RFR: 8212933: Thread-SMR: requesting a VM operation whilst holding a ThreadsListHandle can cause deadlocks (original) (raw)

Robbin Ehn robbin.ehn at oracle.com
Fri Oct 26 14:33:18 UTC 2018

Hi, please review.

When the VM thread executes a handshake it uses different ThreadsLists during the execution. A JavaThread that is armed for the handshake when it is already in the exit path in VM will cancel the handshake. Even if the VM thread cannot see this thread after the initial ThreadsList which where used for arming, the handshake can progress when the exiting thread cancels the handshake.

But if a third thread takes a ThreadsList where the exiting JavaThread is present and tries to execute a VM operation, hence waiting on VM thread to finish the handshake, the JavaThread in the exit path can never reach the handshake cancellation point. VM thread cannot finishes the handshake and the third thread is stuck waiting on the VM thread.

To allow holding a ThreadsList when executing a VM operation we instead let the VM thread use the same ThreadsList over the entire handshake making all armed threads visible to the VM thread at all time. And if VM thread spots a terminated thread it will count that thread is already done by only clearing it's operation.

Passes local stress testing, t1-5 and the deadlock is no longer reproduce-able. Added a jtreg handshake + thread suspend test as a reproducer.

Issue: https://bugs.openjdk.java.net/browse/JDK-8212933 Code: http://cr.openjdk.java.net/~rehn/8212933/v1/webrev/

Thanks, Robbin

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