RFR (S/M) JDK-8010196 NPG: Internal Error: Metaspace allocation lock -- possible deadlock

[Stefan Karlsson]

On 04/08/2013 03:49 PM, Mikael Gerdin wrote:
> Hi
>
> The problem is that when running the G1 garbage collector a call to 
> objArrayOopDesc::obj_at_put can in rare cases cause the G1 dirty card 
> queue buffer to fill up and this will cause G1 to take 
> DirtyCardQ_CBL_mon/16 to return the full buffer and get a new one.
>
> Adding dependencies to a ClassLoaderData is currently protected by the 
> per-metaspace "Metaspace allocation lock"/5 (which protects more than 
> just allocations).
>
> Because I want to avoid messing around with the lock ordering I 
> suggest that we use an ObjectLocker on the _dependencies oop in 
> ClassLoaderData.
> _dependencies is a 2-element objArrayOop, in effect it's an ad-hoc 
> linked list of ClassLoaders/Classes which must be kept alive by this CLD.
>
> Using an ObjectLocker on the head element of the linked list should be 
> at least as good as using the metaspace_lock(). There should not be 
> any new deadlock issues with VM mutexes since any application thread 
> could use a similar synchronized linked list construct.
>
> As a bonus I suggest that we factor out the dependency handling from 
> ClassLoaderData into a inner class: ClassLoaderData::Dependencies and 
> let Dependencies manage the head of the linked list by itself. This 
> should make it more clear that the dependency list is not guarded by 
> the metaspace lock anymore.
>
> Buglinks:
> https://jbs.oracle.com/bugs/browse/JDK-8010196
> http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=8010196
>
> Testing:
> default jprt run
> jdk/test/java/lang/invoke/BigArityTest.java (which could reproduce the 
> issue with -XX:+UseG1GC -XX:G1UpdateBufferSize=1)
>
> Webrevs:
> Incremental:
> http://cr.openjdk.java.net/~mgerdin/8010196/webrev.0-fix/webrev
> http://cr.openjdk.java.net/~mgerdin/8010196/webrev.0-factor-out/webrev
> http://cr.openjdk.java.net/~mgerdin/8010196/webrev.0-rename/webrev
> Everything:
> http://cr.openjdk.java.net/~mgerdin/8010196/webrev.0/webrev

Looks good.

StefanK

>
> Thanks
> /Mikael