how to bring jvm to safepoint

[David Holmes]

Tom Rodriguez said the following on 07/31/10 05:01:
> Actually I think the problem is that the thread is in thread_in_Java
> state. I don't believe you can safepoint from that state, only
> thread_in_vm.

The reported failure:

#  Error: Possible safepoint reached by thread that does not allow it

is here:

void Thread::check_for_valid_safepoint_state(bool potential_vm_operation) {
     // Check if current thread is allowed to block at a safepoint
     if (!(_allow_safepoint_count == 0))
       fatal("Possible safepoint reached by thread that does not allow it");

as far as I can see the only code that touches _allow_safepoint_count 
are the No_Safepoint_Verifier classes.

Also the stack shows:

V  [libjvm.so+0x7ee63d]
j  sun.misc.Unsafe.park(ZJ)V+0

and so we should have transitioned to _thread_in_vm here.

That said, Tony also stated:

 > The reason for the fail is that
 > Thread::check_for_valid_safepoint_state() didn't pass because
 > the currentThread is not a VMThread.

But that is not supported by the error produced.

Tony: why did you make the above comment?

Either way we need to see a decoded stack to get an idea of what is 
going wrong.

Cheers,
David

> 
> tom
> 
> On Jul 30, 2010, at 12:04 AM, David Holmes <David.Holmes at oracle.com>
> wrote:
> 
>> Sorry Tony I keep forgetting that I don't have a way to decode a
>> crash log from a VM that you built yourself. :( However you may be
>> able to decode it yourself please see:
>> 
>> http://blogs.sun.com/dave/entry/a_tool_to_decode_hs
>> 
>> for a perl script.
>> 
>> That said looking at the actual assertion failure I am guessing
>> that you initiated the VM operation from code where a
>> No_Safepoint_verifier is active.
>> 
>> David
>> 
>> Tony Guan said the following on 07/30/10 15:58:
>>> Hi David, Thanks a lot! I am copying the content to below, and
>>> for convenience, the full log file is attached. cat
>>> hs_err_pid30894.log # # A fatal error has been detected by the
>>> Java Runtime Environment: # #  Internal Error 
>>> (/home/tony/software/OpenJDK/jdk7/hotspot/src/share/vm/runtime/thread.cpp:777),
>>>  pid=30894, tid=1082374480 #  Error: Possible safepoint reached
>>> by thread that does not allow it # # JRE version: 7.0 # Java VM:
>>> OpenJDK 64-Bit Server VM (17.0-b07291505-internal-debug mixed
>>> mode linux-amd64 ) # If you would like to submit a bug report,
>>> please visit: #   http://java.sun.com/webapps/bugreport/crash.jsp
>>>  # ---------------  T H R E A D  --------------- Current thread
>>> (0x00000000012ed000):  JavaThread "RMI TCP Connection(idle)"
>>> daemon [_thread_in_Java, id=30929, 
>>> stack(0x000000004073b000,0x000000004083c000)] Stack:
>>> [0x000000004073b000,0x000000004083c000], sp=0x0000000040839d60,
>>> free space=3fb0000000000000000k Native frames: (J=compiled Java
>>> code, j=interpreted, Vv=VM code, C=native code) V
>>> [libjvm.so+0x8cad18] V  [libjvm.so+0x8cbe38] V
>>> [libjvm.so+0x4039f1] V  [libjvm.so+0x87a183] V
>>> [libjvm.so+0x8e1127] V  [libjvm.so+0x4d52ab] V
>>> [libjvm.so+0x7ee63d] j  sun.misc.Unsafe.park(ZJ)V+0 j
>>> java.util.concurrent.locks.LockSupport.parkNanos(Ljava/lang/Object;J)V+20
>>>  j
>>> java.util.concurrent.SynchronousQueue$TransferStack.awaitFulfill(Ljava/util/concurrent/SynchronousQueue$TransferStack$SNode;ZJ)Ljava/util/concurrent/SynchronousQueue$TransferStack$SNode;+174
>>>  j
>>> java.util.concurrent.SynchronousQueue$TransferStack.transfer(Ljava/lang/Object;ZJ)Ljava/lang/Object;+102
>>>  j
>>> java.util.concurrent.SynchronousQueue.poll(JLjava/util/concurrent/TimeUnit;)Ljava/lang/Object;+11
>>>  j
>>> java.util.concurrent.ThreadPoolExecutor.getTask()Ljava/lang/Runnable;+141
>>>  j
>>> java.util.concurrent.ThreadPoolExecutor.runWorker(Ljava/util/concurrent/ThreadPoolExecutor$Worker;)V+17
>>>  j  java.util.concurrent.ThreadPoolExecutor$Worker.run()V+5 j
>>> java.lang.Thread.run()V+11 v  ~StubRoutines::call_stub V
>>> [libjvm.so+0x555ecc] V  [libjvm.so+0x747a34] V
>>> [libjvm.so+0x554b55] V  [libjvm.so+0x5551f0] V
>>> [libjvm.so+0x5553cc] V  [libjvm.so+0x5be6cd] V
>>> [libjvm.so+0x879869] V  [libjvm.so+0x87c1b5] V
>>> [libjvm.so+0x750455] .... (above is the stack, please see the
>>> attachment for more information) Tony (Xiaohua Guan) On Thu, Jul
>>> 29, 2010 at 8:07 PM, David Holmes <David.Holmes at oracle.com>
>>> wrote:
>>>> Tony,
>>>> 
>>>> I don't see anything obviously wrong with what you attempted.
>>>> Can you show the full stack dump from the error.
>>>> 
>>>> David Holmes
>>>> 
>>>> Tony Guan said the following on 07/30/10 07:29:
>>>>> Dear all,
>>>>> 
>>>>> I want to invoke the GC at a certain time, for example, when
>>>>> a certain method is called at runtime, so I created a new
>>>>> subtype of VM_GC_Operation class, and initialized it with an
>>>>> object op, finally, executed using:VMThread::execute(&op)
>>>>> 
>>>>> then I got the following error message:
>>>>> 
>>>>> #  Internal Error
>>>>> 
>>>>> (/home/tony/software/OpenJDK/jdk7/hotspot/src/share/vm/runtime/thread.cpp:777),
>>>>>  pid=29652, tid=1096460624 #  Error: Possible safepoint
>>>>> reached by thread that does not allow it
>>>>> 
>>>>> The reason for the fail is that 
>>>>> Thread::check_for_valid_safepoint_state() didn't pass because
>>>>> the currentThread is not a VMThread.
>>>>> 
>>>>> My question is: how do I bring the current thread to a
>>>>> safepoint? or how do I call a GC operation properly? is there
>>>>> any prerequisite for doing so?
>>>>> 
>>>>> Thanks!
>>>>> 
>>>>> Tony (Xiaohua Guan)