RFR 8247878: Move Management strong oops to OopStorage

[coleen.phillimore at oracle.com]

I've corrected this change with Kim's and David's feedback, and retested 
with tier1-3.  This is much better.

incremental webrev at 
http://cr.openjdk.java.net/~coleenp/2020/8247878.02.incr/webrev
full webrev at http://cr.openjdk.java.net/~coleenp/2020/8247878.02/webrev

Thanks,
Coleen

On 7/17/20 10:50 AM, coleen.phillimore at oracle.com wrote:
>
> Hi Kim, Thank you for reviewing this.
>
> On 7/17/20 5:02 AM, Kim Barrett wrote:
>>> On Jul 16, 2020, at 11:01 AM, coleen.phillimore at oracle.com wrote:
>>>
>>> Summary: Use OopStorage for strong oops stored with memory and 
>>> thread sampling and dumping, and remove oops_do and GC calls.
>>>
>>> These use OopStorageSet::vm_global()  OopStorage for now. I'll 
>>> change the thread sampling oops to use a new OopStorage once the GC 
>>> code is changed to nicely allow additional oop storages.  The memory 
>>> pool oops are never deleted once created, so they should stay in 
>>> vm_global() oop storage.
>>>
>>> Tested with tiers 1-3 (tiers 4-6 with other changes) and 
>>> javax/management tests.  I timed the tests to see if there was any 
>>> noticeable performance difference, and there was not.
>>>
>>> open webrev at 
>>> http://cr.openjdk.java.net/~coleenp/2020/8247878.01/webrev
>>> bug link https://bugs.openjdk.java.net/browse/JDK-8247878
>>>
>>> Thanks,
>>> Coleen
>> ------------------------------------------------------------------------------ 
>>
>> src/hotspot/share/oops/oopHandle.inline.hpp
>>    50   if (peek() != NULL) {
>>
>> Adding that seems like an unrelated change, and it's not clear to me
>> why this is being done.
>
> This was to save null checks in all the callers, particularly here:
>
>  ThreadSnapshot::~ThreadSnapshot() {
> + _blocker_object.release(OopStorageSet::vm_global());
> + _blocker_object_owner.release(OopStorageSet::vm_global());
> + _threadObj.release(OopStorageSet::vm_global());
> +
>
>>
>> ------------------------------------------------------------------------------ 
>>
>> src/hotspot/share/services/lowMemoryDetector.cpp
>>   299   if (_sensor_obj.peek() != NULL) {
>>   300     InstanceKlass* sensorKlass = 
>> Management::sun_management_Sensor_klass(CHECK);
>>   301     Handle sensor_h(THREAD, _sensor_obj.resolve());
>>
>> I see no reason for a peek operation here, and think it just makes the
>> code harder to understand.  Just unconditionally resolve the sensor.
>>
>> Similarly here:
>>   364   if (_sensor_obj.peek() != NULL) {
>>   365     InstanceKlass* sensorKlass = 
>> Management::sun_management_Sensor_klass(CHECK);
>>   366     Handle sensor(THREAD, _sensor_obj.resolve());
>
> I can move the NULL check down after the Handle.  I was mostly keeping 
> the existing pattern.
>>
>> ------------------------------------------------------------------------------ 
>>
>> src/hotspot/share/services/memoryManager.cpp
>>   136       _memory_mgr_obj = 
>> AtomicOopHandle(OopStorageSet::vm_global(), mgr_obj);
>>
>> There is a race here. The handle constructor allocates the oopstorage
>> entry and then does a release_store of the value into it. After (in
>> source order) the handle is constructed, it is copied into
>> _memory_mgr_obj, which is just a copy-assign of the oop* from
>> oopstorage. There's nothing to prevent that copy-assign from being
>> reordered before the store of the value into the oopstorage, either by
>> the compiler or the hardware.
>
> Ok, I see that.
>>
>> The _obj in _memory_mgr_obj is being read without locking and may be
>> written by another thread, so should itself be appropriate atomic.
>> It's not clear what the semantics of this new kind of handle are
>> supposed to be, but I think as used for _memory_mgr_obj there are
>> problems.
>>
>> The same is true for _memory_pool_obj.
>>
>> I think the atomicity here is in the wrong place. The pointee of the
>> oop* doesn't need atomic access, the oop* itself does.  At least I
>> think so; both _memory_mgr_obj and _memory_pool_obj are lazily
>> assigned on first use and never change after that, right?
>
> Yes.  volatile is in the wrong place.
>>
>> One way to do that would be that the type of _memory_mgr_obj is `oop*
>> volatile`, and is initialized as
>>
>>    oop* oop_ptr = ... allocate oopstorage entry ...
>>    NativeAccess<>::oop_store(oop_ptr, value);
>>    Atomic::release_store(&_memory_mgr_obj, oop_ptr);
>>
>> Alternatively, use
>>
>>    volatile OopHandle _memory_mgr_obj;
>>
>>    Atomic::release_store(&_memory_mgr_obj, OopHandle(...));
>>
>> and teach Atomic how to deal with OopHandle by defining a
>> PrimitiveConversions::Translator for it.
>>
>> The declaration would be
>>
>>    volatile OopHandle _memory_mgr_obj;
>>
>> and accesses would be
>>
>>    Atomic::load_acquire(&_memory_mgr_obj).resolve();
>>
>> And AtomicOopHandle isn't useful and should be discarded.
>
> Ok, yes, this is exactly what I want.  And David will be happy because 
> he didn't like AtomicOopHandle.
>
> Thanks for catching this and your help.
> Coleen
>
>>
>> ------------------------------------------------------------------------------ 
>>
>>
>