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

Daniel D. Daugherty daniel.daugherty at oracle.com
Tue Oct 20 14:51:40 UTC 2015

On 10/20/15, 1:53 AM, David Holmes wrote:

Hi Dan,

Great find in getting to the bottom of this one!

Thanks! And thanks for the fast review!

Also, welcome back from vacation... hope you had a blast.

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.

Saw your comment, but I already had the current fix in my stress testing cycle so I went with it...

I could use OrderAccess::storestore(), but I think I'm more fond of "OrderAccess::release_store_ptr(&gBlockList, temp)". To me the semantics are more clear:

release previous stores before storing a new value in this variable

I think OrderAccess::release_store_ptr() is also a little less heavyweight than OrderAccess::storestore(), but I'd have to re-read everything in orderAccess.?pp to nail that down for sure.

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 loadacquire() or a loadLoad() (if using storeStore()).**

Yes, Carsten is right and I was modeling after other ObjectMonitor code that doesn't do this quite right. I'll fix this to use load_acquire().

** 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. :(

Hey it seems to work! (famous last words)

src/share/vm/runtime/vmStructs.cpp

Can you not just define volatilestaticfield?

Yes, I went that way originally and then I changed my mind to avoid colliding with the other format. See below.

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.

It's meant to be odd because it follows the actual decl:

 static ObjectMonitor * volatile gBlockList;

So "static pointer volatile field" is exactly what I have:

 static ObjectMonitor * volatile gBlockList;

=> (static ObjectMonitor *) volatile gBlockList;

i.e. I have a static ObjectMonitor pointer that is volatile

Compared to these two forms:

 static volatile ObjectMonitor * gBlockList;
 static ObjectMonitor volatile * gBlockList;

=> static (volatile ObjectMonitor) * gBlockList; => static (ObjectMonitor volatile) * gBlockList;

i.e. I have a static pointer to a volatile ObjectMonitor.

Hopefully, this makes my reasons a bit more clear...

Dan

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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