RFR 8234613: JavaThread can escape back to Java from an ongoing handshake
David Holmes
david.holmes at oracle.com
Mon Nov 25 22:16:22 UTC 2019
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
>
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