RFR: 8156871: Possible concurrency issue with JVM_AddModuleExports

[Lois Foltan]

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