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

Wed Jul 16 11:50:40 UTC 2014

Hi Mikael,

thanks for the review. Please see comments inline.

On 16.07.2014 10:34, Mikael Gerdin wrote:
> Hi Tobias,
>
> On Wednesday 16 July 2014 10.24.16 Tobias Hartmann wrote:
>> Hi Mikael,
>>
>> thanks for the review. Please see comments inline.
>>
>> On 15.07.2014 13:36, Mikael Gerdin wrote:
>>> Tobias,
>>>
>>> On Tuesday 15 July 2014 09.48.08 Tobias Hartmann wrote:
>>>> 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.
>>> Yep, there are a lot of leftover references to metadata as oops,
>>> especially in some compiler/runtime parts such as MDOs and CompiledICs.
>>>
>>>>> 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.
>>> I'm wondering, is there some way to figure out the IC for the Method*?
>>>
>>> In CompiledStaticCall::emit_to_interp_stub a static_stub_Relocation is
>>> created and from the looks of it it points to the call site through some
>>> setting of a "mark".
>>>
>>> The metadata relocation is emitted just after the static_stub_Relocation,
>>> so one approach (untested) could be to have a case for
>>> static_stub_Relocations, create a CompiledIC.at(reloc->static_call()) and
>>> check if it's a call to interpreted. If it is the advance the
>>> relocIterator to the next position and check that metadata for liveness.
>> The relocation entries for this particular case are [1]. Looking at the
>> static_stub_Relocation (0xffffffff6ea49cc4) we don't know if the stub
>> belongs to an optimized IC or a compiled static call. We would either
>> have to create both CompiledIC.at(..) and compiledStaticCall_at(..) or
>> check the relocation entry for the call (0xffffffff6ea49bc0) requiring
>> another iteration. Only then we are able to look at the
>> metadata_relocation at the next position.
> That's a good point.
>
>> Since we already have case statements for opt_virtual_call_type and
>> static_call_type (at least in nmethod::do_unloading_parallel(..)) I
>> would prefer to infer the Method* from the IC or compiled static call.
>>
>> I adapted the implementation according to Coleen's feedback:
>>
>> http://cr.openjdk.java.net/~thartmann/8029443/webrev.01/
>>
>> What do you think?
> I think your suggested change is fine.
>
> Did you test it with -XX:+UseG1GC to exercise the do_unloading_parallel path
> as well?

I just did some more testing with runThese and -XX:+UseG1GC and it looks 
fine.

Best,
Tobias

>
> /Mikael
>
>> Thanks,
>> Tobias
>>
>>
>> [1] Relocation entries:
>>
>> [...]
>> 154031          @0xffffffff6ea49b3a: 3005
>> 154032 relocInfo at 0xffffffff6ea49b3a [type=3(opt_virtual_call)
>> addr=0xffffffff6ea49bc0 offset=20]
>> [...]
>> 154047          @0xffffffff6ea49b52: f801ffe8500a
>> 154048 relocInfo at 0xffffffff6ea49b56 [type=5(static_stub)
>> addr=0xffffffff6ea49cc4 offset=40 data=-24] |
>> [static_call=0xffffffff6ea49bc0]
>> 154049          @0xffffffff6ea49b58: f003c000
>> 154050 relocInfo at 0xffffffff6ea49b5a [type=12(metadata)
>> addr=0xffffffff6ea49cc4 offset=0 data=3] |
>> [metadata_addr=0xffffffff6ea49d68 *=0xffffffff6ae20960
>> offset=0]metadata_value=0xffffffff6ae20960: {method}
>> {0xffffffff6ae20968} 'newInstance'
>> '([Ljava/lang/Object;)Ljava/lang/Object;' in
>> 'sun/reflect/GeneratedConstructorAccessor3'
>> 154051          @0xffffffff6ea49b5c: f003c007
>> 154052 relocInfo at 0xffffffff6ea49b5e [type=12(metadata)
>> addr=0xffffffff6ea49ce0 offset=28 data=3] |
>> [metadata_addr=0xffffffff6ea49d68 *=0xffffffff6ae20960
>> offset=0]metadata_value=0xffffffff6ae20960: {method}
>> {0xffffffff6ae20968} 'newInstance'
>> '([Ljava/lang/Object;)Ljava/lang/Object;' in
>> 'sun/reflect/GeneratedConstructorAccessor3'
>>
>> 154053          @0xffffffff6ea49b60:
>>> /Mikael
>>>
>>>> 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