RFR: 8154750: Add missing OrderAccess operations to ClassLoaderData lock-free data structures

[David Holmes]

Correction ...

On 28/05/2016 7:58 AM, David Holmes wrote:
> On 28/05/2016 6:59 AM, Kim Barrett wrote:
>>> On May 26, 2016, at 11:39 PM, David Holmes <david.holmes at oracle.com>
>>> wrote:
>>>
>>> Hi Kim,
>>>
>>> Updated webrev:
>>>
>>> http://cr.openjdk.java.net/~dholmes/8154750/webrev.v2/
>>
>> ------------------------------------------------------------------------------
>>
>> src/share/vm/classfile/classLoaderData.cpp
>>  363         // lock-free iteration will either see k or k->next_link,
>> both
>>  364         // of which are stable
>>
>> I think this new comment isn't needed, given Stefan's reply and your
>> plan to add an assert_locked_or_safepoint to this function.
>
> If that works out yes. A JPRT run without the lock and an assertion for
> being at a safepoint passed but I need to run more tests. I'd also like
> Coleen to chime in. :)
>
>> ------------------------------------------------------------------------------
>>
>> src/share/vm/classfile/classLoaderData.cpp
>>  210 void ClassLoaderData::packages_do(void f(PackageEntry*)) {
>>  211   assert_locked_or_safepoint(Module_lock);
>>
>> Lock mismatch.  CLD::packages() protects _packages using the
>> metaspace_lock().
>
> It is the same check that CLDG::packages_do() uses - I just copied the
> assertions into the CLD methods. But you are right this is a problem.
>
> When I started work on this I queried about the different locks used in
> different places:
>
> "The purpose of the Module_lock is to protect access to module specific
> JVM data structures including the ModuleEntryTables and
> PackageEntryTables.  Each ClassLoaderData has its own instances of these
> two tables but they can be accessed by multiple threads.
>
> The metaspace lock, in this case, is used when creating these tables and
> storing their addresses in the classLoaderData's _modules and _packages
> fields."
>
> So basically the metaspace lock controls creation of the tables but once
> created they are modified under the modules lock. But the
> ModuleEntryTable creation actually needs both.
>
> What does that mean for our memory ordering issues? It means that if
> packages_do() is called with the Modules_lock, then it need not
> synchronize with a thread lazily creating the package table under the
> Modules lock.

That should be metaspace_lock.

David
-----


So I can remove the assert in CLD::packages_do and have to
> initially read _packages with load_ptr_acquire.
>
> Well spotted! I'll recheck the other cases as well.
>
> Thanks,
> David
>
>
>
>
>> ------------------------------------------------------------------------------
>>
>>