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8281615: Deadlock caused by jdwp agent #7461
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Original file line number | Diff line number | Diff line change |
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@@ -1,5 +1,5 @@ | ||
/* | ||
* Copyright (c) 2001, 2005, Oracle and/or its affiliates. All rights reserved. | ||
* Copyright (c) 2001, 2022, Oracle and/or its affiliates. All rights reserved. | ||
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. | ||
* | ||
* This code is free software; you can redistribute it and/or modify it | ||
|
@@ -81,14 +81,22 @@ cbTrackingObjectFree(jvmtiEnv* jvmti_env, jlong tag) | |
struct bag * | ||
classTrack_processUnloads(JNIEnv *env) | ||
{ | ||
debugMonitorEnter(classTrackLock); | ||
if (deletedSignatures == NULL) { | ||
// Class tracking not initialized, nobody's interested. | ||
debugMonitorExit(classTrackLock); | ||
return NULL; | ||
return NULL; | ||
} | ||
|
||
/* Allocate new bag outside classTrackLock lock to avoid deadlock. | ||
* | ||
* Note: jvmtiAllocate/jvmtiDeallocate() may be blocked by ongoing safepoints. | ||
* It is dangerous to call them (via bagCreateBag/bagDestroyBag())while holding monitor(s), | ||
* because jvmti may post events, e.g. JVMTI_EVENT_OBJECT_FREE at safepoints and event processing | ||
* code may acquire the same monitor(s), e.g. classTrackLock in cbTrackingObjectFree(), | ||
* which can lead to deadlock. | ||
*/ | ||
struct bag* new_bag = bagCreateBag(sizeof(char*), 10); | ||
debugMonitorEnter(classTrackLock); | ||
struct bag* deleted = deletedSignatures; | ||
deletedSignatures = bagCreateBag(sizeof(char*), 10); | ||
deletedSignatures = new_bag; | ||
debugMonitorExit(classTrackLock); | ||
return deleted; | ||
} | ||
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@@ -194,8 +202,11 @@ classTrack_initialize(JNIEnv *env) | |
void | ||
classTrack_activate(JNIEnv *env) | ||
{ | ||
// Allocate bag outside classTrackLock lock to avoid deadlock. | ||
// See comments in classTrack_processUnloads() for details. | ||
struct bag* new_bag = bagCreateBag(sizeof(char*), 1000); | ||
There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. I don't think there can be any race during activation but this change is harmless. There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. I am not sure classTrack_activate() has the problem, but it is called outside of initialization, inside debug loop ... so I assume that event callback might happen. There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. I agree. This code is called when the debug agent receives a CLASS_UNLOAD EventRequest (which the debug agent handles by requesting JVMTI GC_FINISH events). So there is nothing that prevents There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. In that case the There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. Okay I see from later comments that in fact no such race exists, but we have to synchronize with cbTrackingObjectFree There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. Bah! But then a comment after that says it does exist again. There seem to be issues understanding exactly what the concurrent call sequences can be with this code. There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. I think you might be confusing There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. ...and even with the race, I still I think it is safe for a new |
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debugMonitorEnter(classTrackLock); | ||
deletedSignatures = bagCreateBag(sizeof(char*), 1000); | ||
deletedSignatures = new_bag; | ||
debugMonitorExit(classTrackLock); | ||
} | ||
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@@ -214,12 +225,14 @@ void | |
classTrack_reset(void) | ||
{ | ||
debugMonitorEnter(classTrackLock); | ||
struct bag* to_delete = deletedSignatures; | ||
deletedSignatures = NULL; | ||
debugMonitorExit(classTrackLock); | ||
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if (deletedSignatures != NULL) { | ||
bagEnumerateOver(deletedSignatures, cleanDeleted, NULL); | ||
bagDestroyBag(deletedSignatures); | ||
deletedSignatures = NULL; | ||
// Deallocate bag outside classTrackLock to avoid deadlock. | ||
// See comments in classTrack_processUnloads() for details. | ||
if (to_delete != NULL) { | ||
bagEnumerateOver(to_delete, cleanDeleted, NULL); | ||
bagDestroyBag(to_delete); | ||
} | ||
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debugMonitorExit(classTrackLock); | ||
} |
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Nit: need space after ))
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Fixed. Thanks, @dholmes-ora