RFR: 8038925: Java with G1 crashes in dump_instance_fields using jmap or jcmd without fullgc

[Bengt Rutisson]

Hi Andreas,

The fix looks good.

One comment about the test. It does not verify that no full GC happens. 
The way the test is set up I guess that should not happen and I am not 
sure it is worth the effort to add a check for it. Just wanted to 
mention it if you want to make test more resilient to future changes in 
the JVM that for some reason can trigger a full GC for this test.

I'm fine with leaving the test as it is.

Thanks,
Bengt


On 5/20/14 4:26 PM, Andreas Eriksson wrote:
> Hi,
>
> Could I have a review for this G1 jdk7 only fix please?
> (There is a related problem in jdk8 and jdk9, where an assert can fail 
> because of this problem. I have logged another bug for this.)
>
> Description:
> Due to the marking cleanup reclaiming empty regions, and having stale 
> references a crash can occur when doing a heap dump.
> The code tries to do an is_klass check on the object, which crashes 
> the VM.
> The fix is to add an is_perm check before doing the check, since 
> is_perm will do a bounds check on the oop and if it's in the perm gen 
> we know it's safe to look at it since G1 only ever does full 
> compactions of the perm gen.
>
> For more information, and a more in-depth analysis, please see the 
> jira bug.
>
> http://cr.openjdk.java.net/~aeriksso/8038925/webrev.01/
> https://bugs.openjdk.java.net/browse/JDK-8038925
>
> Regards,
> Andreas