RFR(S): JDK-8129108: nmethod related crash in CMS

Poonam Bajaj Parhar poonam.bajaj at oracle.com
Mon Jun 22 21:19:55 UTC 2015


Hello Mikael,

Thanks for taking a look at the changes.

Yes, you are right, the best place to add the SO_AllCodeCache 
root_scanning option is
CMSCollector::setup_cms_unloading_and_verification_state(). There the 
issue is that
we are adding/removing the root scanning options based on if the 
verification options
are enabled/disabled.

SO_AllCodeCache should get added to the root_scanning_options if we are 
not unloading
classes regardless whether we are verifying or not.

I compared this code with the code of jdk7 and the only part that has 
changed in there is related
to the Perm Gen. I could also reproduce the issue with 7u fastdebug 
build with
-XX:+CMSParallelInitialMarkEnabled and -XX:-VerifyBeforeExit.

Here's the updated webrev:
http://cr.openjdk.java.net/~poonam/8129108/webrev.01/

Failing testcase works fine with these changes.

Thanks,
Poonam

On 6/22/2015 6:41 AM, Mikael Gerdin wrote:
> Poonam,
>
> On 2015-06-20 06:32, Poonam Bajaj Parhar wrote:
>> Hello,
>>
>> Please review this change for bug:
>> JDK-8129108: <https://bugs.openjdk.java.net/browse/JDK-8129108> nmethod
>> related crash in CM
>>
>> There is an assertion failure with fastdebug build with test
>> /nsk.coverage.arguments.arguments002.arguments002/.
>> This failure occurs when CMSCollector::_should_unload_classes is false
>> and the _roots_scanning_options does
>> not contain SO_AllCodeCache:
>>
>> #  Internal Error 
>> (hotspot/src/share/vm/gc/cms/concurrentMarkSweepGeneration.cpp:4472), 
>> pid=16267, tid=0xf36ffb70
>> #  assert(_collector->should_unload_classes() ||
>> (_collector->CMSCollector::roots_scanning_options() &
>> GenCollectedHeap::SO_AllCodeCache)) failed: if we didn't scan the code
>> cache, we have to be ready to drop nmethods with expired weak oops
>>
>>
>> This problem was exposed with the fix ofJDK-8085965:
>> <https://bugs.openjdk.java.net/browse/JDK-8085965> VM hangs in 
>> C2Compiler,
>> that disables CMSClassUnloadingEnabled when -Xnoclassgc or
>> -XX:-ClassUnloading
>> are specified on the command line.
>>
>> CMSCollector::_roots_scanning_options option should contain
>> SO_AllCodeCache to ensure
>> that we scan code cache when we are not unloading classes.
>>
>> Webrev: http://cr.openjdk.java.net/~poonam/8129108/webrev/
>
> This looks very similar to the code in 
> CMSCollector::setup_cms_unloading_and_verification_state
> which is called from CMSCollector::checkpointRootsInitialWork
>
> I think that the if-clause in setup_cms_unloading... needs to be fixed 
> to handle the class unloading case separately from the verification 
> state.
> It's not entirely clear to me exactly which cases the complicated 
> conditions in the if statements try to capture, though. You may want 
> to look at a previous version of the sources (before perm gen removal) 
> to better understand the reasoning behind it.
>
> /Mikael
>
>> Testing: JPRT, testcase 
>> /nsk.coverage.arguments.arguments002.arguments002/
>>
>> Thanks,
>> Poonam
>>



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