RFR 8234613: JavaThread can escape back to Java from an ongoing handshake
David Holmes
david.holmes at oracle.com
Mon Nov 25 06:46:10 UTC 2019
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 ?
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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