code review request for deadlock detection fix (8007476)

[David Holmes]

Hi Dan,

This looks fine to me. I'm a little nervous the removal of the asserts 
as we should detect when an owning thread disappears as it is a major 
problem. But if those asserts are only in the diagnostic paths then it 
is better to let the diagnostics complete as you have done.

On 28/02/2013 12:52 AM, Karen Kinnear wrote:
> Code looks good. I did not study the details (again) for the monitor owner, but
> my memory is that there are other cases in which the monitor owner can be
> temporarily null, so this is will help there also.

Karen, it isn't that the monitor owner is temporarily null, but that the 
thread associated with an owned monitor can not be found. That should 
never happen. Whenever a monitor has an owner the "owner" information 
should lead to a live thread.

David
-----

>
> thanks,
> Karen
>
> On Feb 26, 2013, at 6:54 PM, Daniel D. Daugherty wrote:
>
>> Greetings,
>>
>> I have a fix to make deadlock detection a little more robust in the case
>> of being unable to find the JavaThread associated with an object lock:
>>
>>     8007476 assert(the_owner != NULL) failed: Did not find owning Java
>>             thread for lock word address
>>     http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=8007476
>>     https://jbs.oracle.com/bugs/browse/JDK-8007476
>>
>> Yes, I know that's not supposed to happen, but if and when it does happen,
>> it would be good to be able to diagnose the VM to try and figure out what
>> the heck happened.
>>
>> Here is the webrev URL:
>>
>>     http://cr.openjdk.java.net/~dcubed/8007476-webrev/0-hsx25/
>>
>> There is some sample output in the bug report that shows both regular
>> deadlock detection output and the proposed modified deadlock detection
>> output.
>>
>> Thanks, in advance, for any comments, suggestions or questions.
>>
>> Dan
>>
>> P.S.
>> Yes, I've included a bit of tweaking to JVM/TI code to deal with
>> invalid assertion failures that are discussed in the following bug:
>>
>>     6786879 Race in jvmti GetObjectMonitorUsage could lead to crashes
>>     http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6786879
>>     https://jbs.oracle.com/bugs/browse/JDK-6786879
>>
>> However, I haven't tried to address all the races that are discussed
>> in that bug including the race in the call to
>> JvmtiEnv::is_thread_fully_suspended() where the underlying call to
>> JavaThread::wait_for_ext_suspend_completion() can crash on lock exit.
>> Yikes, that one is nasty.
>