[14] RFR(XS): 8227236: assert(singleton != __null && singleton != declared_interface) failed

[Igor Veresov]

It’d be nice if CI provided a consistent snapshot of the world. Having inconsistencies like that is a bit scary.
Is it because we don’t want to eagerly snapshot the whole hierarchy of the given class?

igor



> On Aug 13, 2019, at 3:36 PM, dean.long at oracle.com wrote:
> 
> Looks OK, but what's the harm in memoizing for shared classes?
> 
> dl
> 
> On 8/13/19 8:40 AM, Vladimir Ivanov wrote:
>> http://cr.openjdk.java.net/~vlivanov/8227236/webrev.00/
>> https://bugs.openjdk.java.net/browse/JDK-8227236
>> 
>> There's a race between ciInstanceKlass::nof_implementors()/implementor()
>> and class loading for shared classes which may manifest as
>> inconsistency between consecutive calls to nof_implementors() and
>> implementor() (ciInstanceKlass::implementor() doesn't cache
>> InstanceKlass::implementor() for shared classes [1]). That's what
>> triggers the assert: the check sees declared_interface->nof_implementors() == 1,
>> but concurrent class loading introduces new implementor class before the
>> assert. Assert hits singleton == declared_interface case since
>> declared_interface->implementor() == declared_interface when
>> declared_interface->nof_implementors() > 1 [2].
>> 
>> The race has been there long before JDK-6986483 [3] (same sequence of
>> nof_implementors()/implementor() calls and the assert in C1 code), but
>> it seems recent JDK changes made it more likely to occur.
>> 
>> Proposed fix is to check for unique implementor using a single
>> implementor() call. If there's any concurrent class loading happening
>> which introduces more implementors, corresponding nmethod dependency
>> will invalidate the nmethod during installation attempt.
>> 
>> Testing: hs-precheckin-comp, tier1, tier2
>> 
>> Thanks!
>> 
>> Best regards,
>> Vladimir Ivanov
>> 
>> [1] http://hg.openjdk.java.net/jdk/jdk/file/9c0715c5bbf3/src/hotspot/share/ci/ciInstanceKlass.cpp#l617
>> 
>> [2] http://hg.openjdk.java.net/jdk/jdk/file/9c0715c5bbf3/src/hotspot/share/ci/ciInstanceKlass.hpp#l72
>> 
>> [3] https://jbs.oracle.com/browse/JDK-6986483
>