RFR: 8078641: MethodHandle.asTypeCache can retain classes from unloading [v3]

[Vladimir Ivanov]

On Fri, 3 Sep 2021 12:51:13 GMT, Peter Levart <plevart at openjdk.org> wrote:

>> Vladimir Ivanov has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Address review comments
>
> src/java.base/share/classes/java/lang/invoke/MethodHandle.java line 877:
> 
>> 875:         }
>> 876:         if (asTypeSoftCache != null) {
>> 877:             atc = asTypeSoftCache.get();
> 
> NPE is possible here too! asTypeSoftCache is a non-volatile field which is read twice. First time in the if (...) condition, 2nd time in the line that de-references it to call .get(). This is a data-race since concurrent thread may be setting this field from null to non-null. Those two reads may get reordered. 1st read may return non-null while 2nd may return null. This can be avoided if the field is read just once by introducing a local variable to store its value.

Fixed.

> src/java.base/share/classes/java/lang/invoke/MethodHandle.java line 878:
> 
>> 876:         if (asTypeSoftCache != null) {
>> 877:             atc = asTypeSoftCache.get();
>> 878:             if (newType == atc.type) {
> 
> NPE is possible here! act can be null as it is a result of SoftReference::get

Good catch! Fixed.

> src/java.base/share/classes/java/lang/invoke/MethodHandle.java line 933:
> 
>> 931:     }
>> 932: 
>> 933:     /* Returns true when {@code loader} keeps {@code mt} either directly or indirectly through the loader delegation chain. */
> 
> Well, to be precise, loader can't keep mt alive. It would be better to say "keeps mt components alive" ...

Fixed.

> src/java.base/share/classes/java/lang/invoke/MethodHandle.java line 948:
> 
>> 946:         if (isBuiltinLoader(defLoader)) {
>> 947:             return true; // built-in loaders are always reachable
>> 948:         }
> 
> No need for special case here. isAncestorLoaderOf(defLoader, loader) already handles this case.

Though the check is redundant, I find the current version clearer.

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PR: https://git.openjdk.java.net/jdk/pull/5246