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

Robbin Ehn robbin.ehn at oracle.com
Sun Oct 28 19:58:19 UTC 2018

Thanks Erik!


On 26/10/2018 16:37, Erik Österlund wrote:
> Hi Robbin,
> Looks good.
> Thanks,
> /Erik
> On 2018-10-26 16:33, Robbin Ehn wrote:
>> 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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