Incorrect obj_is_alive logic? (Was: What does BufferBlob::Interpreter in a JVM crash log mean)

[Krystal Mok]

Hi all,

Comments on bug 7066603, inline below:

On Wed, Nov 16, 2011 at 4:05 AM, Tom Rodriguez <tom.rodriguez at oracle.com>
 wrote:

>
> On Nov 14, 2011, at 2:00 PM, Martin Hare Robertson wrote:
> > The biggest unresolved problem was reported under bug 7066603. Sadly
> this seems to have been marked as low priority.
>
> It seems to have gotten stuck over in the wrong category.  I've moved into
> the proper category and updated it.  It appears CMS isn't visiting some
> regular Java objects held in perm so they are missing from the dump.  The
> recent changes to move Strings and Classes out of perm make this problem
> disappear in JDK7.  So it only affects JDK6 at this point.  I believe the
> bug is in the obj_is_alive logic that CMS uses to visit only live objects
> in perm.
>

Looking at the repro example in bug 7066603, I agree that the problem seem
to lie in obj_is_alive logic.

A quick patch to HotSpot 20.0 [1]:

diff -r f0f676c5a2c6
src/share/vm/gc_implementation/concurrentMarkSweep/compactibleFreeListSpace.cpp
---
a/src/share/vm/gc_implementation/concurrentMarkSweep/compactibleFreeListSpace.cpp
  Tue Mar 15 19:30:16 2011 -0700
+++
b/src/share/vm/gc_implementation/concurrentMarkSweep/compactibleFreeListSpace.cpp
  Wed Nov 16 20:38:29 2011 +0800
@@ -1143,7 +1143,7 @@
         return (dead_map->sizeInBits() == 0) // bit_map has been allocated
                || !dead_map->par_isMarked((HeapWord*) p);
       } else {
-        return false; // We can't say for sure if it's live, so we say
that it's dead.
+        return true; // We can't say for sure if it's live, so we say that
it's alive.
       }
     }
   }

With this patch, re-run the repro, and jhat doesn't complain any more.

I'm not sure yet if this is the right change. It's interesting to find that
in the repro example, all objects missing from the heap dump seem to be
Strings referenced by objects in PermGen; the roots are most probably
static fields.

Regards,
Kris Mok

[1]:
http://hg.openjdk.java.net/hsx/hsx20/master/file/f0f676c5a2c6/src/share/vm/gc_implementation/concurrentMarkSweep/compactibleFreeListSpace.cpp


> tom
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