/* * Copyright (c) 2017, 2022, Oracle and/or its affiliates. All rights reserved. * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * * This code is free software; you can redistribute it and/or modify it * under the terms of the GNU General Public License version 2 only, as * published by the Free Software Foundation. * * This code is distributed in the hope that it will be useful, but WITHOUT * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License * version 2 for more details (a copy is included in the LICENSE file that * accompanied this code). * * You should have received a copy of the GNU General Public License version * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. *
*/
// The '_cnt', '_max' and '_times" fields are enabled via // -XX:+EnableThreadSMRStatistics:
// # of parallel threads in _delete_lock->wait(). // Impl note: Hard to imagine > 64K waiting threads so this could be 16-bit, // but there is no nice 16-bit _FORMAT support.
uint ThreadsSMRSupport::_delete_lock_wait_cnt = 0;
// Max # of parallel threads in _delete_lock->wait(). // Impl note: See _delete_lock_wait_cnt note.
uint ThreadsSMRSupport::_delete_lock_wait_max = 0;
// Flag to indicate when an _delete_lock->notify() is needed. // Impl note: See _delete_lock_wait_cnt note. volatile uint ThreadsSMRSupport::_delete_notify = 0;
// # of threads deleted over VM lifetime. // Impl note: Atomically incremented over VM lifetime so use unsigned for more // range. Unsigned 64-bit would be more future proof, but 64-bit atomic inc // isn't available everywhere (or is it?). volatile uint ThreadsSMRSupport::_deleted_thread_cnt = 0;
// Max time in millis to delete a thread. // Impl note: 16-bit might be too small on an overloaded machine. Use // unsigned since this is a time value. Set via Atomic::cmpxchg() in a // loop for correctness. volatile uint ThreadsSMRSupport::_deleted_thread_time_max = 0;
// Cumulative time in millis to delete threads. // Impl note: Atomically added to over VM lifetime so use unsigned for more // range. Unsigned 64-bit would be more future proof, but 64-bit atomic inc // isn't available everywhere (or is it?). volatile uint ThreadsSMRSupport::_deleted_thread_times = 0;
// The bootstrap list is empty and cannot be freed.
ThreadsList ThreadsSMRSupport::_bootstrap_list{0};
// This is the VM's current "threads list" and it contains all of // the JavaThreads the VM considers to be alive at this moment in // time. The other ThreadsList objects in the VM contain past // snapshots of the "threads list". _java_thread_list is initially // set to _bootstrap_list so that we can detect when we have a very // early use of a ThreadsListHandle.
ThreadsList* volatile ThreadsSMRSupport::_java_thread_list = &_bootstrap_list;
// # of ThreadsLists allocated over VM lifetime. // Impl note: We allocate a new ThreadsList for every thread create and // every thread delete so we need a bigger type than the // _deleted_thread_cnt field.
uint64_t ThreadsSMRSupport::_java_thread_list_alloc_cnt = 1;
// # of ThreadsLists freed over VM lifetime. // Impl note: See _java_thread_list_alloc_cnt note.
uint64_t ThreadsSMRSupport::_java_thread_list_free_cnt = 0;
// Max size ThreadsList allocated. // Impl note: Max # of threads alive at one time should fit in unsigned 32-bit.
uint ThreadsSMRSupport::_java_thread_list_max = 0;
// Max # of nested ThreadsLists for a thread. // Impl note: Hard to imagine > 64K nested ThreadsLists so this could be // 16-bit, but there is no nice 16-bit _FORMAT support.
uint ThreadsSMRSupport::_nested_thread_list_max = 0;
// # of ThreadsListHandles deleted over VM lifetime. // Impl note: Atomically incremented over VM lifetime so use unsigned for // more range. There will be fewer ThreadsListHandles than threads so // unsigned 32-bit should be fine. volatile uint ThreadsSMRSupport::_tlh_cnt = 0;
// Max time in millis to delete a ThreadsListHandle. // Impl note: 16-bit might be too small on an overloaded machine. Use // unsigned since this is a time value. Set via Atomic::cmpxchg() in a // loop for correctness. volatile uint ThreadsSMRSupport::_tlh_time_max = 0;
// Cumulative time in millis to delete ThreadsListHandles. // Impl note: Atomically added to over VM lifetime so use unsigned for more // range. Unsigned 64-bit would be more future proof, but 64-bit atomic inc // isn't available everywhere (or is it?). volatile uint ThreadsSMRSupport::_tlh_times = 0;
// # of parallel ThreadsLists on the to-delete list. // Impl note: Hard to imagine > 64K ThreadsLists needing to be deleted so // this could be 16-bit, but there is no nice 16-bit _FORMAT support.
uint ThreadsSMRSupport::_to_delete_list_cnt = 0;
// Max # of parallel ThreadsLists on the to-delete list. // Impl note: See _to_delete_list_cnt note.
uint ThreadsSMRSupport::_to_delete_list_max = 0;
// 'inline' functions first so the definitions are before first use:
inlinebool ThreadsSMRSupport::is_bootstrap_list(ThreadsList* list) { return list == &_bootstrap_list;
}
inlinevoid ThreadsSMRSupport::update_deleted_thread_time_max(uint new_value) { while (true) {
uint cur_value = _deleted_thread_time_max; if (new_value <= cur_value) { // No need to update max value so we're done. break;
} if (Atomic::cmpxchg(&_deleted_thread_time_max, cur_value, new_value) == cur_value) { // Updated max value so we're done. Otherwise try it all again. break;
}
}
}
// Hash table of pointers found by a scan. Used for collecting hazard // pointers (ThreadsList references). Also used for collecting JavaThreads // that are indirectly referenced by hazard ptrs. An instance of this // class only contains one type of pointer. // class ThreadScanHashtable : public CHeapObj<mtThread> { private: staticunsignedint ptr_hash(void * const& s1) { // 2654435761 = 2^32 * Phi (golden ratio) return (unsignedint)(((uint32_t)(uintptr_t)s1) * 2654435761u);
}
// ResourceHashtable SIZE is specified at compile time so we // use 1031 which is the first prime after 1024. typedef ResourceHashtable<void *, int, 1031,
AnyObj::C_HEAP, mtThread,
&ThreadScanHashtable::ptr_hash> PtrTable;
PtrTable * _ptrs;
public: // ResourceHashtable is passed to various functions and populated in // different places so we allocate it using C_HEAP to make it immune // from any ResourceMarks that happen to be in the code paths.
ThreadScanHashtable() : _ptrs(new (mtThread) PtrTable()) {}
// Closure to gather JavaThreads indirectly referenced by hazard ptrs // (ThreadsList references) into a hash table. This closure handles part 2 // of the dance - adding all the JavaThreads referenced by the hazard // pointer (ThreadsList reference) to the hash table. // class AddThreadHazardPointerThreadClosure : public ThreadClosure { private:
ThreadScanHashtable *_table;
virtualvoid do_thread(Thread *thread) { if (!_table->has_entry((void*)thread)) { // The same JavaThread might be on more than one ThreadsList or // more than one thread might be using the same ThreadsList. In // either case, we only need a single entry for a JavaThread.
_table->add_entry((void*)thread);
}
}
};
// Closure to gather JavaThreads indirectly referenced by hazard ptrs // (ThreadsList references) into a hash table. This closure handles part 1 // of the dance - hazard ptr chain walking and dispatch to another // closure. // class ScanHazardPtrGatherProtectedThreadsClosure : public ThreadClosure { private:
ThreadScanHashtable *_table; public:
ScanHazardPtrGatherProtectedThreadsClosure(ThreadScanHashtable *table) : _table(table) {}
// This code races with ThreadsSMRSupport::acquire_stable_list() which // is lock-free so we have to handle some special situations. //
ThreadsList *current_list = NULL; while (true) {
current_list = thread->get_threads_hazard_ptr(); // No hazard ptr so nothing more to do. if (current_list == NULL) { return;
}
// If the hazard ptr is verified as stable (since it is not tagged), // then it is safe to use. if (!Thread::is_hazard_ptr_tagged(current_list)) break;
// The hazard ptr is tagged as not yet verified as being stable // so we are racing with acquire_stable_list(). This exchange // attempts to invalidate the hazard ptr. If we win the race, // then we can ignore this unstable hazard ptr and the other // thread will retry the attempt to publish a stable hazard ptr. // If we lose the race, then we retry our attempt to look at the // hazard ptr. if (thread->cmpxchg_threads_hazard_ptr(NULL, current_list) == current_list) return;
}
assert(ThreadsList::is_valid(current_list), "current_list="
INTPTR_FORMAT " is not valid!", p2i(current_list));
// The current JavaThread has a hazard ptr (ThreadsList reference) // which might be _java_thread_list or it might be an older // ThreadsList that has been removed but not freed. In either case, // the hazard ptr is protecting all the JavaThreads on that // ThreadsList.
AddThreadHazardPointerThreadClosure add_cl(_table);
current_list->threads_do(&add_cl);
}
};
// Closure to gather hazard ptrs (ThreadsList references) into a hash table. // // Since this closure gathers hazard ptrs that may be tagged, this hash // table of hazard ptrs should only be used for value comparison and not // traversal of the ThreadsList. // class ScanHazardPtrGatherThreadsListClosure : public ThreadClosure { private:
ThreadScanHashtable *_table; public:
ScanHazardPtrGatherThreadsListClosure(ThreadScanHashtable *table) : _table(table) {}
if (thread == NULL) return;
ThreadsList *hazard_ptr = thread->get_threads_hazard_ptr(); if (hazard_ptr == NULL) return; #ifdef ASSERT if (!Thread::is_hazard_ptr_tagged(hazard_ptr)) { // We only validate hazard_ptrs that are not tagged since a tagged // hazard ptr can be deleted at any time.
assert(ThreadsList::is_valid(hazard_ptr), "hazard_ptr=" INTPTR_FORMAT " for thread=" INTPTR_FORMAT " is not valid!", p2i(hazard_ptr),
p2i(thread));
} #endif // In this closure we always ignore the tag that might mark this // hazard ptr as not yet verified. If we happen to catch an // unverified hazard ptr that is subsequently discarded (not // published), then the only side effect is that we might keep a // to-be-deleted ThreadsList alive a little longer.
hazard_ptr = Thread::untag_hazard_ptr(hazard_ptr); if (!_table->has_entry((void*)hazard_ptr)) {
_table->add_entry((void*)hazard_ptr);
}
}
};
// Closure to print JavaThreads that have a hazard ptr (ThreadsList // reference) that contains an indirect reference to a specific JavaThread. // class ScanHazardPtrPrintMatchingThreadsClosure : public ThreadClosure { private:
JavaThread *_thread; public:
ScanHazardPtrPrintMatchingThreadsClosure(JavaThread *thread) : _thread(thread) {}
if (thread == NULL) return;
ThreadsList *current_list = thread->get_threads_hazard_ptr(); if (current_list == NULL) { return;
} // If the hazard ptr is unverified, then ignore it. if (Thread::is_hazard_ptr_tagged(current_list)) return;
// The current JavaThread has a hazard ptr (ThreadsList reference) // which might be _java_thread_list or it might be an older // ThreadsList that has been removed but not freed. In either case, // the hazard ptr is protecting all the JavaThreads on that // ThreadsList, but we only care about matching a specific JavaThread.
JavaThreadIterator jti(current_list); for (JavaThread *p = jti.first(); p != NULL; p = jti.next()) { if (p == _thread) {
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)); break;
}
}
}
};
// Closure to validate hazard ptrs. // class ValidateHazardPtrsClosure : public ThreadClosure { public:
ValidateHazardPtrsClosure() {};
if (thread == NULL) return;
ThreadsList *hazard_ptr = thread->get_threads_hazard_ptr(); if (hazard_ptr == NULL) return; // If the hazard ptr is unverified, then ignore it since it could // be deleted at any time now. if (Thread::is_hazard_ptr_tagged(hazard_ptr)) return;
assert(ThreadsList::is_valid(hazard_ptr), "hazard_ptr=" INTPTR_FORMAT " for thread=" INTPTR_FORMAT " is not valid!", p2i(hazard_ptr),
p2i(thread));
}
};
// Closure to determine if the specified JavaThread is found by // threads_do(). // class VerifyHazardPtrThreadClosure : public ThreadClosure { private: bool _found;
Thread *_self;
if (_thread->get_threads_hazard_ptr() == NULL && _previous == NULL) { // The typical case is first.
acquire_stable_list_fast_path(); return;
}
// The nested case is rare.
acquire_stable_list_nested_path();
}
// Fast path way to acquire a stable ThreadsList. // void SafeThreadsListPtr::acquire_stable_list_fast_path() {
assert(_thread != NULL, "sanity check");
assert(_thread->get_threads_hazard_ptr() == NULL, "sanity check");
ThreadsList* threads;
// Stable recording of a hazard ptr for SMR. This code does not use // locks so its use of the _java_thread_list & _threads_hazard_ptr // fields is racy relative to code that uses those fields with locks. // OrderAccess and Atomic functions are used to deal with those races. // while (true) {
threads = ThreadsSMRSupport::get_java_thread_list();
// Publish a tagged hazard ptr to denote that the hazard ptr is not // yet verified as being stable. Due to the fence after the hazard // ptr write, it will be sequentially consistent w.r.t. the // sequentially consistent writes of the ThreadsList, even on // non-multiple copy atomic machines where stores can be observed // in different order from different observer threads.
ThreadsList* unverified_threads = Thread::tag_hazard_ptr(threads);
_thread->set_threads_hazard_ptr(unverified_threads);
// If _java_thread_list has changed, we have lost a race with // Threads::add() or Threads::remove() and have to try again. if (ThreadsSMRSupport::get_java_thread_list() != threads) { continue;
}
// We try to remove the tag which will verify the hazard ptr as // being stable. This exchange can race with a scanning thread // which might invalidate the tagged hazard ptr to keep it from // being followed to access JavaThread ptrs. If we lose the race, // we simply retry. If we win the race, then the stable hazard // ptr is officially published. if (_thread->cmpxchg_threads_hazard_ptr(threads, unverified_threads) == unverified_threads) { break;
}
}
// A stable hazard ptr has been published letting other threads know // that the ThreadsList and the JavaThreads reachable from this list // are protected and hence they should not be deleted until everyone // agrees it is safe to do so.
_list = threads;
verify_hazard_ptr_scanned();
}
// Acquire a nested stable ThreadsList; this is rare so it uses // reference counting. // void SafeThreadsListPtr::acquire_stable_list_nested_path() {
assert(_thread != NULL, "sanity check");
// The thread already has a hazard ptr (ThreadsList ref) so we need // to create a nested ThreadsListHandle with the current ThreadsList // since it might be different than our current hazard ptr. To remedy // the situation, the ThreadsList pointed to by the pre-existing // stable hazard ptr is reference counted before the hazard ptr may // be released and moved to a new ThreadsList. The old ThreadsList // is remembered in the ThreadsListHandle.
ThreadsList* current_list = _previous->_list; if (EnableThreadSMRStatistics) {
_thread->inc_nested_threads_hazard_ptr_cnt();
} if (!_previous->_has_ref_count) { // Promote the thread's current SafeThreadsListPtr to be reference counted.
current_list->inc_nested_handle_cnt();
_previous->_has_ref_count = true;
} // Clear the hazard ptr so we can go through the fast path below and // acquire a nested stable ThreadsList.
_thread->set_threads_hazard_ptr(NULL);
// We're releasing either a leaf or nested ThreadsListHandle. In either // case, we set this thread's hazard ptr back to NULL and we do it before // _nested_handle_cnt is decremented below.
_thread->set_threads_hazard_ptr(NULL); if (_previous != NULL) { // The ThreadsListHandle being released is a nested ThreadsListHandle. if (EnableThreadSMRStatistics) {
_thread->dec_nested_threads_hazard_ptr_cnt();
} // The previous ThreadsList is stable because the _nested_handle_cnt is // > 0, but we cannot safely make it this thread's hazard ptr again. // The protocol for installing and verifying a ThreadsList as a // thread's hazard ptr is handled by acquire_stable_list_fast_path(). // And that protocol cannot be properly done with a ThreadsList that // might not be the current system ThreadsList.
assert(_previous->_list->_nested_handle_cnt > 0, "must be > zero");
} if (_has_ref_count) { // This thread created a nested ThreadsListHandle after the current // ThreadsListHandle so we had to protect this ThreadsList with a // ref count. We no longer need that protection.
_list->dec_nested_handle_cnt();
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));
}
// After releasing the hazard ptr, other threads may go ahead and // free up some memory temporarily used by a ThreadsList snapshot.
// We use double-check locking to reduce traffic on the system // wide Thread-SMR delete_lock. if (ThreadsSMRSupport::delete_notify()) { // An exiting thread might be waiting in smr_delete(); we need to // check with delete_lock to be sure.
ThreadsSMRSupport::release_stable_list_wake_up(_has_ref_count);
assert(_previous == NULL || ThreadsList::is_valid(_previous->_list), "_previous->_list=" INTPTR_FORMAT " is not valid after calling release_stable_list_wake_up!",
p2i(_previous->_list));
}
}
// Verify that the stable hazard ptr used to safely keep threads // alive is scanned by threads_do() which is a key piece of honoring // the Thread-SMR protocol. void SafeThreadsListPtr::verify_hazard_ptr_scanned() { #ifdef ASSERT
assert(_list != NULL, "_list must not be NULL");
if (ThreadsSMRSupport::is_bootstrap_list(_list)) { // We are early in VM bootstrapping so nothing to do here. return;
}
if ( _thread == VM_Exit::shutdown_thread()) { // The shutdown thread has removed itself from the Threads // list and is safe to have a waiver from this check because // VM_Exit::_shutdown_thread is not set until after the VMThread // has started the final safepoint which holds the Threads_lock // for the remainder of the VM's life. return;
}
if (VMError::is_error_reported_in_current_thread()) { // If there is an error reported by this thread it may use ThreadsList even // if it's unsafe. return;
}
// The closure will attempt to verify that the calling thread can // be found by threads_do() on the specified ThreadsList. If it // is successful, then the specified ThreadsList was acquired as // a stable hazard ptr by the calling thread in a way that honored // the Thread-SMR protocol. // // If the calling thread cannot be found by threads_do() and if // it is not the shutdown thread, then the calling thread is not // honoring the Thread-SMR ptotocol. This means that the specified // ThreadsList is not a stable hazard ptr and can be freed by // another thread from the to-be-deleted list at any time. //
VerifyHazardPtrThreadClosure cl(_thread);
ThreadsSMRSupport::threads_do(&cl, _list);
// If the calling thread is not honoring the Thread-SMR protocol, // then we will either crash in threads_do() above because 'threads' // was freed by another thread or we will fail the assert() below. // In either case, we won't get past this point with a badly placed // ThreadsListHandle.
assert(cl.found(), "Acquired a ThreadsList snapshot from a thread not recognized by the Thread-SMR protocol."); #endif
}
// Shared singleton data for all ThreadsList(0) instances. // Used by _bootstrap_list to avoid static init time heap allocation. // No real entries, just the final NULL terminator. static JavaThread* const empty_threads_list_data[1] = {};
// Result has 'entries + 1' elements, with the last being the NULL terminator. static JavaThread* const* make_threads_list_data(int entries) { if (entries == 0) { return empty_threads_list_data;
}
JavaThread** data = NEW_C_HEAP_ARRAY(JavaThread*, entries + 1, mtThread);
data[entries] = NULL; // Make sure the final entry is NULL. return data;
}
// Add a JavaThread to a ThreadsList. The returned ThreadsList is a // new copy of the specified ThreadsList with the specified JavaThread // appended to the end.
ThreadsList *ThreadsList::add_thread(ThreadsList *list, JavaThread *java_thread) { const uint index = list->_length; const uint new_length = index + 1; const uint head_length = index;
ThreadsList *const new_list = new ThreadsList(new_length);
int ThreadsList::find_index_of_JavaThread(JavaThread *target) { if (target == NULL) { return -1;
} for (uint i = 0; i < length(); i++) { if (target == thread_at(i)) { return (int)i;
}
} return -1;
}
JavaThread* ThreadsList::find_JavaThread_from_java_tid(jlong java_tid) const {
ThreadIdTable::lazy_initialize(this);
JavaThread* thread = ThreadIdTable::find_thread_by_tid(java_tid); if (thread == NULL) { // If the thread is not found in the table find it // with a linear search and add to the table. for (uint i = 0; i < length(); i++) {
thread = thread_at(i);
oop tobj = thread->threadObj(); // Ignore the thread if it hasn't run yet, has exited // or is starting to exit. if (tobj != NULL && java_tid == java_lang_Thread::thread_id(tobj)) {
MutexLocker ml(Threads_lock); // Must be inside the lock to ensure that we don't add a thread to the table // that has just passed the removal point in Threads::remove(). if (!thread->is_exiting()) {
ThreadIdTable::add_thread(java_tid, thread); return thread;
}
}
}
} elseif (!thread->is_exiting()) { return thread;
} return NULL;
}
bool ThreadsList::includes(const JavaThread * const p) const { if (p == NULL) { returnfalse;
} for (uint i = 0; i < length(); i++) { if (thread_at(i) == p) { returntrue;
}
} returnfalse;
}
// Remove a JavaThread from a ThreadsList. The returned ThreadsList is a // new copy of the specified ThreadsList with the specified JavaThread // removed.
ThreadsList *ThreadsList::remove_thread(ThreadsList* list, JavaThread* java_thread) {
assert(list->_length > 0, "sanity");
uint i = (uint)list->find_index_of_JavaThread(java_thread);
assert(i < list->_length, "did not find JavaThread on the list"); const uint index = i; const uint new_length = list->_length - 1; const uint head_length = index; const uint tail_length = (new_length >= index) ? (new_length - index) : 0;
ThreadsList *const new_list = new ThreadsList(new_length);
if (head_length > 0) {
Copy::disjoint_words((HeapWord*)list->_threads, (HeapWord*)new_list->_threads, head_length);
} if (tail_length > 0) {
Copy::disjoint_words((HeapWord*)list->_threads + index + 1, (HeapWord*)new_list->_threads + index, tail_length);
}
// Convert an internal thread reference to a JavaThread found on the // associated ThreadsList. This ThreadsListHandle "protects" the // returned JavaThread *. // // If thread_oop_p is not NULL, then the caller wants to use the oop // after this call so the oop is returned. On success, *jt_pp is set // to the converted JavaThread * and true is returned. On error, // returns false. // bool ThreadsListHandle::cv_internal_thread_to_JavaThread(jobject jthread,
JavaThread ** jt_pp,
oop * thread_oop_p) {
assert(this->list() != NULL, "must have a ThreadsList");
assert(jt_pp != NULL, "must have a return JavaThread pointer"); // thread_oop_p is optional so no assert()
// The JVM_* interfaces don't allow a NULL thread parameter; JVM/TI // allows a NULL thread parameter to signify "current thread" which // allows us to avoid calling cv_external_thread_to_JavaThread(). // The JVM_* interfaces have no such leeway.
oop thread_oop = JNIHandles::resolve_non_null(jthread); // Looks like an oop at this point. if (thread_oop_p != NULL) { // Return the oop to the caller; the caller may still want // the oop even if this function returns false.
*thread_oop_p = thread_oop;
}
JavaThread *java_thread = java_lang_Thread::thread(thread_oop); if (java_thread == NULL) { // The java.lang.Thread does not contain a JavaThread * so it has // not yet run or it has died. returnfalse;
} // Looks like a live JavaThread at this point.
if (java_thread != JavaThread::current()) { // jthread is not for the current JavaThread so have to verify // the JavaThread * against the ThreadsList. if (EnableThreadSMRExtraValidityChecks && !includes(java_thread)) { // Not on the JavaThreads list so it is not alive. returnfalse;
}
}
// Return a live JavaThread that is "protected" by the // ThreadsListHandle in the caller.
*jt_pp = java_thread; returntrue;
}
void ThreadsSMRSupport::add_thread(JavaThread *thread){
ThreadsList *new_list = ThreadsList::add_thread(get_java_thread_list(), thread); if (EnableThreadSMRStatistics) {
inc_java_thread_list_alloc_cnt();
update_java_thread_list_max(new_list->length());
} // Initial _java_thread_list will not generate a "Threads::add" mesg.
log_debug(thread, smr)("tid=" UINTX_FORMAT ": Threads::add: new ThreadsList=" INTPTR_FORMAT, os::current_thread_id(), p2i(new_list));
ThreadsList *old_list = xchg_java_thread_list(new_list);
free_list(old_list); if (ThreadIdTable::is_initialized()) {
jlong tid = SharedRuntime::get_java_tid(thread);
ThreadIdTable::add_thread(tid, thread);
}
}
// set_delete_notify() and clear_delete_notify() are called // under the protection of the delete_lock, but we also use an // Atomic operation to ensure the memory update is seen earlier than // when the delete_lock is dropped. // void ThreadsSMRSupport::clear_delete_notify() {
Atomic::dec(&_delete_notify);
}
bool ThreadsSMRSupport::delete_notify() { // Use load_acquire() in order to see any updates to _delete_notify // earlier than when delete_lock is grabbed. return (Atomic::load_acquire(&_delete_notify) != 0);
}
// Safely free a ThreadsList after a Threads::add() or Threads::remove(). // The specified ThreadsList may not get deleted during this call if it // is still in-use (referenced by a hazard ptr). Other ThreadsLists // in the chain may get deleted by this call if they are no longer in-use. void ThreadsSMRSupport::free_list(ThreadsList* threads) {
assert_locked_or_safepoint(Threads_lock);
if (is_bootstrap_list(threads)) { // The bootstrap list cannot be freed and is empty so // it does not need to be scanned. Nothing to do here.
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)); return;
}
threads->set_next_list(_to_delete_list);
_to_delete_list = threads; if (EnableThreadSMRStatistics) {
_to_delete_list_cnt++; if (_to_delete_list_cnt > _to_delete_list_max) {
_to_delete_list_max = _to_delete_list_cnt;
}
}
// Gather a hash table of the current hazard ptrs:
ThreadScanHashtable *scan_table = new ThreadScanHashtable();
ScanHazardPtrGatherThreadsListClosure scan_cl(scan_table);
threads_do(&scan_cl);
OrderAccess::acquire(); // Must order reads of hazard ptr before reads of // nested reference counters
// Walk through the linked list of pending freeable ThreadsLists // and free the ones that are not referenced from hazard ptrs.
ThreadsList* current = _to_delete_list;
ThreadsList* prev = NULL;
ThreadsList* next = NULL; bool threads_is_freed = false; while (current != NULL) {
next = current->next_list(); if (!scan_table->has_entry((void*)current) && current->_nested_handle_cnt == 0) { // This ThreadsList is not referenced by a hazard ptr. if (prev != NULL) {
prev->set_next_list(next);
} if (_to_delete_list == current) {
_to_delete_list = next;
}
log_debug(thread, smr)("tid=" UINTX_FORMAT ": ThreadsSMRSupport::free_list: threads=" INTPTR_FORMAT " is freed.", os::current_thread_id(), p2i(current)); if (current == threads) threads_is_freed = true; delete current; if (EnableThreadSMRStatistics) {
_java_thread_list_free_cnt++;
_to_delete_list_cnt--;
}
} else {
prev = current;
}
current = next;
}
if (!threads_is_freed) { // Only report "is not freed" on the original call to // free_list() for this ThreadsList.
log_debug(thread, smr)("tid=" UINTX_FORMAT ": ThreadsSMRSupport::free_list: threads=" INTPTR_FORMAT " is not freed.", os::current_thread_id(), p2i(threads));
}
// Return true if the specified JavaThread is protected by a hazard // pointer (ThreadsList reference). Otherwise, returns false. // bool ThreadsSMRSupport::is_a_protected_JavaThread(JavaThread *thread) {
assert_locked_or_safepoint(Threads_lock);
// Gather a hash table of the JavaThreads indirectly referenced by // hazard ptrs.
ThreadScanHashtable *scan_table = new ThreadScanHashtable();
ScanHazardPtrGatherProtectedThreadsClosure scan_cl(scan_table);
threads_do(&scan_cl);
OrderAccess::acquire(); // Must order reads of hazard ptr before reads of // nested reference counters
// Walk through the linked list of pending freeable ThreadsLists // and include the ones that are currently in use by a nested // ThreadsListHandle in the search set.
ThreadsList* current = _to_delete_list; while (current != NULL) { if (current->_nested_handle_cnt != 0) { // 'current' is in use by a nested ThreadsListHandle so the hazard // ptr is protecting all the JavaThreads on that ThreadsList.
AddThreadHazardPointerThreadClosure add_cl(scan_table);
current->threads_do(&add_cl);
}
current = current->next_list();
}
// Wake up portion of the release stable ThreadsList protocol; // uses the delete_lock(). // void ThreadsSMRSupport::release_stable_list_wake_up(bool is_nested) { constchar* log_str = is_nested ? "nested hazard ptr" : "regular hazard ptr";
// Note: delete_lock is held in smr_delete() for the entire // hazard ptr search so that we do not lose this notify() if // the exiting thread has to wait. That code path also holds // Threads_lock (which was grabbed before delete_lock) so that // threads_do() can be called. This means the system can't start a // safepoint which means this thread can't take too long to get to // a safepoint because of being blocked on delete_lock. //
MonitorLocker ml(ThreadsSMRSupport::delete_lock(), Monitor::_no_safepoint_check_flag); if (ThreadsSMRSupport::delete_notify()) { // Notify any exiting JavaThreads that are waiting in smr_delete() // that we've released a ThreadsList.
ml.notify_all();
log_debug(thread, smr)("tid=" UINTX_FORMAT ": ThreadsSMRSupport::release_stable_list notified %s", os::current_thread_id(), log_str);
}
}
void ThreadsSMRSupport::remove_thread(JavaThread *thread) {
ThreadsList *new_list = ThreadsList::remove_thread(ThreadsSMRSupport::get_java_thread_list(), thread); if (EnableThreadSMRStatistics) {
ThreadsSMRSupport::inc_java_thread_list_alloc_cnt(); // This list is smaller so no need to check for a "longest" update.
}
// Final _java_thread_list will not generate a "Threads::remove" mesg.
log_debug(thread, smr)("tid=" UINTX_FORMAT ": Threads::remove: new ThreadsList=" INTPTR_FORMAT, os::current_thread_id(), p2i(new_list));
// See note for clear_delete_notify(). // void ThreadsSMRSupport::set_delete_notify() {
Atomic::inc(&_delete_notify);
}
// Safely delete a JavaThread when it is no longer in use by a // ThreadsListHandle. // void ThreadsSMRSupport::smr_delete(JavaThread *thread) {
elapsedTimer timer; if (EnableThreadSMRStatistics) {
timer.start();
}
while (true) {
{ // Will not make a safepoint check because this JavaThread // is not on the current ThreadsList.
MutexLocker ml(Threads_lock); // Cannot use a MonitorLocker helper here because we have // to drop the Threads_lock first if we wait.
ThreadsSMRSupport::delete_lock()->lock_without_safepoint_check(); // Set the delete_notify flag after we grab delete_lock // and before we scan hazard ptrs because we're doing // double-check locking in release_stable_list().
ThreadsSMRSupport::set_delete_notify();
if (!is_a_protected_JavaThread(thread)) { // This is the common case.
ThreadsSMRSupport::clear_delete_notify();
ThreadsSMRSupport::delete_lock()->unlock(); break;
} if (!has_logged_once) {
has_logged_once = true;
log_debug(thread, smr)("tid=" UINTX_FORMAT ": ThreadsSMRSupport::wait_until_not_protected: thread=" INTPTR_FORMAT " is not deleted.", os::current_thread_id(), p2i(thread)); if (log_is_enabled(Debug, os, thread)) {
ScanHazardPtrPrintMatchingThreadsClosure scan_cl(thread);
threads_do(&scan_cl);
ThreadsList* current = _to_delete_list; while (current != NULL) { if (current->_nested_handle_cnt != 0 && current->includes(thread)) {
log_debug(thread, smr)("tid=" UINTX_FORMAT ": ThreadsSMRSupport::wait_until_not_protected: found nested hazard pointer to thread=" INTPTR_FORMAT, os::current_thread_id(), p2i(thread));
}
current = current->next_list();
}
}
}
} // We have to drop the Threads_lock to wait or delete the thread
if (EnableThreadSMRStatistics) {
_delete_lock_wait_cnt++; if (_delete_lock_wait_cnt > _delete_lock_wait_max) {
_delete_lock_wait_max = _delete_lock_wait_cnt;
}
} // Wait for a release_stable_list() call before we check again. No // safepoint check, no timeout, and not as suspend equivalent flag // because this JavaThread is not on the Threads list.
ThreadsSMRSupport::delete_lock()->wait_without_safepoint_check(); if (EnableThreadSMRStatistics) {
_delete_lock_wait_cnt--;
}
ThreadsSMRSupport::clear_delete_notify();
ThreadsSMRSupport::delete_lock()->unlock(); // Retry the whole scenario.
}
}
// Apply the closure to all threads in the system, with a snapshot of // all JavaThreads provided by the list parameter. void ThreadsSMRSupport::threads_do(ThreadClosure *tc, ThreadsList *list) {
list->threads_do(tc);
Threads::non_java_threads_do(tc);
}
// Apply the closure to all threads in the system. void ThreadsSMRSupport::threads_do(ThreadClosure *tc) {
threads_do(tc, _java_thread_list);
}
// Debug, logging, and printing stuff at the end:
// Print SMR info for a SafeThreadsListPtr to a given output stream. void SafeThreadsListPtr::print_on(outputStream* st) { if (this == _thread->_threads_list_ptr) { // The top level hazard ptr.
st->print(" _threads_hazard_ptr=" INTPTR_FORMAT, p2i(_list));
} else { // Nested hazard ptrs.
st->print(", _nested_threads_hazard_ptr=" INTPTR_FORMAT, p2i(_list));
}
}
// Print SMR info for a thread to a given output stream. void ThreadsSMRSupport::print_info_on(const Thread* thread, outputStream* st) {
ThreadsList* hazard_ptr = thread->get_threads_hazard_ptr(); if (hazard_ptr != NULL) {
st->print(" _threads_hazard_ptr=" INTPTR_FORMAT, p2i(hazard_ptr));
} if (EnableThreadSMRStatistics && thread->_threads_list_ptr != NULL) { // The count is only interesting if we have a _threads_list_ptr.
st->print(", _nested_threads_hazard_ptr_cnt=%u", thread->_nested_threads_hazard_ptr_cnt);
} if (SafepointSynchronize::is_at_safepoint() || Thread::current() == thread) { // It is only safe to walk the list if we're at a safepoint or the // calling thread is walking its own list.
SafeThreadsListPtr* current = thread->_threads_list_ptr; if (current != NULL) { // Skip the top nesting level as it is always printed above.
current = current->previous();
} while (current != NULL) {
current->print_on(st);
current = current->previous();
}
}
}
// Print Threads class SMR info. void ThreadsSMRSupport::print_info_on(outputStream* st) { bool needs_unlock = false; if (Threads_lock->try_lock_without_rank_check()) { // We were able to grab the Threads_lock which makes things safe for // this call, but if we are error reporting, then a nested error // could happen with the Threads_lock held.
needs_unlock = true;
}
ThreadsList* saved_threads_list = NULL;
{
ThreadsListHandle tlh; // make the current ThreadsList safe for reporting
saved_threads_list = tlh.list(); // save for later comparison
if (_to_delete_list != NULL) { if (Threads_lock->owned_by_self()) { // Only safe if we have the Threads_lock.
st->print_cr("_to_delete_list=" INTPTR_FORMAT ", length=%u, elements={",
p2i(_to_delete_list), _to_delete_list->length());
print_info_elements_on(st, _to_delete_list);
st->print_cr("}"); for (ThreadsList *t_list = _to_delete_list->next_list();
t_list != NULL; t_list = t_list->next_list()) {
st->print("next-> " INTPTR_FORMAT ", length=%u, elements={",
p2i(t_list), t_list->length());
print_info_elements_on(st, t_list);
st->print_cr("}");
}
} else {
st->print_cr("_to_delete_list=" INTPTR_FORMAT, p2i(_to_delete_list));
st->print_cr("Skipping _to_delete_list fields and contents for safety.");
}
} if (EnableThreadSMRStatistics) {
st->print_cr("_java_thread_list_alloc_cnt=" UINT64_FORMAT ", " "_java_thread_list_free_cnt=" UINT64_FORMAT ", " "_java_thread_list_max=%u, " "_nested_thread_list_max=%u",
_java_thread_list_alloc_cnt,
_java_thread_list_free_cnt,
_java_thread_list_max,
_nested_thread_list_max); if (_tlh_cnt > 0) {
st->print_cr("_tlh_cnt=%u" ", _tlh_times=%u" ", avg_tlh_time=%0.2f" ", _tlh_time_max=%u",
_tlh_cnt, _tlh_times,
((double) _tlh_times / _tlh_cnt),
_tlh_time_max);
} if (_deleted_thread_cnt > 0) {
st->print_cr("_deleted_thread_cnt=%u" ", _deleted_thread_times=%u" ", avg_deleted_thread_time=%0.2f" ", _deleted_thread_time_max=%u",
_deleted_thread_cnt, _deleted_thread_times,
((double) _deleted_thread_times / _deleted_thread_cnt),
_deleted_thread_time_max);
}
st->print_cr("_delete_lock_wait_cnt=%u, _delete_lock_wait_max=%u",
_delete_lock_wait_cnt, _delete_lock_wait_max);
st->print_cr("_to_delete_list_cnt=%u, _to_delete_list_max=%u",
_to_delete_list_cnt, _to_delete_list_max);
} if (needs_unlock) {
Threads_lock->unlock();
} else { if (_java_thread_list != saved_threads_list) {
st->print_cr("The _java_thread_list has changed from " INTPTR_FORMAT " to " INTPTR_FORMAT " so some of the above information may be stale.",
p2i(saved_threads_list), p2i(_java_thread_list));
}
}
}
// Print ThreadsList elements (4 per line). void ThreadsSMRSupport::print_info_elements_on(outputStream* st, ThreadsList* t_list) {
uint cnt = 0;
JavaThreadIterator jti(t_list); for (JavaThread *jt = jti.first(); jt != NULL; jt = jti.next()) {
st->print(INTPTR_FORMAT, p2i(jt)); if (cnt < t_list->length() - 1) { // Separate with comma or comma-space except for the last one. if (((cnt + 1) % 4) == 0) { // Four INTPTR_FORMAT fit on an 80 column line so end the // current line with just a comma.
st->print_cr(",");
} else { // Not the last one on the current line so use comma-space:
st->print(", ");
}
} else { // Last one so just end the current line.
st->cr();
}
cnt++;
}
}
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