Questions on concurrent class unloading

[Zhengyu Gu]

On 4/8/19 2:49 PM, Erik Osterlund wrote:
> Hi Zhengyu,
> 
> The code is called by concurrent GC threads, so is_ConcurrentGC_thread() should return true.
> 
E.g ZNMethodUnlinkClosure is only used by ZNMethodUnlinkTask, and the 
task is executed by worker threads, which are not concurrent gc thread. No?

I also added assert(false, "debug") inside under nm->is_unloading() 
branch, it never fired.

Thanks,

-Zhengyu



> is_unloading() returns true if the nmethod has a dead oop due to GC.
> 
> /Erik
> 
>> On 8 Apr 2019, at 17:27, Zhengyu Gu <zgu at redhat.com> wrote:
>>
>> Hi,
>>
>> I am studying concurrent class unloading in ZGC, it looks to me that nothing gets unlinked (ZNMethod::unlink())and purged (ZNMethod::purge()).
>>
>> Inside ZNMethodUnlinkClosure::do_nmethod() [1]
>>
>> It appears that nm->is_unloading() can never be true. Otherwise, assertion should fail inside nm->flush_dependencies(false), cause neither Universe::heap()->is_gc_active() (true for STW GC) nor is_ConcurrentGC_thread() is true.
>>
>> Similar with ZNMethodPurgeClosure [2] , nm->make_unloaded() has similar assertion.
>>
>> What did I miss?
>>
>> Thanks,
>>
>> -Zhengyu
>>
>>
>> [1] http://hg.openjdk.java.net/jdk/jdk/file/542735f2a53e/src/hotspot/share/gc/z/zNMethod.cpp#l279
>>
>> [2] http://hg.openjdk.java.net/jdk/jdk/file/542735f2a53e/src/hotspot/share/gc/z/zNMethod.cpp#l357
>>
>>
>>
>