[sh/jdk8u] MetadataOnStackMark::record() is not MT-safe

[Zhengyu Gu]

Tested 11u, does not seem affected by this bug.

-Zhengyu

On 7/29/21 10:22 AM, Roman Kennke wrote:
> Ok.
> 
> How about later JDKs? Could they be affected, too?
> 
> Thanks,
> Roman
> 
>> This is not a Shenandoah specific bug, I believe G1 suffers the same 
>> problem. The reason to fix Shenandoah first, in hope that we can have 
>> a build for our customer to test this fix, as we don't have a 
>> reproducer. I can move the fix to jdk8u if you think otherwise.
>>
>> Apparently, MetadataOnStackMark::record() was never designed for MT, 
>> but G1 and Shenandoah's parallel cleaning code can call 
>> nmethod::do_unloading_parallel() from different worker threads, and 
>> nmethod::do_unloading_parallel() in turn calls 
>> nmethod::clean_ic_if_metadata_is_dead() which may call 
>> Metadata::mark_on_stack() to record metadata to MetadataOnStackMark().
>>
>> Calling MetadataOnStackMark::record() without synchronization, may 
>> corrupt internal buffer, which can result fatal error in 
>> MetadataOnStackMark's destructor, when it walks the buffers.
>>
>>
>> This problem is hard to reproduce, cause it is only possible when 
>> there are redefined classes.
>>
>>
>> Webrev: 
>> http://cr.openjdk.java.net/~zgu/shenandoah/metadata_on_stack/webrev.00/
>>
>> Thanks,
>>
>> -Zhengyu
>>
>