RFR 8234613: JavaThread can escape back to Java from an ongoing handshake

[David Holmes]

Hi Patricio,

v2 looks good!

Thanks,
David

On 26/11/2019 4:48 am, Patricio Chilano wrote:
> Hi David,
> 
> On 11/25/19 1:46 AM, David Holmes wrote:
>> Hi Patricio,
>>
>> On 23/11/2019 4:25 am, Patricio Chilano wrote:
>>> Hi,
>>>
>>> This patch aims to address a current bug where, given the right 
>>> combination of handshakes and external suspend/resume, a JavaThread 
>>> can transition from a safe state back to Java without blocking for a 
>>> still-in-progress handshake. In the description of the bug I added an 
>>> example, tracing the state changes of the JavaThread as it goes 
>>> through the different transitions until it escapes the handshake. 
>>> Currently, the window of time for this issue to happen is so small 
>>> that we do not see actual failures running tests. Running test 
>>> SuspendAtExit.java and adding some small delay before restoring the 
>>> JavaThread state in java_suspend_self_with_safepoint_check() can 
>>> demonstrate the issue.
>>
>> Good catch. This highlights how difficult it is to see where all the 
>> thread-state-transitions are and reason about what can and can't 
>> happen in a given sequence of code.
>>
>>> The proposed fix is to check again if we have a pending/in-progress 
>>> handshake operation after executing ~ThreadInVMForHandshake().
>>
>> Minor nit but given we end up calling process_self_inner only after it 
>> was determined the current _handshake has_operation(), then the while 
>> loop should really be a do-while loop ?
> Right, changed!
> 
> Here is v2:
> http://cr.openjdk.java.net/~pchilanomate/8234613/v02/webrev/ 
> <http://cr.openjdk.java.net/~pchilanomate/8234613/v02/webrev/src/hotspot/share/runtime/handshake.cpp.udiff.html>
> 
> Thanks for looking at this David!
> 
> Patricio
>> Thanks,
>> David
>> -----
>>
>>> Tested with mach5, tiers1-6 on all platforms (Linux, macOS, Windows 
>>> and Solaris).
>>>
>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8234613
>>> Webrev: http://cr.openjdk.java.net/~pchilanomate/8234613/v01/webrev/
>>>
>>> Thanks,
>>> Patricio
>