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

[Vladimir Ivanov]

> Relaxing the check is not the fix. ATP::select_task() should be fixed -
> exclude safepoints in it.
OK. I planned to fix select_task separately (filed RFE to track that), 
since people complains about intermittent failures during testing due to 
this bug and I have some concerns with the fix I had. But I'll try to 
fix it as part of this change then.

> 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.
It's not always the case. MDO can be created first (e.g. compilation can 
force MDO creation [1]) and there's no need in MethodCounters (see 
InterpreterGenerator::generate_counter_incr [2]).

> And removing stale tasks could be done in an other place, outside MCQ lock.
I looked again closely at the code and I think I found a sweet spot.

Updated webrev:
http://cr.openjdk.java.net/~vlivanov/8023461/webrev.03/

Testing: JPRT, VM testbase, bigapps, jtreg

Best regards,
Vladimir Ivanov

[1]
Method::build_interpreter_method_data(methodHandle, Thread*)
ciMethod::ensure_method_data(methodHandle)
ciMethod::ensure_method_data()
GraphBuilder::try_inline_full(ciMethod*, bool, Bytecodes::Code, 
Instruction*)

[2] 
http://hg.openjdk.java.net/jdk9/hs-comp/hotspot/file/tip/src/cpu/x86/vm/templateInterpreter_x86_32.cpp#l340

> 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