RFR: 8281615: Deadlock caused by jdwp agent [v2]

[Zhengyu Gu]

On Tue, 15 Feb 2022 06:36:36 GMT, Chris Plummer <cjplummer at openjdk.org> wrote:

>> src/jdk.jdwp.agent/share/native/libjdwp/classTrack.c line 100:
>> 
>>> 98:     struct bag* deleted = deletedSignatures;
>>> 99:     deletedSignatures = NULL;
>>> 100:     debugMonitorExit(classTrackLock);
>> 
>> This looks risky as the critical section is broken and the NULL deleted signatures exposed. If `cbTrackingObjectFree` occurs while this is true then you will lose the record of the deleted signature.
>> 
>> Alternatively you could allow for lock-free reading of `deletedSignatures`, preemptively allocate a new bad if needed then take the lock. Or even use the lock to read `deletedSignatures` to determine if a new bag is needed, then drop the lock, create the bag, take the lock and re-check everything.
>
> Agreed.

Fixed

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PR: https://git.openjdk.java.net/jdk/pull/7461