Request for review: 6922246: Deadlock during intensive class loading in parallel

Coleen Phillimore coleen.phillimore at
Wed Oct 20 07:24:54 PDT 2010

Thank you for the discussion.   I am reimplementing this with a CAS as 
Tom suggested.  I guess it's not such a simple fix, anyway, see below:

David Holmes wrote:
> Igor Veresov said the following on 10/20/10 18:31:
>> On 10/20/10 12:16 AM, David Holmes wrote:
>>> Igor Veresov said the following on 10/20/10 16:52:
>>>> There's no question a store-store barrier is needed after 291 for
>>>> non-TSO if we were to make it lock-free (with the current code monitor
>>>> lock should contain such barrier anyway).
>>> There's no requirement for a storestore barrier when acquiring a lock.
>>> You will happen to get that on sparc and x86 if the lock acquisition
>>> involves a CAS, but that is not true on other archs. In general 
>>> terms it
>>> is considered safe to move stores into a critical section at which time
>>> it can be reordered with other stores inside the critical section.
>> Yes, that's what I meant. Basically a lock should guarantee that all 
>> stores before the lock precede stores in the critical section, which 
>> is essentially a storestore barrier.
> But my point is that locks in general don't guarantee this.
Does a CAS guarantee this?  If there isn't a guarantee, I will add an 
Atomic::storestore(), plus make _method_data volatile.

>>> Right. If it were not safe/correct for more than one thread to create a
>>> MDO, and have all but one thrown away, then we'd have a different
>>> problem to solve. Which reminds we, if another thread does beat us to
>>> storing the MDO then we need to delete the one we created.
>> Yes, perhaps we can try to deallocate. Although I suppose the race 
>> should occur infrequently and GC will take care of the unused copy 
>> anyway.
> Ah! Is this a GC'able oop we are creating? I had assumed it was a 
> C/C++ object.
Right now the extra MDO would get GC'ed because nothing references it 
and it's in the PermGen.  But we're moving everything out of the 
PermGen. When it's not allocated in the PermGen, this extra MDO will get 
deallocated when the memory region associated with it's classloader is 
deallocated.  I think we should revisit this thing though at that point 
because it would be leaked with the bootclassloader.

> David
>> igor
>>> Cheers,
>>> David
>>>> igor
>>>> On 10/19/10 3:59 PM, David Holmes wrote:
>>>>> Hi Coleen,
>>>>> coleen phillimore said the following on 10/20/10 08:11:
>>>>>> Summary: allocate methodData before taking out the MethodData_lock
>>>>>> open webrev at
>>>>>> bug link at
>>>>>> Thanks to Ramki for the diagnosis and suggestion for the fix.
>>>>> This is a variation on the unsafe double-checked-locking idiom.
>>>>> 286 if (method->method_data() == NULL) {
>>>>> 291 methodDataOop method_data = oopFactory::new_methodData(method,
>>>>> CHECK);
>>>>> 292 MutexLocker ml(MethodData_lock, THREAD);
>>>>> 295 if (method->method_data() == NULL) {
>>>>> 296 method->set_method_data(method_data);
>>>>> In general terms any stores associated with the method_data 
>>>>> construction
>>>>> are not guaranteed to be visible by the time set_method_data is 
>>>>> called.
>>>>> A second thread could then see a non-null method->method_data() but
>>>>> access uninitialized method_data values.
>>>>> A general solution would need memory barriers before 286 and after 
>>>>> 291
>>>>> to ensure correctness on all platforms. A simpler, though perhaps
>>>>> heavier solution would be to use a MutexUnlocker around the 
>>>>> allocation:
>>>>> MutexLocker ml(MethodData_lock, THREAD);
>>>>> if (method->method_data() == NULL) {
>>>>> methodDataOop method_data = NULL;
>>>>> { // need new block for scope
>>>>> MutexUnLocker ml(MethodData_lock, THREAD);
>>>>> method_data = oopFactory::new_methodData(method, CHECK);
>>>>> }
>>>>> // someone might have beaten us to it
>>>>> if (method->method_data() == NULL) {
>>>>> method->set_method_data(method_data);
>>>>> }
>>>>> }
>>>>> A lock-less solution as Tom proposed would also require suitable
>>>>> memory-barriers.
>>>>> David

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