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

[Zhengyu Gu]

Does it sound like that all the hash tables with lock-free lookup, such 
as SymbolTable, etc., are exposed with the same issues?

-Zhengyu


On 05/31/2016 06:08 PM, Coleen Phillimore wrote:
>
> This pattern also exists in
>
> InstanceKlass::mask_for()
>
> which is primarily called walking frames (could be in parallel) during 
> a safepoint.
>
> Could you apply your change there also?
>
> Thanks,
> Coleen
>
>
> On 5/31/16 5:57 PM, Coleen Phillimore wrote:
>>
>>
>> On 5/27/16 5:58 PM, 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. :)
>>
>> Yes, please remove this comment too.
>>
>>>
>>>> ------------------------------------------------------------------------------ 
>>>>
>>>> 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."
>>>
>>
>> Right, metaspace_lock protects the fields of ClassLoaderData (and is 
>> reused to protect the metaspace also).
>>
>>> 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. So I can remove the assert in CLD::packages_do and 
>>> have to initially read _packages with load_ptr_acquire.
>>
>> http://cr.openjdk.java.net/~dholmes/8154750/webrev.v3/src/share/vm/classfile/classLoaderData.cpp.udiff.html 
>>
>>
>> Can you put a comment in modules_do to describe why you don't need 
>> load_ptr_acquire? in modules_do and here:
>>
>> + // Check if _modules got allocated while we were waiting for this 
>> lock.
>> + if ((modules = _modules) == NULL) {
>>
>> The racing thread has done a store_release so you don't need another 
>> load_acquire after you get the lock?
>>
>> I've reviewed this and this looks really good.  Thank you for fixing 
>> this!
>>
>> I guess the alternative to the double check locking algorithm is to 
>> always take the lock but I think this would be slower.  But maybe 
>> not.  Anyway, we can copy this design pattern now from your changes.
>>
>> Thanks,
>> Coleen
>>
>>
>>>
>>> Well spotted! I'll recheck the other cases as well.
>>>
>>> Thanks,
>>> David
>>>
>>>
>>>
>>>
>>>> ------------------------------------------------------------------------------ 
>>>>
>>>>
>>
>