RFR JDK-8232222: Set state to 'linked' when an archived class is restored at runtime

coleen.phillimore at oracle.com coleen.phillimore at oracle.com
Tue Jun 9 02:46:20 UTC 2020

Hi Jiangi,

I apologize for jumping in at this late stage of this change.  I've seen 
the emails but there's been a lot of discussion which is hard to follow.

I have some concerns with setting the state to "linked" since the 
changes that Erik Osterlund is working on would require reinitializing 
the itable and vtables when you load the shared class.  See the JEP for 
New Invoke Bindings https://bugs.openjdk.java.net/browse/JDK-8221828. 
We would have to remove this optimization.  Erik is planning on getting 
this work into JDK 16 since we have a functionally complete version.  See:

Also, I haven't figured out why you are enabling this optimization if 
JVMTI is requested, since the optimization seems to have minor 
benefits.  And I'm concerned with threads observing the class as linked 
but I don't see any bugs there.  By setting the state to "linked" we are 
skipping these steps:

linking super classes and interfaces - can we assume that they are 
already linked when ik->restore_unshareable_info is called ?
check_verification_constraints - presumably OK for NULL CLD, this has a 
quick exit
link_methods - this is already called in restore_unshareable_info so it 
has a quick exit
check_linking_constraints - presumably OK for NULL CLD, this has a quick 
initializing the vtable - will need to revert the change for new invoke 
bindings and maybe valhalla.

link_class_impl doesn't really do that much for boot class loader.

I imagine that this change is so that potentially CDS classes can be 
pre-initialized so that more in the mirror can be shared, which sounds 
difficult to do except maybe for some classes.  Is this being discussed 
on the Project Leyden thread?


On 6/8/20 9:02 PM, Jiangli Zhou wrote:
> Hi Ioi,
> After incorporating David's suggestion of locking init_lock for
> posting ClassPrepare events, do you have other concerns about the
> change? I hope we are finally able to move on with an inclusive and
> right solution that works for broad usages, particular on the cloud
> spectrum.
> Best,
> Jiangli
> On Thu, Jun 4, 2020 at 3:14 PM Jiangli Zhou <jianglizhou at google.com> wrote:
>> Hi David,
>> On Wed, Jun 3, 2020 at 9:59 PM David Holmes <david.holmes at oracle.com> wrote:
>>> Ioi pointed out that my proposal was incomplete and that it would need
>>> to be more like:
>>> if (is_shared() &&
>>>       JvmtiExport::should_post_class_prepare() &&
>>>       !BytecodeVerificationLocal &&
>>>       loader_data->is_the_null_class_loader_data()) {
>>>       Handle h_init_lock(THREAD, init_lock());
>>>       ObjectLocker ol(h_init_lock, THREAD, h_init_lock() != NULL);
>>>       set_init_state(linked);
>>>       >>> call JVMTI
>>>       return true;
>>>     }
>>> This alleviates any concerns about behavioural changes to JVM TI, and
>>> also allows JVM TI enabled code to partially benefit from the
>>> pre-linking optimisation.
>>> Otherwise I agree with Ioi that any behaviour change to JVM TI needs to
>>> be justified by significant performance gains.
>> Thanks a lot for the input and suggestion! Locking the init_lock for
>> the JVMTI ClassPrepre event here sounds ok to me. The ClassDefine is
>> normally posted before the ClassPrepare. That's why the change was
>> made in systemDictionary.cpp instead of within
>> InstanceKlass::restore_unshareable_info() function, to keep the same
>> events ordering for any given class. I added the 'init_lock' locking
>> code for post_class_prepare(), and kept the code in
>> systemDictionary.cpp in webreve.03 below.  Not changing the JVMTI
>> events ordering feels safer to me. Would the following be ok to
>> everyone?
>>    http://cr.openjdk.java.net/~jiangli/8232222/webrev.03/
>> I also changed the InstanceKlass::restore_unshareable_info() to set
>> _init_state via set_init_state API as you suggested. We can get away
>> without locking the init_lock for setting the flag itself.
>> Best regards,
>> Jiangli
>>> David
>>> -----
>>> On 4/06/2020 8:42 am, David Holmes wrote:
>>>> Correction ...
>>>> On 3/06/2020 5:19 pm, David Holmes wrote:
>>>>> On 3/06/2020 3:44 pm, Ioi Lam wrote:
>>>>>> On 6/2/20 10:16 PM, David Holmes wrote:
>>>>>>> Hi Ioi,
>>>>>>> On 3/06/2020 2:55 pm, Ioi Lam wrote:
>>>>>>>> On 5/27/20 11:13 PM, David Holmes wrote:
>>>>>>>>> Hi Jiangli,
>>>>>>>>> On 28/05/2020 11:35 am, Ioi Lam wrote:
>>>>>>>>>>> I was going to take the suggestion, but realized that it would add
>>>>>>>>>>> unnecessary complications for archived boot classes with class
>>>>>>>>>>> pre-initialization support. Some agents may set
>>>>>>>>>>> JvmtiExport::should_post_class_prepare(). It's worthwhile to
>>>>>>>>>>> support
>>>>>>>>>>> class pre-init uniformly for archived boot classes with
>>>>>>>>>>> JvmtiExport::should_post_class_prepare() enabled or disabled.
>>>>>>>>>> This would introduce behavioral changes when JVMTI is enabled:
>>>>>>>>>> + The order of JvmtiExport::post_class_prepare is different than
>>>>>>>>>> before
>>>>>>>>>> + JvmtiExport::post_class_prepare may be called for a class that
>>>>>>>>>> was not called before (if the class is never linked during run time)
>>>>>>>>>> + JvmtiExport::post_class_prepare was called inside the
>>>>>>>>>> init_lock, now it's called outside of the init_lock
>>>>>>>>> I have to say I share Ioi's concerns here. This change will impact
>>>>>>>>> JVM TI agents in a way we can't be sure of. From a specification
>>>>>>>>> perspective I think we are fine as linking can be lazy or eager,
>>>>>>>>> so there's no implied order either. But this would be a
>>>>>>>>> behavioural change that will be observable by agents. (I'm less
>>>>>>>>> concerned about the init_lock situation as it seems potentially
>>>>>>>>> buggy to me to call out to an agent with the init_lock held in the
>>>>>>>>> first place! I find it hard to imagine an agent only working
>>>>>>>>> correctly if the init_lock is held.)
>>>>>>>> David,
>>>>>>>> The init_lock has a serializing effect. The callback for a subclass
>>>>>>>> will not be executed until the callback for its super class has
>>>>>>>> been finished.
>>>>>>> Sorry I don't see that is the case. The init_lock for the subclass
>>>>>>> is distinct from the init_lock of the superclass, and linking of
>>>>>>> subclasses and superclasses is independent.
>>>>>> In InstanceKlass::link_class_impl, you first link all of your super
>>>>>> classes.
>>>>>> If another thread is already linking your super class, you will block
>>>>>> on that superclass's init_lock.
>>>>> The point is that there is already a race in terms of the execution of
>>>>> the two callbacks. So while this change can certainly produce a
>>>>> different result to what would previously be seen, such a result is
>>>>> already possible in the general case.
>>>>>> Of course, I may be wrong and my analysis may be bogus. But I hope
>>>>>> you can appreciate that this is not going to be a trivial change to
>>>>>> analyze.
>>>>> Yes I agree. While in general ordering of the class_prepare callbacks
>>>>> is not guaranteed for independent classes, if a given application
>>>>> explicitly loads and links classes in a known order then it can
>>>>> (reasonably) expect its callbacks to execute in that order. If this
>>>>> change means classes will now be linked in an order independent of
>>>>> what the normal runtime order would be then that could be a problem
>>>>> for existing agents.
>>>>> So where does this leave us? The change is within spec, but could
>>>>> trigger changes in agent behaviour that we can't really evaluate
>>>>> a-priori. So as you say we should have a fairly good reason for doing
>>>>> this. I can easily envisage that pre-linking when no callbacks are
>>>>> enabled would be a performance boost. But with callbacks enabled and
>>>>> consuming CPU cycles any benefit from pre-linking could be lost in the
>>>>> noise.
>>>>> What if we did as Ioi suggested and only set the class as linked in
>>>>> restore_unshareable_info if !JvmtiExport::should_post_class_prepare();
>>>>> and in addition in InstanceKlass::link_class_imp we added an
>>>>> additional check at the start:
>>>>> // Pre-linking at load time may have been disabled for shared classes,
>>>>> // but we may be able to do it now.
>>>>> if (JvmtiExport::should_post_class_prepare() &&
>>>>>       !BytecodeVerificationLocal &&
>>>>>       loader_data->is_the_null_class_loader_data()) {
>>>>>     _init_state = linked;
>>>>> }
>>>> There should obviously be a check for is_shared() in there as well.
>>>> David
>>>> -----
>>>>> ?
>>>>> That avoids the problem of changing the JVM TI callback behaviour, but
>>>>> also shortens the link time path when the callbacks are enabled.
>>>>> Hope I got that right. :)
>>>>> David
>>>>> -----
>>>>>> Thanks
>>>>>> - Ioi
>>>>>>> David
>>>>>>> -----
>>>>>>>> With the proposed patch, the callback for both the super class and
>>>>>>>> subclass can proceed in parallel. So if an agent performs class
>>>>>>>> hierarchy analysis, for example, it may need to perform extra
>>>>>>>> synchronization.
>>>>>>>> This is just one example that I can think of. I am sure there are
>>>>>>>> other issues that we have not thought about.
>>>>>>>> The fact is we are dealing with arbitrary code in the callbacks,
>>>>>>>> and we are changing the conditions of how they are called. The
>>>>>>>> calls happen inside very delicate code (class loading, system
>>>>>>>> dictionary). I am reluctant to do the due diligence, which is
>>>>>>>> substantial, of verifying that this is a safe change, unless we
>>>>>>>> have a really compelling reason to do so.
>>>>>>>> Thanks
>>>>>>>> - Ioi

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