RFR: 8281615: Deadlock caused by jdwp agent

[Chris Plummer]

On Mon, 14 Feb 2022 14:27:45 GMT, Zhengyu Gu <zgu at openjdk.org> wrote:

> There are scenarios that JDWP agent can deadlock on `classTrackLock` monitor. Following is the scenario in bug report.
> 
> **Java Thread** 
> -   loads a class and post `JVMTI_EVENT_CLASS_PREPARE` event
> - JDWP event callback handler calls `classTrack_processUnloads()` to handle the event.
> - `classTrack_processUnloads()` takes `classTrackLock` lock, then tries to allocate a new bag under the lock.
> - bag allocation code calls` jvmtiAllocate()`, which may be blocked by ongoing safepoint due to state transition.
> 
> If the safepoint is GC safepoint (prior to JDK16) or `VM_JvmtiPostObjectFree`  safepoint (JDK16 or later)
> 
> **VM Thread**
> - post `JVMTI_EVENT_OBJECT_FREE`
> - JDWP event callback handler calls `cbTrackingObjectFree()` to handle the event
> - `cbTrackingObjectFree()` tries to acquire `classTrackLock` lock, leads to deadlock
> 
> From my research, there are three events that may be posted at safepoints, `JVMTI_EVENT_GARBAGE_COLLECTION_START`, `JVMTI_EVENT_GARBAGE_COLLECTION_FINISH` and  `JVMTI_EVENT_OBJECT_FREE`, but only  `JVMTI_EVENT_OBJECT_FREE` is relevant to JDWP agent.
> 
> The solution I purpose here, is simply move allocation/deallocation code outside of `classTrackLock` lock.
> 
> 
> Test:
> - [x] tier1 
> - [x] vmTestbase_nsk_jdi
> - [x] vmTestbase_nsk_jdwp
> - [x] vmTestbase_nsk_jvmti

Slight correction to the following in  your description:
> JDWP event callback handler calls classTrack_processUnloads() to handle the event.

classTrack_processUnloads() isn't called to handle the event. It's called as a side affect of handling the event.

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