RFR: 8231289: Disentangle JvmtiRawMonitor from ObjectMonitor and clean it up

David Holmes david.holmes at oracle.com
Thu Oct 3 10:13:21 UTC 2019

Hi Dan,

On 3/10/2019 3:20 am, Daniel D. Daugherty wrote:
> Sorry for the delay in reviewing this one... I've been playing whack-a-mole
> with Robbin's MoCrazy test and my AsyncMonitorDeflation bits...

No problem - your contribution made the wait worthwhile :)

> On 9/24/19 1:09 AM, David Holmes wrote:
>> Bug: https://bugs.openjdk.java.net/browse/JDK-8231289
>> webrev: http://cr.openjdk.java.net/~dholmes/8231289/webrev/
> src/hotspot/share/prims/jvmtiEnv.cpp
>      Thanks for removing the PROPER_TRANSITIONS stuff. That was old
>      and crufty stuff.
> src/hotspot/share/prims/jvmtiEnvBase.cpp
>      No comments.
> src/hotspot/share/prims/jvmtiRawMonitor.cpp
>      L39:   new (ResourceObj::C_HEAP, mtInternal) 
> GrowableArray<JvmtiRawMonitor*>(1,true);
>          nit - need a space between ',' and 'true'.
>          Update: leave for your follow-up bug.

Fixed now so I don't forget later. :)

> src/hotspot/share/prims/jvmtiRawMonitor.hpp
>      No comments.
> src/hotspot/share/runtime/objectMonitor.hpp
>      Glad I added those 'protected for JvmtiRawMonitor' in one
>      of my recent cleanup bugs. Obviously I'll have to merge
>      with Async Monitor Deflation. :-)
> src/hotspot/share/runtime/thread.cpp
>      No comments.
> src/hotspot/share/runtime/thread.hpp
>      No comments.
> src/hotspot/share/services/threadService.cpp
>      L397:     waitingToLockMonitor = jt->current_pending_monitor();
>      L398:     if (waitingToLockMonitor == NULL) {
>      L399:       // we can only be blocked on a raw monitor if not 
> blocked on an ObjectMonitor
>      L400:       waitingToLockRawMonitor = 
> jt->current_pending_raw_monitor();
>      L401:     }
>          JVM/TI has this event handler:
>            typedef void (JNICALL *jvmtiEventMonitorContendedEnter)
>                (jvmtiEnv *jvmti_env,
>                 JNIEnv* jni_env,
>                 jthread thread,
>                 jobject object);
>          This event handler is called after set_current_pending_monitor()
>          and if the event handler uses a RawMonitor, then it possible for
>          for the thread to show up as blocked on both a Java monitor and
>          a JVM/TI RawMonitor.

Oh that is interesting - good catch! So that means the current code is 
broken because the raw monitor will replace the ObjectMonitor as the 
pending monitor and then set it back to NULL, thus losing the fact the 
thread is actually pending on the ObjectMonitor. And of course while the 
pending monitor is the raw monitor that totally messes up the deadlock 
detection as the ObjectMonitor is missing from consideration. :(

This also probably means that you can have a pending raw monitor at the 
same time as you have a "Blocker" as I'm pretty sure there are various 
JVM TI event handlers that may execute between the Blocker being set and 
the actual park. So that would be an additional breakage in the existing 

Back to my code and I have two problems. The second, which is easy to 
address, is the deadlock printing code. I'll hoist the 
waitingToLockRawMonitor chunk to the top so it is executed independent 
of the waitingToLockMonitor value (which remains in an if/else 
relationship with the waitingToLockBlocker). But now that we might print 
two "records" at a time I have to make additional changes to get 
meaningful output for the current thread (which is handled as a common 
code after the if/else block to finish whichever record was being 
printed). Also I can no longer use "continue" as the 3 outcomes are not 
mutually exclusive - so this could get a bit messy. :(

So definitely a v2 webrev on the way.

But before that I need to solve my first problem - and I don't know how. 
Now that it is apparent that a thread can be blocked on both a raw 
monitor and an ObjectMonitor at the same time, I have no idea how to 
actually account for this in the deadlock detection code. That code has 
a while loop that expects to at most find either a locked ObjectMonitor 
or j.u.c Blocker, and it adds the owner thread to the cycle detection, 
then moves on. But now I can have two different owner threads in the 
same loop iteration. I don't know how to account for that.

Given that it seems to me that the current code is already broken if we 
encounter these conditions, then perhaps all I can do is handle the 
other cases, where the blocking reasons are mutually exclusive, and not 
try to fix things? i.e. leave lines #434 to #440 as they are in webrev 
v1 - which implies no change to line #398 except the comment ... ??

> test/hotspot/jtreg/vmTestbase/nsk/jvmti/RawMonitorWait/rawmnwait005/rawmnwait005.cpp 
>      No comments.
> Thumbs up! The only non-nit I have is the setting of 
> waitingToLockRawMonitor
> on L400 and the corresponding comment on L399. Everything else is a nit.
> I don't need to see a new webrev.

If only that were true :(


> Thanks for tackling this disentangle issue!
> Dan
>> The earlier attempt to rewrite JvmtiRawMonitor as a simple wrapper 
>> around PlatformMonitor proved not so simple and ultimately had too 
>> many issues due to the need to support Thread.interrupt.
>> I'd previously stated in the bug report:
>> "In the worst-case I suppose we could just copy ObjectMonitor to a new 
>> class and have JvmtiRawMonitor continue to extend that (with some 
>> additional minor adjustments) - or even just inline it all as needed."
>> but hadn't looked at it in detail. Richard Reingruber did look at it 
>> and pointed out that it is actually quite simple - we barely use any 
>> actual code from ObjectMonitor, mainly just the state. So thanks 
>> Richard! :)
>> So this change basically copies or moves anything needed by 
>> JvmtiRawMonitor from ObjectMonitor, breaking the connection between 
>> the two. We also copy and simplify ObjectWaiter, turning it into a 
>> QNode internal class. There is then a lot of cleanup that was applied 
>> (and a lot more that could still be done):
>> - Removed the never implemented/used PROPER_TRANSITIONS ifdefs
>> - Fixed the disconnect between the types of non-JavaThreads expected 
>> by the upper layer code and lower layer code
>> - cleaned up and simplified return codes
>> - consolidated code that is identical for JavaThreads and 
>> non-JavaThreads (e.g. notify/notifyAll).
>> - removed used of TRAPS/THREAD where not appropriate and replaced with 
>> "Thread * Self" in the style of the rest of the code
>> - changed recursions to be int rather than intptr_t (a "fixme" in the 
>> ObjectMonitor code)
>> I have not changed the many style flaws with this code:
>> - Capitalized names
>> - extra spaces before ;
>> - ...
>> but could do so if needed. I wanted to try and keep it more obvious 
>> that the fundamental functional code is actually unmodified.
>> There is one aspect that requires further explanation: the notion of 
>> current pending monitor. The "current pending monitor" is stored in 
>> the Thread and used by a number of introspection APIs for things like 
>> finding monitors, doing deadlock detection, etc. The JvmtiRawMonitor 
>> code would also set/clear itself as "current pending monitor". Most 
>> uses of the current pending monitor actually, explicitly or 
>> implicitly, ignore the case when the monitor is a JvmtiRawMonitor 
>> (observed by the fact the mon->object() query returns NULL). The 
>> exception to that is deadlock detection where raw monitors are at 
>> least partially accounted for. To preserve that I added the notion of 
>> "current pending raw monitor" and updated the deadlock detection code 
>> to use that.
>> The test:
>> test/hotspot/jtreg/vmTestbase/nsk/jvmti/RawMonitorWait/rawmnwait005/rawmnwait005.cpp 
>> was updated because I'd noticed previously that it was the only test 
>> that used interrupt with raw monitors, but was in fact broken: the 
>> test thread is a daemon thread so the main thread could terminate the 
>> VM immediately after the interrupt() call, thus you would never know 
>> if the interruption actually worked as expected.
>> Testing:
>>  - tiers 1 - 3
>>  - vmTestbase/nsk/monitoring/  (for deadlock detection**)
>>  - vmTestbase/nsk/jdwp
>>  - vmTestbase/nsk/jdb/
>>  - vmTestbase/nsk/jdi/
>>  - vmTestbase/nsk/jvmti/
>>  - serviceability/jvmti/
>>  - serviceability/jdwp
>>  - JDK: java/lang/management
>> ** There are no existing deadlock related tests involving 
>> JvmtiRawMonitor. It would be interesting/useful to add them to the 
>> existing nsk/monitoring tests that cover synchronized and JNI locking. 
>> But it's a non-trivial enhancement that I don't really have time to do.
>> Thanks,
>> David
>> -----

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