RFR 8171971: Fix timing bug in JVM management of package export lists

[Karen Kinnear]

Harold,

Thank you for the review. Looks good.

Can you possibly changed packageEntry.hpp lines 60->61
  “Because transitions are only allowed from less exposure to greater exposure, the transition from
qualifiedly exported to not exported would be considered a backward direction. Therefore the implementation
considers a package as qualifiedly exported even if its export-list exists but is empty."

I believe that the forward transition restriction will make it cleaner for us to make these into lock-free snapshots
going forward.

thanks,
Karen

> On Jan 20, 2017, at 8:56 AM, harold seigel <harold.seigel at oracle.com> wrote:
> 
> Thanks!
> 
> Harold
> 
> 
> On 1/20/2017 8:55 AM, Lois Foltan wrote:
>> Looks good.
>> Lois
>> 
>> On 1/19/2017 6:38 PM, harold seigel wrote:
>>> Hi,
>>> 
>>> Here is an updated webrev containing the changes that you suggested.
>>> 
>>> http://cr.openjdk.java.net/~hseigel/bug_8171971.2/webrev/index.html
>>> 
>>> Could you take another look?
>>> 
>>> Thanks, Harold
>>> 
>>> On 1/19/2017 1:37 PM, harold seigel wrote:
>>>> Hi Karen, Lois,
>>>> 
>>>> Thanks for the review!  Please see comments in-line.
>>>> 
>>>> Harold
>>>> 
>>>> 
>>>> On 1/19/2017 11:54 AM, Karen Kinnear wrote:
>>>>> Harold,
>>>>> 
>>>>> Thank you for doing this. And for the comments on the states.
>>>>> 
>>>>> Looks good. I believe that the changes you have made have fixed the timing holes. Thank you.
>>>>> 
>>>>> 1) packageEntry.hpp: 145: set_unqual_exported()
>>>>> It would be helpful to have a comment that this explicitly sets PKG_EXP_UNQUALIFIED and clears PKG_EXP_ALLUNNAMED.
>>>> Done
>>>>> 
>>>>> I realized my concept of states is slightly different,
>>>>> with AllUnnamed, NamedExports and AllUnnamed+NamedExports being different states. In future, when we rework this
>>>>> for performance, let’s revisit the states.
>>>> Agreed
>>>>> 
>>>>>> On Jan 19, 2017, at 10:14 AM, Lois Foltan <lois.foltan at oracle.com <mailto:lois.foltan at oracle.com>> wrote:
>>>>>> 
>>>>>> 
>>>>>> On 1/18/2017 10:05 AM, harold seigel wrote:
>>>>>>> Hi,
>>>>>>> 
>>>>>>> Please review this fix for the package export timing holes discussed in JDK-8171971.  The fix reduces the number of PackageEntry fields that are used to maintain a package's export state and uses the Module_lock to protect all access to these fields.
>>>>>>> 
>>>>>>> Also, in cases where a package transitions from having qualified exports to being unqualifiedly exported, it fixes the cleanup of its qualified export list by removing the _exported_pending_delete field and using just is_unqual_exported() to determine when the qualified exports list can be purged (at a safepoint).
>>>>>>> 
>>>>>>> Open Webrev: http://cr.openjdk.java.net/~hseigel/bug_8171971/webrev/ <http://cr.openjdk.java.net/%7Ehseigel/bug_8171971/webrev/>
>>>>>>> 
>>>>>>> JBS Bug: https://bugs.openjdk.java.net/browse/JDK-8171971
>>>>>>> 
>>>>>>> The fix was tested with the hotspot, java/lang, java/util, java/io, JFR, and other JTReg tests, the JCK lang and VM tests, RBT tier2 - tier5 tests on LinuxX64, and the colocated and non-colocated NSK tests.
>>>>>> 
>>>>>> Hi Harold,
>>>>>> 
>>>>>> Looks good! A couple of comments in src/share/vm/classfile/packageEntry.hpp:
>>>>>> 
>>>>>> - line #133, the method has_qual_exports_list().  A comment should be added ahead of this method to indicate that even if the _qualified_exports list is empty the package is still considered qualifiedly exported and a similar comment should be added to PackageEntry::purge_qualified_exports().  During a GC safepoint, it could be the scenario that all modules on a given package's qualified export list die, thus leaving an allocated but empty _qualified_exports field.  If the _qualified_exports GrowableArray was deleted in that scenario, then according to the new _exports_flag and a NULL value for _qualified_exports, the package would erroneously not be considered qualifiedly exported anymore.
>>>> Good point.  I added the comments.
>>>>> So not deleting the GrowableArray today is deliberate, because the transition from qualifiedly exported to not exported is not currently valid.
>>>>> Agreed, we may want to revisit that in future.
>>>>> 
>>>>> 2) Harold, could you possibly add that state change to the comment to the “A package cannot transition from”, i.e. after line 58?
>>>> Done
>>>>>> This can be reworked in the future to have the _exports_flag contain definitions for all possible states.  Relying on the state of whether _qualified_exports field is NULL or not can be problematic, but I think your changes do improve upon ensuring a PackageEntry maintains a valid state.
>>>>>> 
>>>>>> - line #86 - please add a blank line for readability
>>>> okay
>>>>>> - line #139 & #143 please out the return expression in parentheses.
>>>> (okay)
>>>>>> 
>>>>>> Thanks,
>>>>>> Lois
>>>>>> 
>>>>>>> 
>>>>>>> Thanks, Harold
>>>>> 
>>>>> thanks,
>>>>> Karen
>>>>> 
>>>> 
>>> 
>> 
>