RFR 8029178: Parallel class loading test anonymous-simple gets SIGSEGV in Metaspace:, :contains

[Mikael Gerdin]

Coleen,

On Thursday 02 January 2014 12.25.25 Coleen Phillmore wrote:
> Summary: Metaspace::contains cannot look at purged metaspaces while CMS
> concurrently deallocates them.
> 
> Removed 2 calls to is_metaspace_object where the object may be in a
> deallocated metaspace.  Removed walking virtual space lists for
> determining contains because the virtual space list can change
> concurrently with the walk.   CLDG::contains is slower but no slowdowns
> with testing were observed.
> 
> Tested by SQE testbase tests, jtreg tests.   Functional testing by
> parallel class loading tests and nsk/coverage/arguments/arguments008
> (ie. calls Method::is_valid_method)
> 
> open webrev at http://cr.openjdk.java.net/~coleenp/8029178/

--- old/src/share/vm/code/dependencies.cpp	2014-01-02 
12:04:06.325014397 -0500
+++ new/src/share/vm/code/dependencies.cpp	2014-01-02 
12:04:05.877014397 -0500
@@ -655,8 +655,6 @@
   } else {
     o = _deps->oop_recorder()->metadata_at(i);
   }
-  assert(o == NULL || o->is_metaspace_object(),
-         err_msg("Should be metadata " PTR_FORMAT, o));
   return o;
 }
Why did you remove only this call to is_metaspace_object and no other?
This and none of the other callers of is_metaspace_object seem to be in any hot 
path so I agree with your assessment that this should not be a performance issue.

It seems to me that the core of the change is to not check the _unloading list in 
CLDG::contains and then change all calls to go through CLDG::contains, the CLD and 
then the SpaceManagers and Chunks, that seems like a reasonable approach.

I like that you put CLDG::contains in PRODUCT in os::print_location!

The old code incorrectly calls the static Metaspace::contains for each CLD in 
CLDG::contains but calls it like metaspace_or_null->contains(x) so it's not strange 
that it was found to be slow :)

/Mikael

> bug link https://bugs.openjdk.java.net/browse/JDK-8029178
> 
> Thanks,
> Coleen