Questions on concurrent class unloading

[Erik Osterlund]

Hi Zhengyu,

The code is called by concurrent GC threads, so is_ConcurrentGC_thread() should return true.

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
> 
> 
>