RFR 8085965: VM hangs in C2Compiler

Poonam Bajaj Parhar poonam.bajaj at oracle.com
Thu Jun 11 18:12:02 UTC 2015


Hello Jon,

Here's the updated webrev:
http://cr.openjdk.java.net/~poonam/8085965/webrev.01/

Thanks,
Poonam

On 6/11/2015 6:45 AM, Poonam Bajaj Parhar wrote:
> Hello Jon,
>
> Thanks for taking a look at the code changes:
>
> On 6/10/2015 6:42 PM, Jon Masamitsu wrote:
>> Poonam,
>>
>> http://cr.openjdk.java.net/~poonam/8085965/webrev/src/share/vm/gc/cms/concurrentMarkSweepGeneration.cpp.frames.html
>>
>> I think it reads more easily if it is coded like this
>>
>> if (!ClassUnloading) {
>>   _should_unload_classes = false;
>> } else if ((_full_gc_requested && 
>> ExplicitGCInvokesConcurrentAndUnloadsClasses) {
>>    _should_unload_classes = true
>>  } else if (CMSClassUnloadingEnabled) {
>>   // Condition 2.a above 2678
>>  // Disjuncts 2.b.(i,ii,iii) above
>>    _should_unload_classes = (concurrent_cycles_since_last_unload() >=
>> CMSClassUnloadingMaxInterval)
>>                                                  || 
>> _cmsGen->is_too_full();
>>  }
>>
>> I might not have cut-and-pasted it right but I think you can see what 
>> I mean.
>>
> I understand, will make this change.
>
>> Also fix the block comment above to say that ClassUnloading overides 
>> the other
>> conditions.
>>
> yes.
>
>> http://cr.openjdk.java.net/~poonam/8085965/webrev/src/share/vm/runtime/arguments.cpp.frames.html
>>
>>
>> This catches the case where -Xnoclassgc is used but does not catch if 
>> -XX:-ClassUnloading is used.  Yes?
>> 2752     } else if (match_option(option, "-Xnoclassgc")) {
>> 2753       FLAG_SET_CMDLINE(bool, ClassUnloading, false);
>> 2754       FLAG_SET_CMDLINE(bool, CMSClassUnloadingEnabled, false);
>>
>> Instead if you add in "share/vm/runtime/arguments.cpp" to the method
>>
>> 1262 void Arguments::set_cms_and_parnew_gc_flags() ;
>>
>> if (!ClassUnloading) {
>> FLAG_SET_CMDLINE(bool, CMSClassUnloadingEnabled, false);
>> }
>>
> Oops! I missed it; will add it.
>
> Thanks,
> Poonam
>
>
>> That should catch both -Xnoclassgc and -XX:-ClassUnloading
>>
>> Jon
>>
>> On 6/10/2015 2:20 PM, Poonam Bajaj Parhar wrote:
>>> Please review the code changes to fix:
>>> JDK-8085965: <https://bugs.openjdk.java.net/browse/JDK-8085965> VM 
>>> hangs in C2Compiler
>>>
>>> Problem and fix:
>>> In JDK8, CMSClassUnloadingEnabled option which is used to control 
>>> the class-unloading in CMS, was enabled by default. But if the user 
>>> specifies -Xnoclassgc or -XX:-ClassUnloading on the command line 
>>> then classes get unloaded but updating of subklasses/siblings links 
>>> gets skipped by the ClassUnloading check in the following function:
>>>
>>> /void Klass::clean_weak_klass_links(BoolObjectClosure* is_alive, bool /
>>> /clean_alive_klasses) {/
>>> /   if (!ClassUnloading) {/
>>> /     return;/
>>> /   }
>>>
>>> /which corrupts the class hierarchy links causing hangs and crashes.
>>>
>>> This fix honors -XX:-ClassUnloading and -Xnoclassgc options and 
>>> disables class unloading with CMS.
>>>
>>> Webrev: http://cr.openjdk.java.net/~poonam/8085965/webrev/
>>>
>>> Thanks,
>>> Poonam
>>>
>>
>

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