RFR 8055008: Clean up code that saves the previous versions of redefined classes
Daniel D. Daugherty
daniel.daugherty at oracle.com
Wed Aug 20 22:37:50 UTC 2014
On 8/20/14 9:54 AM, Coleen Phillimore wrote:
>
> Hi, it appears that my code is wrong and maybe the existing code is
> wrong also. I have a spec question below.
Rely embedded below...
>
> On 8/19/14, 7:52 PM, serguei.spitsyn at oracle.com wrote:
>> On 8/19/14 3:53 PM, Daniel D. Daugherty wrote:
>>>
>>> On 8/19/14 3:39 PM, Daniel D. Daugherty wrote:
>>>> On 8/15/14 2:18 PM, Coleen Phillimore wrote:
>>>>> Summary: Use scratch_class to find EMCP methods for breakpoints if
>>>>> the old methods are still running
>>>>>
>>>>> See bug for more details. This fix also includes a change to
>>>>> nmethod::metadata_do() to not walk the _method multiple times (the
>>>>> _method is added to the metadata section of the nmethod), and an
>>>>> attempt to help the sweeper clean up these scratch_class instance
>>>>> classes sooner.
>>>>>
>>>>> Tested with nsk tests, java/lang/instrument tests and jck tests
>>>>> (which include some jvmti tests).
>>>>>
>>>>> open webrev at http://cr.openjdk.java.net/~coleenp/8055008/
>>>>
>>>> src/share/vm/oops/instanceKlass.hpp
>>>> line 1047 // RedefineClass support
>>>> Should be 'RedefineClasses'.
>>>>
>>>> line 1049: void mark_newly_obsolete_methods(Array<Method*>*
>>>> old_methods,
>>>> int emcp_method_count);
>>>> The 'obsolete' part of the function name does not match with
>>>> the 'emcp' part of the parameter name. EMCP methods are 'old',
>>>> but not 'obsolete'.
>>>>
>>>> Update: OK, I think I get it. We want to mark methods that are
>>>> newly transitioning from EMCP -> obsolete and the
>>>> emcp_method_count
>>>> parameter tells us if there is any work to do.
>>>>
>>>> src/share/vm/oops/instanceKlass.cpp
>>>> line 3023: } // pvw is cleaned up
>>>> 'pvw' no longer exists so comment is stale.
>>>>
>>>> line 3455: // check the previous versions array
>>>> This comment should move above this line:
>>>>
>>>> 3453 for (; pv_node != NULL; ) {
>>>>
>>>> and 'array' should change to 'list'.
>>>>
>>>> Sorry for the bad placement of the original comment.
>>>>
>>>> line 3463: last->link_previous_versions(pv_node);
>>>> So now we've overwritten the previous value of
>>>> last->previous_versions. How does that InstanceKlass
>>>> get freed? Maybe a short comment?
>>>>
>>>> line 3484: // Mark the emcp method as obsolete if it's not
>>>> executing
>>>> I'm not sure about whether this is a good idea. When we had a
>>>> redefine make a method obsolete before, we knew that we could
>>>> make all older but previously EMCP methods obsolete because
>>>> the new method version did make them obsolete.
>>>>
>>>> With this optimization, we're saying that no thread is
>>>> executing
>>>> the method so we're going to make it obsolete even if the
>>>> current
>>>> redefine did not do so. I worry about a method call that is in
>>>> the early stages of assembling the call stack being caught
>>>> calling a method that is now obsolete but not because of a
>>>> redefine made it obsolete. Just FYI, one of the tracing flags
>>>> catches unexpected calls to obsolete methods today and it does
>>>> catch the current system's legitimate race.
>>>
>>> JVM/TI has an isMethodObsolete() API:
>>>
>>> jvmtiError
>>> IsMethodObsolete(jvmtiEnv* env,
>>> jmethodID method,
>>> jboolean* is_obsolete_ptr)
>>>
>>> It would be possible for an agent to observe a method changing from
>>> not obsolete to obsolete when that's not expected. I suspect that
>>> this would be a spec violation.
>>
>> I agree that this looks like a spec violation.
>> The emcp methods by definition are non-obsolete,
>> or in opposite, the obsolete methods are non-emcp:
>> http://docs.oracle.com/javase/8/docs/platform/jvmti/jvmti.html#obsoleteMethods
>>
>> Old method versions may become obsolete, not emcp:
>> http://docs.oracle.com/javase/7/docs/platform/jvmti/jvmti.html#RedefineClasses
>>
>> But maybe I'm missing something...
>
> If an EMCP method is not running, should we save it on a previous
> version list anyway so that we can make it obsolete if it's redefined
> and made obsolete?
Interesting question. The problem with an EMCP method is that it might
not be running right now, but we could have a Java thread that's in
the process of invoking it that is stopped on a safepoint. We resume
the world and the Java thread finishes calling the EMCP method...
It's really hard to catch in-progress uses of jmethodIDs and make
sure that the in-progress use switches from the EMCP method to the
latest version of the method. A rarely seen race, but it does happen...
> Currently we don't save previous versions of methods that are not
> running. We didn't before permgen elimination either. If GC didn't
> find the EMCP method, we would remove the entry.
Not quite true for the pre-PermGen-Removal (PGR) world. We used to
save weak refs for all of the EMCP methods in the previous version
info. As the EMCP methods became collectible we removed them from
the previous version info. This means if GC could find the EMCP
method anywhere (stack, jmethodID, JNI handle, etc), then it stayed
alive. This means that even if no thread was currently executing
an EMCP method, an in-progress call to that method could still
complete and poof now we have the EMCP method back on a stack
somewhere...
Dan
>
> Thanks,
> Coleen
>
>>
>>
>> Thanks,
>> Serguei
>>
>>>
>>> Dan
>>>
>>>
>>>>
>>>> line 3527: // clear out any matching EMCP method entries the
>>>> hard way.
>>>> Perhaps "mark" instead of "clear out"?
>>>>
>>>> old line 3659: if (!method->is_obsolete() &&
>>>> new line 3546: if (method->is_emcp() &&
>>>> The old code is correct. The old code won't remark a method
>>>> that
>>>> was already made obsolete so there won't be more than one
>>>> trace
>>>> message for that operation.
>>>>
>>>> line 3581: // stack, and set emcp methods on the stack.
>>>> In comments 'emcp' should be 'EMCP'. We did not do that in the
>>>> code because of HotSpot's variable name style.
>>>>
>>>> Also, set what on EMCP methods on the stack?
>>>>
>>>> line 3591: ... If emcp method from
>>>> line 3592: // a previous redefinition may be made obsolete by
>>>> this redefinition.
>>>> Having trouble parsing this comment.
>>>>
>>>> src/share/vm/oops/method.hpp
>>>> line 693: // emcp methods (equivalent method except constant
>>>> pool is different)
>>>> line 694: // that are old but not obsolete or deleted.
>>>> Perhaps:
>>>>
>>>> // EMCP methods are old but not obsolete or deleted.
>>>> Equivalent
>>>> // Modulo Constant Pool means the method is equivalent except
>>>> // the constant pool and instructions that access the constant
>>>> // pool might be different.
>>>>
>>>> src/share/vm/prims/jvmtiImpl.cpp
>>>> No comments.
>>>>
>>>> src/share/vm/prims/jvmtiRedefineClasses.cpp
>>>> No comments.
>>>>
>>>> src/share/vm/code/nmethod.cpp
>>>> So in the original code f(_method) was being called two extra
>>>> times? (once in the while-loop and once at the end) So I'm
>>>> guessing that f(_method) is properly called when the rest of
>>>> the metadata is handled in the nmethod (line 2085)?
>>>>
>>>> src/share/vm/memory/universe.cpp
>>>> No comments (resisting 'The Walking Dead' ref...)
>>>>
>>>> test/runtime/RedefineTests/RedefineFinalizer.java
>>>> No comments.
>>>>
>>>> test/runtime/RedefineTests/RedefineRunningMethods.java
>>>> line 44: " while (!stop) { count2++; }" +
>>>> line 53: while (!stop) { count1++; }
>>>> line 56: while (!stop) { count2++; }
>>>>
>>>> These may not behave well on OSes with bad threading
>>>> models. You might want to add a helper function that
>>>> sleeps for 10ms and have each of these loops call it
>>>> so the test more well behaved.
>>>>
>>>> Dan
>>>>
>>>>
>>>>> bug link https://bugs.openjdk.java.net/browse/JDK-8055008
>>>>>
>>>>> Thanks,
>>>>> Coleen
>>>>
>>>>
>>>
>>
>
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