[9] RFR(S): 8029443: 'assert(klass->is_loader_alive(_is_alive)) failed: must be alive' during VM_CollectForMetadataAllocation

[Tobias Hartmann]

Hi Vladimir,

> Impressive work, Tobias!

Thanks! Took me a while to figure out what's happening.

> So before the permgen removal embedded method* were oops and they were 
> processed in relocInfo::oop_type loop.

Okay, good to know. That explains why the terms oops and metadata are 
used interchangeably at some points in the code.

> May be instead of specializing opt_virtual_call_type and 
> static_call_type call site you can simple add a loop for 
> relocInfo::metadata_type (similar to oop_type loop)?

The problem with iterating over relocInfo::metadata_type is that we 
don't know to which stub, i.e., to which IC the Method* pointer belongs. 
Since we don't want to unload the entire method but only clear the 
corresponding IC, we need this information.

Thanks,
Tobias

>
> Thanks,
> Vladimir
>
> On 7/14/14 4:56 AM, Tobias Hartmann wrote:
>> Hi,
>>
>> please review the following patch for JDK-8029443.
>>
>> Bug: https://bugs.openjdk.java.net/browse/JDK-8029443
>> Webrev: http://cr.openjdk.java.net/~thartmann/8029443/webrev.00/
>>
>> *Problem*
>> After the tracing/marking phase of GC, nmethod::do_unloading(..) checks
>> if a nmethod can be unloaded because it contains dead oops. If class
>> unloading occurred we additionally clear all ICs where the cached
>> metadata refers to an unloaded klass or method. If the nmethod is not
>> unloaded, nmethod::verify_metadata_loaders(..) finally checks if all
>> metadata is alive. The assert in CheckClass::check_class fails because
>> the nmethod contains Method* metadata corresponding to a dead Klass.
>> The Method* belongs to a to-interpreter stub [1] of an optimized
>> compiled IC. Normally we clear those stubs prior to verification to
>> avoid dangling references to Method* [2], but only if the stub is not in
>> use, i.e. if the IC is not in to-interpreted mode. In this case the
>> to-interpreter stub may be executed and hand a stale Method* to the
>> interpreter.
>>
>> *Solution
>> *The implementation of nmethod::do_unloading(..) is changed to clean
>> compiled ICs and compiled static calls if they call into a
>> to-interpreter stub that references dead Method* metadata.
>>
>> The patch was affected by the G1 class unloading changes (JDK-8048248)
>> because the method nmethod::do_unloading_parallel(..) was added. I
>> adapted the implementation as well.
>> *
>> Testing
>> *Failing test (runThese)
>> JPRT
>>
>> Thanks,
>> Tobias
>>
>> [1] see CompiledStaticCall::emit_to_interp_stub(..)
>> [2] see nmethod::verify_metadata_loaders(..),
>> static_stub_reloc()->clear_inline_cache() clears the stub