200 // Placeholder objects represent classes currently being loaded.
201 // All threads examining the placeholder table must hold the
202 // SystemDictionary_lock, so we don't need special precautions
203 // on store ordering here.
204 PlaceholderEntry* PlaceholderTable::add_entry(unsigned int hash,
205                                               Symbol* class_name, ClassLoaderData* loader_data,
206                                               Symbol* supername){
207   assert_locked_or_safepoint(SystemDictionary_lock);
208   assert(class_name != NULL, "adding NULL obj");
209 
210   // Both readers and writers are locked so it's safe to just
211   // create the placeholder and insert it in the list without a membar.
212   PlaceholderEntry* entry = new_entry(hash, class_name, loader_data, supername);
213   int index = hash_to_index(hash);
214   Hashtable<Symbol*, mtClass>::add_entry(index, entry);
215   return entry;
216 }
217 
218 
219 // Remove a placeholder object.
220 void PlaceholderTable::remove_entry(unsigned int hash,
221                                     Symbol* class_name,
222                                     ClassLoaderData* loader_data) {
223   assert_locked_or_safepoint(SystemDictionary_lock);
224   int index = hash_to_index(hash);
225   PlaceholderEntry** p = bucket_addr(index);
226   while (*p != NULL) {
227     PlaceholderEntry *probe = *p;
228     if (probe->hash() == hash && probe->equals(class_name, loader_data)) {
229       // Delete entry
230       *p = probe->next();
231       free_entry(probe);
232       return;
233     }
234     p = probe->next_addr();
235   }
236 }
237 
238 PlaceholderEntry* PlaceholderTable::get_entry(unsigned int hash,
239                                               Symbol* class_name,
240                                               ClassLoaderData* loader_data) {
241   assert_locked_or_safepoint(SystemDictionary_lock);
242 
243   int index = hash_to_index(hash);
244   for (PlaceholderEntry *place_probe = bucket(index);
245                          place_probe != NULL;
246                          place_probe = place_probe->next()) {
247     if (place_probe->hash() == hash &&
248         place_probe->equals(class_name, loader_data)) {
249       return place_probe;
250     }
251   }
252   return NULL;
253 }
254 
255 Symbol* PlaceholderTable::find_entry(unsigned int hash,
256                                      Symbol* class_name,
257                                      ClassLoaderData* loader_data) {
258   PlaceholderEntry* probe = get_entry(hash, class_name, loader_data);
259   return (probe != NULL ? probe->klassname() : NULL);
260 }
261 
262 static const char* action_to_string(PlaceholderTable::classloadAction action) {
263   switch (action) {
264   case PlaceholderTable::LOAD_INSTANCE: return "LOAD_INSTANCE";
265   case PlaceholderTable::LOAD_SUPER:    return "LOAD_SUPER";
266   case PlaceholderTable::DEFINE_CLASS:  return "DEFINE_CLASS";
267  }
268  return "";
269 }
270 
271 inline void log(PlaceholderEntry* entry, const char* function, PlaceholderTable::classloadAction action) {
272   if (log_is_enabled(Debug, class, load, placeholders)) {
273     LogTarget(Debug, class, load, placeholders) lt;
274     ResourceMark rm;
275     LogStream ls(lt);
276     ls.print("%s %s ", function, action_to_string(action));
277     entry->print_entry(&ls);
278   }
279 }
280 
281 // find_and_add returns probe pointer - old or new
282 // If no entry exists, add a placeholder entry
283 // If entry exists, reuse entry
284 // For both, push SeenThread for classloadAction
285 // If LOAD_SUPER, this is used for circularity detection for instanceklass loading.
286 PlaceholderEntry* PlaceholderTable::find_and_add(unsigned int hash,
287                                                  Symbol* name,
288                                                  ClassLoaderData* loader_data,
289                                                  classloadAction action,
290                                                  Symbol* supername,
291                                                  JavaThread* thread) {
292   assert(action != LOAD_SUPER || supername != NULL, "must have a super class name");
293   PlaceholderEntry* probe = get_entry(hash, name, loader_data);
294   if (probe == NULL) {
295     // Nothing found, add place holder
296     probe = add_entry(hash, name, loader_data, supername);
297   } else {
298     if (action == LOAD_SUPER) {
299       probe->set_supername(supername);
300     }
301   }
302   probe->add_seen_thread(thread, action);
303   log(probe, "find_and_add", action);
304   return probe;
305 }
306 
307 
308 // placeholder is used to track class loading internal states
309 // placeholder existence now for loading superclass/superinterface
310 // superthreadQ tracks class circularity, while loading superclass/superinterface
311 // loadInstanceThreadQ tracks load_instance_class calls
312 // definer() tracks the single thread that owns define token
313 // defineThreadQ tracks waiters on defining thread's results
314 // 1st claimant creates placeholder
315 // find_and_add adds SeenThread entry for appropriate queue
316 // All claimants remove SeenThread after completing action
317 // On removal: if definer and all queues empty, remove entry
318 // Note: you can be in both placeholders and systemDictionary
319 // Therefore - must always check SD first
320 // Ignores the case where entry is not found
321 void PlaceholderTable::find_and_remove(unsigned int hash,
322                                        Symbol* name, ClassLoaderData* loader_data,
323                                        classloadAction action,
324                                        JavaThread* thread) {
325     assert_locked_or_safepoint(SystemDictionary_lock);
326     PlaceholderEntry *probe = get_entry(hash, name, loader_data);
327     if (probe != NULL) {
328        log(probe, "find_and_remove", action);
329        probe->remove_seen_thread(thread, action);
330        // If no other threads using this entry, and this thread is not using this entry for other states
331        if ((probe->superThreadQ() == NULL) && (probe->loadInstanceThreadQ() == NULL)
332           && (probe->defineThreadQ() == NULL) && (probe->definer() == NULL)) {
333          remove_entry(hash, name, loader_data);
334        }
335     }
336   }
337 
338 PlaceholderTable::PlaceholderTable(int table_size)
339     : Hashtable<Symbol*, mtClass>(table_size, sizeof(PlaceholderEntry)) {
340 }
341 
342 void PlaceholderEntry::verify() const {
343   guarantee(loader_data() != NULL, "Must have been setup.");
344   guarantee(loader_data()->class_loader() == NULL || loader_data()->class_loader()->is_instance(),
345             "checking type of _loader");
346   guarantee(instance_klass() == NULL
347             || instance_klass()->is_instance_klass(),
348             "checking type of instance_klass result");
349 }
350 
351 void PlaceholderTable::verify() {
352   verify_table<PlaceholderEntry>("Placeholder Table");
353 }
354 
355 
356 // Note, doesn't append a cr
357 // Can't call this print_on because HashtableEntry doesn't initialize its vptr
358 // and print_on is a virtual function so the vptr call crashes.
359 void PlaceholderEntry::print_entry(outputStream* st) const {
360   klassname()->print_value_on(st);
361   if (loader_data() != NULL) {
362     st->print(", loader ");
363     loader_data()->print_value_on(st);
364   }
365   if (supername() != NULL) {
366     st->print(", supername ");
367     supername()->print_value_on(st);
368   }
369   if (definer() != NULL) {
370     st->print(", definer ");
371     definer()->print_value_on(st);
372   }
373   if (instance_klass() != NULL) {
374     st->print(", InstanceKlass ");
375     instance_klass()->print_value_on(st);
376   }
377   st->cr();
378   st->print("loadInstanceThreadQ threads:");
379   loadInstanceThreadQ()->print_action_queue(st);
380   st->cr();
381   st->print("superThreadQ threads:");
382   superThreadQ()->print_action_queue(st);
383   st->cr();
384   st->print("defineThreadQ threads:");
385   defineThreadQ()->print_action_queue(st);
386   st->cr();
387 }
388 
389 void PlaceholderTable::print_on(outputStream* st) const {
390   st->print_cr("Placeholder table (table_size=%d, placeholders=%d)",
391                 table_size(), number_of_entries());
392   for (int pindex = 0; pindex < table_size(); pindex++) {
393     for (PlaceholderEntry* probe = bucket(pindex);
394                            probe != NULL;
395                            probe = probe->next()) {
396       st->print("%4d: placeholder ", pindex);
397       probe->print_entry(st);
398     }
399   }
400 }
401 
402 void PlaceholderTable::print() const { return print_on(tty); }