本文为死磕Synchronized底层实现第三篇文章，内容为轻量级锁实现。
轻量级锁并不复杂，其中很多内容在偏向锁一文中已提及过，与本文内容会有部分重叠。
另外轻量级锁的背景和基本流程在概论中已有讲解。强烈建议在看过两篇文章的基础下阅读本文。
本系列文章将对HotSpot的synchronized锁实现进行全面分析，内容包括偏向锁、轻量级锁、重量级锁的加锁、解锁、锁升级流程的原理及源码分析，希望给在研究synchronized路上的同学一些帮助。
本文分为两个部分：
1.轻量级锁获取流程
2.轻量级锁释放流程
本人看的JVM版本是jdk8u。
轻量级锁获取流程
下面开始轻量级锁获取流程分析，代码在bytecodeInterpreter.cpp#1816。
<pre style="box-sizing: inherit; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; font-size: 13px; word-break: initial; word-wrap: initial; white-space: pre; overflow: auto; margin: 16px 0px 14px; padding: 14px 15px 12px; border-radius: 3px; border: none; display: block; line-height: 1.6; background: rgb(246, 246, 246); color: rgb(61, 70, 77); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">CASE(_monitorenter): {
oop lockee = STACK_OBJECT(-1);
...
if (entry != NULL) {
...
// 上面省略的代码中如果CAS操作失败也会调用到InterpreterRuntime::monitorenter
// traditional lightweight locking
if (!success) {
// 构建一个无锁状态的Displaced Mark Word
markOop displaced = lockee->mark()->set_unlocked();
// 设置到Lock Record中去
entry->lock()->set_displaced_header(displaced);
bool call_vm = UseHeavyMonitors;
if (call_vm || Atomic::cmpxchg_ptr(entry, lockee->mark_addr(), displaced) != displaced) {
// 如果CAS替换不成功，代表锁对象不是无锁状态，这时候判断下是不是锁重入
// Is it simple recursive case?
if (!call_vm && THREAD->is_lock_owned((address) displaced->clear_lock_bits())) {
entry->lock()->set_displaced_header(NULL);
} else {
// CAS操作失败则调用monitorenter
CALL_VM(InterpreterRuntime::monitorenter(THREAD, entry), handle_exception);
}
}
}
UPDATE_PC_AND_TOS_AND_CONTINUE(1, -1);
} else {
istate->set_msg(more_monitors);
UPDATE_PC_AND_RETURN(0); // Re-execute
}
}</pre>
如果锁对象不是偏向模式或已经偏向其他线程，则success为false。这时候会构建一个无锁状态的mark word设置到Lock Record中去，我们称Lock Record中存储对象mark word的字段叫Displaced Mark Word。
如果当前锁的状态不是无锁状态，则CAS失败。如果这是一次锁重入，那直接将Lock Record的 Displaced Mark Word设置为null。
我们看个demo，在该demo中重复3次获得锁，
<pre style="box-sizing: inherit; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; font-size: 13px; word-break: initial; word-wrap: initial; white-space: pre; overflow: auto; margin: 16px 0px 14px; padding: 14px 15px 12px; border-radius: 3px; border: none; display: block; line-height: 1.6; background: rgb(246, 246, 246); color: rgb(61, 70, 77); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">synchronized(obj){
synchronized(obj){
synchronized(obj){
}
}
}</pre>
假设锁的状态是轻量级锁，下图反应了mark word和线程栈中Lock Record的状态，可以看到右边线程栈中包含3个指向当前锁对象的Lock Record。其中栈中最高位的Lock Record为第一次获取锁时分配的。其Displaced Mark word的值为锁对象的加锁前的mark word，之后的锁重入会在线程栈中分配一个Displaced Mark word为null的Lock Record。
《死磕Synchronized底层实现--轻量级锁》
为什么JVM选择在线程栈中添加Displaced Mark word为null的Lock Record来表示重入计数呢？首先锁重入次数是一定要记录下来的，因为每次解锁都需要对应一次加锁，解锁次数等于加锁次数时，该锁才真正的被释放，也就是在解锁时需要用到说锁重入次数的。一个简单的方案是将锁重入次数记录在对象头的mark word中，但mark word的大小是有限的，已经存放不下该信息了。另一个方案是只创建一个Lock Record并在其中记录重入次数，Hotspot没有这样做的原因我猜是考虑到效率有影响：每次重入获得锁都需要遍历该线程的栈找到对应的Lock Record，然后修改它的值。
所以最终Hotspot选择每次获得锁都添加一个Lock Record来表示锁的重入。
接下来看看InterpreterRuntime::monitorenter方法
<pre style="box-sizing: inherit; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; font-size: 13px; word-break: initial; word-wrap: initial; white-space: pre; overflow: auto; margin: 16px 0px 14px; padding: 14px 15px 12px; border-radius: 3px; border: none; display: block; line-height: 1.6; background: rgb(246, 246, 246); color: rgb(61, 70, 77); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">IRT_ENTRY_NO_ASYNC(void, InterpreterRuntime::monitorenter(JavaThread* thread, BasicObjectLock* elem))
...
Handle h_obj(thread, elem->obj());
assert(Universe::heap()->is_in_reserved_or_null(h_obj()),
"must be NULL or an object");
if (UseBiasedLocking) {
// Retry fast entry if bias is revoked to avoid unnecessary inflation
ObjectSynchronizer::fast_enter(h_obj, elem->lock(), true, CHECK);
} else {
ObjectSynchronizer::slow_enter(h_obj, elem->lock(), CHECK);
}
...
IRT_END</pre>
fast_enter的流程在偏向锁一文已经分析过，如果当前是偏向模式且偏向的线程还在使用锁，那会将锁的mark word改为轻量级锁的状态，同时会将偏向的线程栈中的Lock Record修改为轻量级锁对应的形式。代码位置在biasedLocking.cpp#212。
<pre style="box-sizing: inherit; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; font-size: 13px; word-break: initial; word-wrap: initial; white-space: pre; overflow: auto; margin: 16px 0px 14px; padding: 14px 15px 12px; border-radius: 3px; border: none; display: block; line-height: 1.6; background: rgb(246, 246, 246); color: rgb(61, 70, 77); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">// 线程还存活则遍历线程栈中所有的Lock Record
GrowableArray<MonitorInfo> cached_monitor_info = get_or_compute_monitor_info(biased_thread);
BasicLock* highest_lock = NULL;
for (int i = 0; i < cached_monitor_info->length(); i++) {
MonitorInfo* mon_info = cached_monitor_info->at(i);
// 如果能找到对应的Lock Record说明偏向的线程还在执行同步代码块中的代码
if (mon_info->owner() == obj) {
...
// 需要升级为轻量级锁，直接修改偏向线程栈中的Lock Record。为了处理锁重入的case，在这里将Lock Record的Displaced Mark Word设置为null，第一个Lock Record会在下面的代码中再处理
markOop mark = markOopDesc::encode((BasicLock*) NULL);
highest_lock = mon_info->lock();
highest_lock->set_displaced_header(mark);
} else {
...
}
}
if (highest_lock != NULL) {
// 修改第一个Lock Record为无锁状态，然后将obj的mark word设置为指向该Lock Record的指针
highest_lock->set_displaced_header(unbiased_prototype);
obj->release_set_mark(markOopDesc::encode(highest_lock));
...
} else {
...
}</pre>
我们看slow_enter的流程。
<pre style="box-sizing: inherit; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; font-size: 13px; word-break: initial; word-wrap: initial; white-space: pre; overflow: auto; margin: 16px 0px 14px; padding: 14px 15px 12px; border-radius: 3px; border: none; display: block; line-height: 1.6; background: rgb(246, 246, 246); color: rgb(61, 70, 77); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">void ObjectSynchronizer::slow_enter(Handle obj, BasicLock* lock, TRAPS) {
markOop mark = obj->mark();
assert(!mark->has_bias_pattern(), "should not see bias pattern here");
// 如果是无锁状态
if (mark->is_neutral()) {
//设置Displaced Mark Word并替换对象头的mark word
lock->set_displaced_header(mark);
if (mark == (markOop) Atomic::cmpxchg_ptr(lock, obj()->mark_addr(), mark)) {
TEVENT (slow_enter: release stacklock) ;
return ;
}
} else
if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
assert(lock != mark->locker(), "must not re-lock the same lock");
assert(lock != (BasicLock*)obj->mark(), "don't relock with same BasicLock");
// 如果是重入，则设置Displaced Mark Word为null
lock->set_displaced_header(NULL);
return;
}
...
// 走到这一步说明已经是存在多个线程竞争锁了 需要膨胀为重量级锁
lock->set_displaced_header(markOopDesc::unused_mark());
ObjectSynchronizer::inflate(THREAD, obj())->enter(THREAD);
}</pre>
轻量级锁释放流程
<pre style="box-sizing: inherit; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; font-size: 13px; word-break: initial; word-wrap: initial; white-space: pre; overflow: auto; margin: 16px 0px 14px; padding: 14px 15px 12px; border-radius: 3px; border: none; display: block; line-height: 1.6; background: rgb(246, 246, 246); color: rgb(61, 70, 77); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">CASE(_monitorexit): {
oop lockee = STACK_OBJECT(-1);
CHECK_NULL(lockee);
// derefing's lockee ought to provoke implicit null check
// find our monitor slot
BasicObjectLock* limit = istate->monitor_base();
BasicObjectLock* most_recent = (BasicObjectLock) istate->stack_base();
// 从低往高遍历栈的Lock Record
while (most_recent != limit ) {
// 如果Lock Record关联的是该锁对象
if ((most_recent)->obj() == lockee) {
BasicLock lock = most_recent->lock();
markOop header = lock->displaced_header();
// 释放Lock Record
most_recent->set_obj(NULL);
// 如果是偏向模式，仅仅释放Lock Record就好了。否则要走轻量级锁or重量级锁的释放流程
if (!lockee->mark()->has_bias_pattern()) {
bool call_vm = UseHeavyMonitors;
// header!=NULL说明不是重入，则需要将Displaced Mark Word CAS到对象头的Mark Word
if (header != NULL || call_vm) {
if (call_vm || Atomic::cmpxchg_ptr(header, lockee->mark_addr(), lock) != lock) {
// CAS失败或者是重量级锁则会走到这里，先将obj还原，然后调用monitorexit方法
most_recent->set_obj(lockee);
CALL_VM(InterpreterRuntime::monitorexit(THREAD, most_recent), handle_exception);
}
}
}
//执行下一条命令
UPDATE_PC_AND_TOS_AND_CONTINUE(1, -1);
}
//处理下一条Lock Record
most_recent++;
}
// Need to throw illegal monitor state exception
CALL_VM(InterpreterRuntime::throw_illegal_monitor_state_exception(THREAD), handle_exception);
ShouldNotReachHere();
}</pre>
轻量级锁释放时需要将Displaced Mark Word替换到对象头的mark word中。如果CAS失败或者是重量级锁则进入到InterpreterRuntime::monitorexit方法中。
<pre style="box-sizing: inherit; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; font-size: 13px; word-break: initial; word-wrap: initial; white-space: pre; overflow: auto; margin: 16px 0px 14px; padding: 14px 15px 12px; border-radius: 3px; border: none; display: block; line-height: 1.6; background: rgb(246, 246, 246); color: rgb(61, 70, 77); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">//%note monitor_1
IRT_ENTRY_NO_ASYNC(void, InterpreterRuntime::monitorexit(JavaThread* thread, BasicObjectLock* elem))
Handle h_obj(thread, elem->obj());
...
ObjectSynchronizer::slow_exit(h_obj(), elem->lock(), thread);
// Free entry. This must be done here, since a pending exception might be installed on
//释放Lock Record
elem->set_obj(NULL);
...
IRT_END</pre>
monitorexit调用完slow_exit方法后,就释放Lock Record。
<pre style="box-sizing: inherit; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; font-size: 13px; word-break: initial; word-wrap: initial; white-space: pre; overflow: auto; margin: 16px 0px 14px; padding: 14px 15px 12px; border-radius: 3px; border: none; display: block; line-height: 1.6; background: rgb(246, 246, 246); color: rgb(61, 70, 77); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">void ObjectSynchronizer::slow_exit(oop object, BasicLock* lock, TRAPS) {
fast_exit (object, lock, THREAD) ;
}
void ObjectSynchronizer::fast_exit(oop object, BasicLock* lock, TRAPS) {
...
markOop dhw = lock->displaced_header();
markOop mark ;
if (dhw == NULL) {
// 重入锁，什么也不做
...
return ;
}
mark = object->mark() ;
// 如果是mark word==Displaced Mark Word即轻量级锁，CAS替换对象头的mark word
if (mark == (markOop) lock) {
assert (dhw->is_neutral(), "invariant") ;
if ((markOop) Atomic::cmpxchg_ptr (dhw, object->mark_addr(), mark) == mark) {
TEVENT (fast_exit: release stacklock) ;
return;
}
}
//走到这里说明是重量级锁或者解锁时发生了竞争，膨胀后调用重量级锁的exit方法。
ObjectSynchronizer::inflate(THREAD, object)->exit (true, THREAD) ;
}</pre>
该方法中先判断是不是轻量级锁，如果是轻量级锁则将替换mark word，否则膨胀为重量级锁并调用exit方法，相关逻辑将在重量级锁的文章中讲解。
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