RR(XS): 7154963 crash in JvmtiEnvBase::get_current_contended_monitor()

[Dmitry Samersoff]

David,

We had off-line conversation with Sergey, and it's my bad to change the
order under if.

Thank you for catching it.

-Dmitry

On 2013-09-12 15:54, David Holmes wrote:
> Hi Sergey,
> 
> On 12/09/2013 8:34 PM, Sergey Gabdurakhmanov wrote:
>> Hello,
>>
>> Here is webrev of changes I'm about to integrate:
>>
>> webrev: http://cr.openjdk.java.net/~sgabdura/7154963/webrev.00/
>> bug: http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=7154963
>>
>> The fix check that the _java_thread parameter is valid when it is
>> possible that the JavaThread has exited after the initial checks were
>> made in generated/jvmtifiles/jvmtiEnter.cpp:
>> jvmti_GetCurrentContendedMonitor().
>>
>> The fix checked by customer on JDK6. Crash is not reproducible with this
>> fix.
> 
> I don't think this fix is safe. If the thread has terminated and been
> deallocated then the code:
> 
> _java_thread->threadObj() != NULL && !_java_thread->is_exiting()
> 
> may be using a pointer that no longer points to a valid thread object.
> (This is unfortunately true in a number of places in the VM.)
> 
> Also threadObj() is never set to NULL after the thread is initialized so
> that condition seems unnecessary if the thread was already determined to
> be alive when the initial checks were made.
> 
> I would think it sufficient to only check
> Threads::includes(_java_thread) - as if that is true then the thread is
> alive and while it may be terminating it can't complete that action as
> the Threads_lock is needed for Threads::remove(this), and we hold the
> Threads_lock as this is a VM op and we are at a safepoint.
> 
> That all said, after reading the bug report (again) I'm unclear what
> role this thread is actually playing in getCurrentContendedMonitor. If
> it is not the current thread, what is it? The owner of the monitor which
> we consider contended?
> 
> David
> -----
> 
>> BR,
>> Sergey


-- 
Dmitry Samersoff
Oracle Java development team, Saint Petersburg, Russia
* I would love to change the world, but they won't give me the sources.