Tutorial :synchronized section does not block!



Question:

I've noticed something very strange yesterday. It seems that two threads are entering two synchronized blocks locking on the same object at the same time.

The class (MyClass) containing the relevant code looks similar to this:

private static int[]    myLock  = new int[0];    protected static int methodA(final long handle, final byte[] sort) {      synchronized (myLock) {          return xsMethodA(handle, sort);      }  }    protected static int methodB(final long handle) {      synchronized (myLock) {          return xsMethodB(handle);      }  }  

I created a thread dump of my application running the above class and was very surprised as I saw this:

"http-8080-136" daemon prio=10 tid=0x00000000447df000 nid=0x70ed waiting for monitor entry [0x00007fd862aea000]     java.lang.Thread.State: BLOCKED (on object monitor)      at com.MyClass.methodA(MyClass.java:750)      - locked <0x00007fd8a6b8c790> (a [I)      at com.SomeOtherClass.otherMethod(SomeOtherClass.java:226)      ...    "http-8080-111" daemon prio=10 tid=0x00007fd87d1a0000 nid=0x70c8 waiting for monitor entry [0x00007fd86e15f000]     java.lang.Thread.State: BLOCKED (on object monitor)      at com.MyClass.methodB(MyClass.java:991)      - locked <0x00007fd8a6b8c790> (a [I)      at com.SomeOtherClass.yetAnotherMethod(SomeOtherClass.java:3231)      ...  

(I changed the class and method names for the case of simplicity, so don't get confused by the silly names.)

It seems that thread http-8080-136 and http-8080-111 have both acquired the lock on myLock. It is the same object as the object address is the same: 0x00007fd8a6b8c790. The Java Runtime Specification says this about the synchronized keyword:

A synchronized statement acquires a mutual-exclusion lock (§17.1) on behalf of the executing thread, executes a block, then releases the lock. While the executing thread owns the lock, no other thread may acquire the lock. [The Java Language Specification, 14.19]

So how is this even possible?

There are another 44 threads in the thread dump "waiting" for the lock. This is how it looks like if a thread is waiting:

"http-8080-146" daemon prio=10 tid=0x00007fd786dab000 nid=0x184b waiting for monitor entry [0x00007fd8393b6000]     java.lang.Thread.State: BLOCKED (on object monitor)      at com.MyClass.methodC(MyClass.java:750)      - waiting to lock <0x00007fd8a6b8c790> (a [I)      at com.SomeOtherClass.yetAnoterMethod2(SomeOtherClass.java:226)  


Solution:1

I've asked the same question on the hotspot-dev mailing list and received a very goot answer from Christopher Phillips:


Hi Eduard

I think its the thread dump that is misleading.

If you really think that the 2 are in the lock simultaneously you should probably get a gcore (which is externally consistent).

The state you see "waiting for monitor entry" is actually MONITOR_WAIT which can represent the following code before actual acquisition of a hot lock : (also see OSThreadContendState in osThread.hpp) called from: src/share/vm/runtime/synchronizer.cpp

3413      OSThreadContendState osts(Self->osthread());  3414      ThreadBlockInVM tbivm(jt);  3415  3416      Self->set_current_pending_monitor(this);  3417  3418      // TODO-FIXME: change the following for(;;) loop to straight-line code.  3419      for (;;) {  3420        jt->set_suspend_equivalent();  3421        // cleared by handle_special_suspend_equivalent_condition()  3422        // or java_suspend_self()  3423  3424        EnterI (THREAD) ;  3425  3426        if (!ExitSuspendEquivalent(jt)) break ;  3427  3428        //  3429        // We have acquired the contended monitor, but while we were  3430        // waiting another thread suspended us. We don't want to enter  3431        // the monitor while suspended because that would surprise the  3432        // thread that suspended us.  

Chris


Solution:2

How was the thread dump taken? If the threads were not paused the lock ownership could have changed between dumping one thread and the next.


Solution:3

I think the relevant information is: "waiting for monitor entry", which is the same for both threads. Since both threads (in the thread dump) are marked deamon threads, I guess there must also be a main-thread running at the same time. Is it possible that the main-thread is the current monitor owner who blocks the other two threads?


Solution:4

They haven't acquired lock, otherwise you would see xsMethodA or xsMethodB in a stacktrace.


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