RFR: 8060130: Simplify the synchronization of defining and getting java.lang.Package

[David M. Lloyd]

On 10/14/2014 07:22 PM, Mandy Chung wrote:
>
> On 10/13/2014 5:50 AM, David M. Lloyd wrote:
>> On 10/10/2014 07:31 PM, Mandy Chung wrote:
>>>
>>> On 10/10/2014 8:10 AM, Claes Redestad wrote:
>>>> Hi all,
>>>>
>>>> please review this patch which attempts to clean up synchronization
>>>> and improve scalability when
>>>> defining and getting java.lang.Package objects.
>>>
>>> I agree with David that getting Package objects are not performance
>>> critical. On the other hand, the code defining/getting Packages is
>>> old and deserves some cleanup especially the synchronization part.
>>
>> I have a little more information on this subject.  We've a possible
>> (and somewhat likely) deadlock which occurs because one thread can
>> attempt to define a system class while holding the
>> java.lang.Package#pkgs lock, while another thread can attempt to get a
>> package while defining a system class (while holding the class loader
>> lock).  I do not recall whether parallel class loading alleviates this
>> issue.  We solved the problem by loading Packages.getPackages() in
>> early (single-threaded) bootstrap.
>>
>
> Do you recall what JDK version you observed this possible deadlock? I
> wonder if the fix for 7001933 [1] in JDK 7 and 6u25 resolved the
> deadlock problem you ran into.
>
> [1] http://hg.openjdk.java.net/jdk9/dev/jdk/rev/4a7da412db38

The change would have been observed in early 2011 so I think it was 
probably a JDK 6 version before 25 (which I did not install until April 
of that year) - looks like most likely candidate based on my JDK 
directory is 1.6.0_22.

>> So from my perspective, just getting rid of the synchronization on
>> that field alone makes this change worthwhile.
>
> Yes that's what I think too.
>
> Mandy

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- DML