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List:       openjdk-serviceability-dev
Subject:    Re: RFR: 8231289: Disentangle JvmtiRawMonitor from ObjectMonitor and clean it up
From:       "serguei.spitsyn () oracle ! com" <serguei ! spitsyn () oracle ! com>
Date:       2019-10-07 17:53:42
Message-ID: d17d9739-29fc-5043-4be1-b00913ac50d1 () oracle ! com
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On 10/7/19 04:23, David Holmes wrote:
> Hi Serguei,
>
> On 7/10/2019 6:42 pm, serguei.spitsyn@oracle.com wrote:
>> Hi David,
>>
>> Suppose, T1 thread is blocked on a raw monitor hold by thread T3 in 
>> JVMTI
>> MonitorContendedEntered callback on an object monitor hold by thread T2.
>>
>> In theory, if only raw monitor is used in further deadlock detection 
>> analysis
>> then a possible dependency loop with T1 an T2 involved won't be 
>> detected.
>> For full analysis the dependency chain through object monitor have to 
>> be also analyzed.
>> This would add extra complexity into the deadlock detection code for 
>> very rare corner case.
>>
>> I do not care much about this, and so, like the approach.
>
> To be clear the case you describe is not handled by either the current 
> code nor my proposed update.

Yes, of course.
Sorry, I have not made it clear.

Thanks,
Serguei

>> The fix itself looks good to me.
>
> Thanks.
>
> David
>
>> Thanks,
>> Serguei
>>
>>
>> On 10/3/19 20:58, David Holmes wrote:
>>> Okay, to allow for me to make forward progress here I've decided to 
>>> follow the "principle of least brokenness" ;-)
>>>
>>> Recap: Because of JVMTI event callbacks it is possible for a thread 
>>> to set its current pending monitor as a JvmtiRawMonitor when it was 
>>> already set for an ObjectMonitor. This is broken in at least two ways:
>>> - when the raw monitor use completes the pending monitor is set to 
>>> NULL, not restored to the ObjectMonitor
>>> - whilst the raw monitor is seen as the pending monitor the 
>>> ObjectMonitor is not considered by the deadlock detection logic
>>>
>>> Ignoring what I'm currently doing with jvmtiRawMonitor, I do not 
>>> know how to fix the above brokenness and it is out of scope for what 
>>> I am trying to do.
>>>
>>> So what I propose is to make things no more broken than they are 
>>> now, and actually improve things a little:
>>> - the pending JvmtiRawMonitor is given preference over the 
>>> ObjectMonitor in the deadlock detection code (this emulates current 
>>> situation of the raw monitor overwriting the pending ObjectMonitor)
>>> - we no longer lose the fact we were also pending on an ObjectMonitor
>>> - the stack printing code in the deadlock detector prints 
>>> information about both the raw monitor and the ObjectMonitor
>>>
>>> Updated webrev:
>>>
>>> http://cr.openjdk.java.net/~dholmes/8231289/webrev.v2/
>>>
>>> The only change is threadService.cpp
>>>
>>> Thanks,
>>> David
>>

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