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/*
* Copyright (c) 2017, 2018, 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.
*
*/
#include "precompiled.hpp"
#include "logging/logStream.hpp"
#include "memory/allocation.inline.hpp"
#include "runtime/jniHandles.inline.hpp"
#include "runtime/thread.inline.hpp"
#include "runtime/threadSMR.inline.hpp"
#include "runtime/vm_operations.hpp"
#include "services/threadService.hpp"
#include "utilities/copy.hpp"
#include "utilities/globalDefinitions.hpp"
#include "utilities/ostream.hpp"
#include "utilities/resourceHash.hpp"
#include "utilities/vmError.hpp"
Monitor* ThreadsSMRSupport::_delete_lock =
new Monitor(Monitor::special, "Thread_SMR_delete_lock",
false /* allow_vm_block */,
Monitor::_safepoint_check_never);
// 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;
ThreadsList* volatile ThreadsSMRSupport::_java_thread_list = new ThreadsList(0);
// # 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;
ThreadsList* ThreadsSMRSupport::_to_delete_list = NULL;
// # 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:
inline void ThreadsSMRSupport::add_deleted_thread_times(uint add_value) {
Atomic::add(add_value, &_deleted_thread_times);
}
inline void ThreadsSMRSupport::inc_deleted_thread_cnt() {
Atomic::inc(&_deleted_thread_cnt);
}
inline void ThreadsSMRSupport::inc_java_thread_list_alloc_cnt() {
_java_thread_list_alloc_cnt++;
}
inline void 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(new_value, &_deleted_thread_time_max, cur_value) == cur_value) {
// Updated max value so we're done. Otherwise try it all again.
break;
}
}
}
inline void ThreadsSMRSupport::update_java_thread_list_max(uint new_value) {
if (new_value > _java_thread_list_max) {
_java_thread_list_max = new_value;
}
}
inline ThreadsList* ThreadsSMRSupport::xchg_java_thread_list(ThreadsList* new_list) {
return (ThreadsList*)Atomic::xchg(new_list, &_java_thread_list);
}
// 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:
static bool ptr_equals(void * const& s1, void * const& s2) {
return s1 == s2;
}
static unsigned int ptr_hash(void * const& s1) {
// 2654435761 = 2^32 * Phi (golden ratio)
return (unsigned int)(((uint32_t)(uintptr_t)s1) * 2654435761u);
}
int _table_size;
// ResourceHashtable SIZE is specified at compile time so our
// dynamic _table_size is unused for now; 1031 is the first prime
// after 1024.
typedef ResourceHashtable<void *, int, &ThreadScanHashtable::ptr_hash,
&ThreadScanHashtable::ptr_equals, 1031,
ResourceObj::C_HEAP, mtThread> 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(int table_size) : _table_size(table_size), _ptrs(new (ResourceObj::C_HEAP, mtThread) PtrTable()) {}
~ThreadScanHashtable() { delete _ptrs; }
bool has_entry(void *pointer) {
int *val_ptr = _ptrs->get(pointer);
return val_ptr != NULL && *val_ptr == 1;
}
void add_entry(void *pointer) {
_ptrs->put(pointer, 1);
}
};
// 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;
public:
AddThreadHazardPointerThreadClosure(ThreadScanHashtable *table) : _table(table) {}
virtual void 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) {}
virtual void do_thread(Thread *thread) {
assert_locked_or_safepoint(Threads_lock);
if (thread == NULL) return;
// 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;
}
// 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.
//
class ScanHazardPtrGatherThreadsListClosure : public ThreadClosure {
private:
ThreadScanHashtable *_table;
public:
ScanHazardPtrGatherThreadsListClosure(ThreadScanHashtable *table) : _table(table) {}
virtual void do_thread(Thread* thread) {
assert_locked_or_safepoint(Threads_lock);
if (thread == NULL) return;
ThreadsList *threads = thread->get_threads_hazard_ptr();
if (threads == NULL) {
return;
}
// 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.
threads = Thread::untag_hazard_ptr(threads);
if (!_table->has_entry((void*)threads)) {
_table->add_entry((void*)threads);
}
}
};
// 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) {}
virtual void do_thread(Thread *thread) {
assert_locked_or_safepoint(Threads_lock);
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 determine if the specified JavaThread is found by
// threads_do().
//
class VerifyHazardPtrThreadClosure : public ThreadClosure {
private:
bool _found;
Thread *_self;
public:
VerifyHazardPtrThreadClosure(Thread *self) : _found(false), _self(self) {}
bool found() const { return _found; }
virtual void do_thread(Thread *thread) {
if (thread == _self) {
_found = true;
}
}
};
// Acquire a stable ThreadsList.
//
void SafeThreadsListPtr::acquire_stable_list() {
assert(_thread != NULL, "sanity check");
_needs_release = true;
_previous = _thread->_threads_list_ptr;
_thread->_threads_list_ptr = this;
if (_thread->get_threads_hazard_ptr() == 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 _smr_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 _smr_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");
assert(_thread->get_threads_hazard_ptr() != NULL,
"cannot have a NULL regular hazard ptr when acquiring a nested hazard ptr");
// 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();
}
current_list->inc_nested_handle_cnt();
_previous->_has_ref_count = true; // promote SafeThreadsListPtr to be reference counted
_thread->_threads_hazard_ptr = NULL; // clear the hazard ptr so we can go through the fast path below
if (EnableThreadSMRStatistics && _thread->nested_threads_hazard_ptr_cnt() > ThreadsSMRSupport::_nested_thread_list_max) {
ThreadsSMRSupport::_nested_thread_list_max = _thread->nested_threads_hazard_ptr_cnt();
}
acquire_stable_list_fast_path();
verify_hazard_ptr_scanned();
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));
}
// Release a stable ThreadsList.
//
void SafeThreadsListPtr::release_stable_list() {
assert(_thread != NULL, "sanity check");
assert(_thread->_threads_list_ptr == this, "sanity check");
_thread->_threads_list_ptr = _previous;
if (_has_ref_count) {
// If a SafeThreadsListPtr has been promoted to use reference counting
// due to nesting of ThreadsListHandles, then the reference count must be
// decremented, at which point it may be freed. The forgotten value of
// the list no longer matters at this point and should already be NULL.
assert(_thread->get_threads_hazard_ptr() == NULL, "sanity check");
if (EnableThreadSMRStatistics) {
_thread->dec_nested_threads_hazard_ptr_cnt();
}
_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));
} else {
// The normal case: a leaf ThreadsListHandle. This merely requires setting
// the thread hazard ptr back to NULL.
assert(_thread->get_threads_hazard_ptr() != NULL, "sanity check");
_thread->set_threads_hazard_ptr(NULL);
}
// 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);
}
}
// 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");
// 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.
//
// Note: 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.
//
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() || _thread == VM_Exit::shutdown_thread(), "Acquired a ThreadsList snapshot from a thread not recognized by the Thread-SMR protocol.");
#endif
}
// 'entries + 1' so we always have at least one entry.
ThreadsList::ThreadsList(int entries) :
_length(entries),
_next_list(NULL),
_threads(NEW_C_HEAP_ARRAY(JavaThread*, entries + 1, mtThread)),
_nested_handle_cnt(0)
{
*(JavaThread**)(_threads + entries) = NULL; // Make sure the extra entry is NULL.
}
ThreadsList::~ThreadsList() {
FREE_C_HEAP_ARRAY(JavaThread*, _threads);
}
// 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);
if (head_length > 0) {
Copy::disjoint_words((HeapWord*)list->_threads, (HeapWord*)new_list->_threads, head_length);
}
*(JavaThread**)(new_list->_threads + index) = java_thread;
return new_list;
}
void ThreadsList::dec_nested_handle_cnt() {
// The decrement needs to be MO_ACQ_REL. At the moment, the Atomic::dec
// backend on PPC does not yet conform to these requirements. Therefore
// the decrement is simulated with an Atomic::sub(1, &addr).
// Without this MO_ACQ_REL Atomic::dec simulation, the nested SMR mechanism
// is not generally safe to use.
Atomic::sub(1, &_nested_handle_cnt);
}
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 {
for (uint i = 0; i < length(); i++) {
JavaThread* 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 && !thread->is_exiting() &&
java_tid == java_lang_Thread::thread_id(tobj)) {
// found a match
return thread;
}
}
return NULL;
}
void ThreadsList::inc_nested_handle_cnt() {
// The increment needs to be MO_SEQ_CST. At the moment, the Atomic::inc
// backend on PPC does not yet conform to these requirements. Therefore
// the increment is simulated with a load phi; cas phi + 1; loop.
// Without this MO_SEQ_CST Atomic::inc simulation, the nested SMR mechanism
// is not generally safe to use.
intx sample = OrderAccess::load_acquire(&_nested_handle_cnt);
for (;;) {
if (Atomic::cmpxchg(sample + 1, &_nested_handle_cnt, sample) == sample) {
return;
} else {
sample = OrderAccess::load_acquire(&_nested_handle_cnt);
}
}
}
bool ThreadsList::includes(const JavaThread * const p) const {
if (p == NULL) {
return false;
}
for (uint i = 0; i < length(); i++) {
if (thread_at(i) == p) {
return true;
}
}
return false;
}
// 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);
}
return new_list;
}
ThreadsListHandle::ThreadsListHandle(Thread *self) : _list_ptr(self, /* acquire */ true) {
assert(self == Thread::current(), "sanity check");
if (EnableThreadSMRStatistics) {
_timer.start();
}
}
ThreadsListHandle::~ThreadsListHandle() {
if (EnableThreadSMRStatistics) {
_timer.stop();
uint millis = (uint)_timer.milliseconds();
ThreadsSMRSupport::update_tlh_stats(millis);
}
}
// 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.
return false;
}
// 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.
return false;
}
}
// Return a live JavaThread that is "protected" by the
// ThreadsListHandle in the caller.
*jt_pp = java_thread;
return true;
}
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);
}
// 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 (OrderAccess::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);
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;
}
}
// Hash table size should be first power of two higher than twice the length of the ThreadsList
int hash_table_size = MIN2((int)get_java_thread_list()->length(), 32) << 1;
hash_table_size--;
hash_table_size |= hash_table_size >> 1;
hash_table_size |= hash_table_size >> 2;
hash_table_size |= hash_table_size >> 4;
hash_table_size |= hash_table_size >> 8;
hash_table_size |= hash_table_size >> 16;
hash_table_size++;
// Gather a hash table of the current hazard ptrs:
ThreadScanHashtable *scan_table = new ThreadScanHashtable(hash_table_size);
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));
}
delete scan_table;
}
// 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);
// Hash table size should be first power of two higher than twice
// the length of the Threads list.
int hash_table_size = MIN2((int)get_java_thread_list()->length(), 32) << 1;
hash_table_size--;
hash_table_size |= hash_table_size >> 1;
hash_table_size |= hash_table_size >> 2;
hash_table_size |= hash_table_size >> 4;
hash_table_size |= hash_table_size >> 8;
hash_table_size |= hash_table_size >> 16;
hash_table_size++;
// Gather a hash table of the JavaThreads indirectly referenced by
// hazard ptrs.
ThreadScanHashtable *scan_table = new ThreadScanHashtable(hash_table_size);
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();
}
bool thread_is_protected = false;
if (scan_table->has_entry((void*)thread)) {
thread_is_protected = true;
}
delete scan_table;
return thread_is_protected;
}
// Wake up portion of the release stable ThreadsList protocol;
// uses the delete_lock().
//
void ThreadsSMRSupport::release_stable_list_wake_up(bool is_nested) {
const char* 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.
//
MonitorLockerEx 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));
ThreadsList *old_list = ThreadsSMRSupport::xchg_java_thread_list(new_list);
ThreadsSMRSupport::free_list(old_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) {
assert(!Threads_lock->owned_by_self(), "sanity");
bool has_logged_once = false;
elapsedTimer timer;
if (EnableThreadSMRStatistics) {
timer.start();
}
while (true) {
{
// No safepoint check because this JavaThread is not on the
// Threads list.
MutexLockerEx ml(Threads_lock, Mutex::_no_safepoint_check_flag);
// Cannot use a MonitorLockerEx 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::smr_delete: 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::smr_delete: 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(Mutex::_no_safepoint_check_flag, 0,
!Mutex::_as_suspend_equivalent_flag);
if (EnableThreadSMRStatistics) {
_delete_lock_wait_cnt--;
}
ThreadsSMRSupport::clear_delete_notify();
ThreadsSMRSupport::delete_lock()->unlock();
// Retry the whole scenario.
}
if (ThreadLocalHandshakes) {
// The thread is about to be deleted so cancel any handshake.
thread->cancel_handshake();
}
delete thread;
if (EnableThreadSMRStatistics) {
timer.stop();
uint millis = (uint)timer.milliseconds();
ThreadsSMRSupport::inc_deleted_thread_cnt();
ThreadsSMRSupport::add_deleted_thread_times(millis);
ThreadsSMRSupport::update_deleted_thread_time_max(millis);
}
log_debug(thread, smr)("tid=" UINTX_FORMAT ": ThreadsSMRSupport::smr_delete: thread=" INTPTR_FORMAT " is deleted.", os::current_thread_id(), p2i(thread));
}
// 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));
}
}
// Log Threads class SMR info.
void ThreadsSMRSupport::log_statistics() {
LogTarget(Info, thread, smr) log;
if (log.is_enabled()) {
LogStream out(log);
print_info_on(&out);
}
}
// Print SMR info for a thread to a given output stream.
void ThreadsSMRSupport::print_info_on(const Thread* thread, outputStream* st) {
if (thread->_threads_hazard_ptr != NULL) {
st->print(" _threads_hazard_ptr=" INTPTR_FORMAT, p2i(thread->_threads_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) {
// Only grab the Threads_lock if we don't already own it and if we
// are not reporting an error.
// Note: Not grabbing the Threads_lock during error reporting is
// dangerous because the data structures we want to print can be
// freed concurrently. However, grabbing the Threads_lock during
// error reporting can be equally dangerous since this thread might
// block during error reporting or a nested error could leave the
// Threads_lock held. The classic no win scenario.
//
MutexLockerEx ml((Threads_lock->owned_by_self() || VMError::is_error_reported()) ? NULL : Threads_lock);
st->print_cr("Threads class SMR info:");
st->print_cr("_java_thread_list=" INTPTR_FORMAT ", length=%u, "
"elements={", p2i(_java_thread_list),
_java_thread_list->length());
print_info_elements_on(st, _java_thread_list);
st->print_cr("}");
if (_to_delete_list != NULL) {
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("}");
}
}
if (!EnableThreadSMRStatistics) {
return;
}
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);
}
// 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++;
}
}