RFR(XXS): 8162945: HotspotDiagnosticMXBean getFlags erroneously reports OutOfMemory

[Markus Gronlund]

Greetings,

 

Please review this small fix to address some new issues seen in testing where 
OOM is erroneously being reported:

 

Bug: https://bugs.openjdk.java.net/browse/JDK-8162945

 

Changeset: 

 

# HG changeset patch

# User mgronlun

# Date 1470141649 -7200

#      Tue Aug 02 14:40:49 2016 +0200

# Node ID e06e6474ff5f5798f9249aeaa6b33ec6aa021563

# Parent  9672159305d72f5dd430a3edd4b97c4e5bc816e0

[mq]: 8162945

 

diff --git a/src/jdk.management/share/native/libmanagement_ext/Flag.c 
b/src/jdk.management/share/native/libmanagement_ext/Flag.c

--- a/src/jdk.management/share/native/libmanagement_ext/Flag.c

+++ b/src/jdk.management/share/native/libmanagement_ext/Flag.c

@@ -142,7 +142,7 @@

             continue;

         }

-        if (valueObj == NULL) {

+        if (valueObj == NULL && !(globals[i].type == 
JMM_VMGLOBAL_TYPE_JSTRING)) {

             free(globals);

             JNU_ThrowOutOfMemoryError(env, 0);

             return 0;

 

 

Summary:

 

OOM's have manifested after the following change was integrated: 

8162524: src/jdk.management/share/native/libmanagement_ext/Flag.c doesn't 
handle JNI exceptions 

The following check was then added to 
jdk\src\jdk.management\share\native\libmanagement_ext\flags.c: 

        if (valueObj == NULL) { 
            free(globals); 
            JNU_ThrowOutOfMemoryError(env, 0); 
            return 0; 
        } 

However, valueObj is not always the direct result of a failed JNI allocation, 
for example:

        case JMM_VMGLOBAL_TYPE_JSTRING: 
            valueObj = globals[i].value.l; 

The valueObj here (a JSTRING) is coming a VM allocation, more specifically from 
:

 

services/management.cpp jmm_GetVMGlobals() 
... 
add_global_entry() 

} else if (flag->is_ccstr()) { 
    Handle str = java_lang_String::create_from_str(flag->get_ccstr(), 
CHECK_false); <<-- 
    global->value.l = (jobject)JNIHandles::make_local(env, str()); 
    global->type = JMM_VMGLOBAL_TYPE_JSTRING; 

For certain ccstr flags, such as  the "HeapDumpFlag" for example, the ccstr() 
is NULL (i.e. not set). 

So returning a NULL is fine here, the JNI NULL check validation needs to take 
this into account.

 

Thanks

Markus