RFR 8055008: Clean up code that saves the previous versions of redefined classes
serguei.spitsyn at oracle.com
serguei.spitsyn at oracle.com
Wed Aug 27 11:29:36 UTC 2014
Dan,
Sorry for the big delay in reply...
It is because I did not have my arguments ready. :(
Please, see below.
On 8/20/14 3:37 PM, Daniel D. Daugherty wrote:
> 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...
It has to be invariant that if an EMCP is not running at redefinition time
then it has to be gone and can not become running in the future.
Otherwise, everything becomes overcomplicated.
This also means that non-running EMCP method can never be made obsolete.
The JVMTI spec is clear about the jmethodIDs and obsolete methods:
Obsolete Methods
The functions |RetransformClasses|
<http://docs.oracle.com/javase/7/docs/platform/jvmti/jvmti.html#RetransformClasses>
and |RedefineClasses|
<http://docs.oracle.com/javase/7/docs/platform/jvmti/jvmti.html#RedefineClasses>
can cause new versions of methods to be installed. An original version
of a method is considered equivalent to the new version if:
* their bytecodes are the same except for indices into the constant
pool and
* the referenced constants are equal.
An original method version which is not equivalent to the new method
version is called obsolete and is assigned a new method ID; the original
method ID now refers to the new method version. A method ID can be
tested for obsolescence with |IsMethodObsolete|
<http://docs.oracle.com/javase/7/docs/platform/jvmti/jvmti.html#IsMethodObsolete>.
From the above, it seems that the EMCP methods do not get new jmethodIDs.
It means that any use of jmethodID that referred to the EMCP method
before the redefinition
after redefinition should refer to the new (non-EMCP) method version.
We must treat it as a bug if this invariant is broken.
However, if an EMCP method is already running (there is a frame on the
stack)
then it can not be replaced with the new method version.
Next redefinition can make such EMCP method to become obsolete, and so,
it should get new methodID if it is still running.
>
>
>> 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...
Do we have any test that demonstrate this behavior?
Should we treat this behavior as a bug?
If we decide to treat it as a bug then is it possible to fix such issues?
Thanks,
Serguei
>
> 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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