RFR (XXS): 8023461: Thread holding lock at safepoint that vm can block on: MethodCompileQueue_lock

[Vladimir Kozlov]

Relaxing the check is not the fix. ATP::select_task() should be fixed - 
exclude safepoints in it.
The update_rate() could be not called if there are no MethodCounters 
(other places may need to be fixed for that). Actually I don't 
understand how we have compilation request for a method without 
MethodCounters. Counters should be created before that.
And removing stale tasks could be done in an other place, outside MCQ lock.

Thanks,
Vladimir K

On 5/12/14 11:15 AM, Vladimir Ivanov wrote:
> http://cr.openjdk.java.net/~vlivanov/8023461/webrev.02/
> https://bugs.openjdk.java.net/browse/JDK-8023461
>
> CompileQueue::get() acquires MethodCompileQueue lock before extracting a
> task from the compilation queue. In Tiered mode it calls
> AdvancedThresholdPolicy::select_task, which can safepoint in multiple
> places. It fires an assert which checks there are no VM locks held VM
> thread can block on.
>
> I checked the code and didn't find any scenarios where VM acquires MCQ
> lock and submits a compilation task from a safepoint, so I decided to
> relax the assert in Thread::check_for_valid_safepoint_state for now and
> exclude MethodCompileQueue in Tiered mode from the check.
>
> I tried to rewrite ATP::select_task to avoid safepoints, but didn't
> succeed. I filed a RFE to further investigate that path (JDK-8042925
> [3]: Consider avoiding safepoints in ...::select_task). Eventually I
> want ATP::select_task to become safepoint-free and then strengthen the
> check back.
>
> For the problems spotted during analysis, I filed 2 followup bugs:
>    [1] JDK-8042924: Possible memory leak during MethodCounters
> initialization
>    [2] JDK-8042926: AdvancedThresholdPolicy::update_rate should use
> handles for Method*
>
> Best regards,
> Vladimir Ivanov
>
> [1] https://bugs.openjdk.java.net/browse/JDK-8042924
> [2] https://bugs.openjdk.java.net/browse/JDK-8042926
> [3] https://bugs.openjdk.java.net/browse/JDK-8042925