RFR: JDK-8251397 NPE on ClassValue.ClassValueMap.cacheArray

Paul Sandoz paul.sandoz at oracle.com
Wed Aug 19 15:57:59 UTC 2020

Hi Galder,

Thanks for doing the fix and the work verifying.

I also verified using a fence fixes the jcstress test. Similarly, I found I could only reproduce the issue in HotSpot when running a test more like ClassValue (the double checked locking pattern when publishing to a plan field) with the options -XX:+StressLCM -XX:+StressGCM. I analyzed assembler output (-XX:+PrintOptoAssembly) to observe the publish store occurring before some stores to fields. It’s highly likely that with HotSpot we got lucky in this case :-)

I don’t object to the use of a release fence (LoadStore|StoreStore), but I believe we could also use a StoreStore fence in this case. Perhaps the ARM folks have a stronger opinion on this?

I marginally prefer placing the fence in the initializeMap, since the relationship between construction and publish is very clear. In either case I recommend adding a comment on why the fence is required e.g.:

// Place a Store fence as the last operation of the constructor to emulate
// ClassValueMap containing final fields. This ensures it can be
// published safely in the non-volatile field Class.classValueMap, (see initializeMap)
// since stores to the fields of ClassValueMap will not be reordered
// to occur after the store to the field type.classValueMap

I also noticed we can change the field Class.classValueMap to be @Stable, but I think we should follow up on that investigation separately. Relatedly, I had an idea to modify HotSpot so it places a StoreStore fence directly before the store to a @Stable field.


> On Aug 19, 2020, at 4:53 AM, Galder Zamarreno <galder at redhat.com> wrote:
> Hi all,
> I've created patch [1] to fix the ClassValue$ClassValueMap NPE bug in [2].
> The bug has been discussed by other members in the list in thread [3].
> The patch follows the simple fix suggested by Doug and others in that
> exchange, e.g. [4]. That is, it adds a release fence
> to ClassValue$ClassValueMap constructor to avoid the NPE.
> To verify the fix, I ran the jcstress test that Paul posted in [5] and
> played around with the difference fixes suggested in the thread. Adding the
> release fence did indeed fix the jcstress test.
> To further verify the issue, I've successfully run both the tier1 tests and
> the Quarkus native testsuite with a Mandrel 20.1 built with JDK 11.0.8
> version patched with the fix (higher JDKs not supported yet). Note that
> this NPE happens on rare occasions.
> The patch applies cleanly to JDK 11.
> Galder
> [1] Webrev:
> http://cr.openjdk.java.net/~sgehwolf/webrevs/galder/JDK-8251397/01/webrev/
> [2] Bug: https://bugs.openjdk.java.net/browse/JDK-8251397
> [3]
> https://mail.openjdk.java.net/pipermail/core-libs-dev/2020-August/068086.html
> [4]
> https://mail.openjdk.java.net/pipermail/core-libs-dev/2020-August/068126.html
> [5]
> https://mail.openjdk.java.net/pipermail/core-libs-dev/2020-August/068110.html

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