RFR (XS) 8047212: fix race between ObjectMonitor alloc and verification code (original) (raw)

David Holmes david.holmes at oracle.com
Tue Oct 20 07:53:01 UTC 2015

Hi Dan,

Great find in getting to the bottom of this one!

On 20/10/2015 1:02 PM, Daniel D. Daugherty wrote:

Greetings,

I have a fix for a long standing race between the lock-free ObjectMonitor verification code and the normal (locked) ObjectMonitor block allocation code path. For this fix, I would like at least a Runtime team reviewer and a Serviceability team reviewer. Thanks! JDK-8047212 runtime/ParallelClassLoading/bootstrap/random/inner-complex assert(ObjectSynchronizer::verifyobjmonisinpool(inf)) failed: monitor is invalid https://bugs.openjdk.java.net/browse/JDK-8047212 Webrev URL: http://cr.openjdk.java.net/~dcubed/8047212-webrev/0-jdk9-hs-rt/

src/share/vm/runtime/synchronizer.cpp

I would have just used OrderAccess::storeStore() as per my comment in the CR. But as Carsten says a "release" should be paired with an "acquire". Which suggests that in the other code that reads these variables we also need either the load_acquire() or a loadLoad() (if using storeStore()).**

** This symmetry is largely missing in our lock-free code, and I think we've been relying on "volatile" to act as a compiler barrier. :(

src/share/vm/runtime/vmStructs.cpp

Can you not just define volatile_static_field? Why does the ptr aspect need to come into it? Also "static pointer volatile field" sounds really odd, it is a static, volatile field that happens to be a pointer-type.

Thanks, David

Testing: Aurora Adhoc RT-SVC nightly batch 4 inner-complex fastdebug parallel runs for 4+ days and 600K iterations without seeing this failure; the experiment is still running; final results to be reported at the end of the review cycle JPRT -testset hotspot

This fix: - makes ObjectMonitor::gBlockList volatile - uses "OrderAccess::releasestoreptr(&gBlockList, temp)" to make sure the new block updates happen before gBlockList is changed to refer to the new block - add SA support for a "static pointer volatile" field like: static ObjectMonitor * volatile gBlockList; See the following link for a nice description of what "volatile" means in the different positions on a variable/parameter decl line: http://www.embedded.com/electronics-blogs/beginner-s-corner/4023801/Introduction-to-the-Volatile-Keyword

Thanks, in advance, for any comments, questions or suggestions. Dan

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