RFR: 8156871: Possible concurrency issue with JVM_AddModuleExports

[Lois Foltan]

Thank you for the review Jiangli!
Lois

On 6/14/2016 7:09 PM, Jiangli Zhou wrote:
> Hi Lois,
>
> The removal of is_unqual_exported() check before set_exported() looks good. I’ll leave it to you and Karen to decide whether to include the tightening of the assertion in this fix or address it separately.
>
> Thanks,
> Jiangli
>
>> On Jun 14, 2016, at 2:11 PM, Lois Foltan <lois.foltan at oracle.com> wrote:
>>
>>
>> On 6/14/2016 2:57 PM, Karen Kinnear wrote:
>>> Lois,
>>>
>>> Thank you for the fix and the test.
>>>
>>> Couple of questions:
>>> 1) packageEntry.hpp
>>> - can you please modify comment “not exported” first line “the package has not been explicitly qualified to a particular module” …
>>>     - maybe you could change this to say “the package does not have qualified or unqualified exports”. Then the qualified exports comment
>>> takes the all unnamed into account.
>> Thanks Karen for the review!  Made the change above.
>>> 2) set_unqual_exported()
>>> Do you really want assert_locked_or_safepoint(Module_lock)?
>>> I could see reducing the export level at a safe point - e.g. if you unload a class loader and a module, then I could see reducing
>>> the exports list or even turning it into null.
>>> But with today’s design I was expecting that the following transitions could not happen at a safe point and only with the lock (i.e. transitions
>>> that increase the export):
>>>     add_qexport
>>>     set_is_exported_allUnnamed (which currently acquires the lock)
>>>     set_unqual_exported() (which currently acquires the lock)
>>>     set_unqual_exported()
>> I see your point, you are correct.  I have made that change not only for PackageEntry::set_unqual_exported() but for PackageEntry::add_qexport() as well.
>> The only mechanism today for transitioning a package that has been qualifiedly exported to be unqualifiedly exported, (widening its exportability to all),  is via a call to JVM_AddModuleExportsToAll. During a safepoint, even if all modules on a package's export list die, that package is still considered qualifiedly exported.
>>
>> New webrev:
>> http://cr.openjdk.java.net/~lfoltan/bug_jdk8156871.1/webrev/
>>
>> Thanks,
>> Lois
>>
>>> thanks,
>>> Karen
>>>
>>>
>>>> On Jun 10, 2016, at 10:25 AM, Lois Foltan <lois.foltan at oracle.com> wrote:
>>>>
>>>> Hello,
>>>>
>>>> Please review the work to finalize a concurrency issue when setting the exported state of a PackageEntry.  The work completed in bug https://bugs.openjdk.java.net/browse/JDK-8152404 actually had the side effect of fixing this bug as well.  Prior to JDK-8152404, each PackageEntry determined its exported state via two flags, one of which was a general _is_exported flag followed by a another more specific state of exportability flag that determined if the package was exported qualifiedly or not to a given module.  Checking these two flags, as the PackageEntry::is_* methods used to do without the Module_lock, was problematic and yielded a situation where a call to add a module to a PackageEntry's qualified exported entry list failed because it was determined that the package was unqualifiedly exported when it really was not.
>>>>
>>>> To complete this fix, I have removed the is_unqual_exported call prior to setting a PackageEntry's exported list.  The method PackageEntry::set_exported determines what the current package export state is and acts correctly.  Also, the test case has been added in this webrev since it is a good stress test case for JVM module support.
>>>>
>>>> Passes JPRT, java/lang, java/util RBT hotspot nightly.
>>>>
>>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8156871
>>>> Open webrev: http://cr.openjdk.java.net/~lfoltan/bug_jdk8156871/webrev/
>>>>
>>>> Thanks,
>>>> Lois