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

David Holmes david.holmes at oracle.com
Thu Jun 4 04:57:23 UTC 2020

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);
     >>> 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.


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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