On Mon, 14 Feb 2022 14:27:45 GMT, Zhengyu Gu <z...@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. ------------- PR: https://git.openjdk.java.net/jdk/pull/7461
