RFR: JDK-8222545: Safe klass asserts

[Per Liden]

On 04/18/2019 06:21 PM, Roman Kennke wrote:
> 
> 
> Am 18.04.19 um 17:34 schrieb Stefan Karlsson:
>> On 2019-04-18 17:29, Roman Kennke wrote:
>>>
>>>
>>> Am 18.04.19 um 16:34 schrieb Stefan Karlsson:
>>>> On 2019-04-18 15:13, Roman Kennke wrote:
>>>>>>>>> To add a little more detail, I could move the change up into 
>>>>>>>>> is_objArray(), but I don't want to expose it to any non-assert 
>>>>>>>>> paths. Therefore I could do 2 different impls there, guarded by 
>>>>>>>>> #ifdef ASSERT but I don't think it's a good idea to behave 
>>>>>>>>> differently under ASSERT, that kindof defeats the point of 
>>>>>>>>> assert, right?
>>>>>>>>>
>>>>>>>>> What do you think ?
>>>>>>>>
>>>>>>>> I don't follow your argument. Under asserts you need to access 
>>>>>>>> the klass pointer "safely" but otherwise you do not. So there 
>>>>>>>> are two behaviours related to accessing the klass pointer 
>>>>>>>> anyway. I'd rather see that encapsulated in the accessor.
>>>>>>>>
>>>>>>>> I assume it's not just asserts but any debug only code that 
>>>>>>>> wants to access the klass pointer.
>>>>>>>
>>>>>>> In general, for any runtime calls into oopDesc::klass() the 
>>>>>>> access should be safe. The acrobatics is only necessary for 
>>>>>>> *GC-internal* 
>>>>>>
>>>>>> This is the part I don't quite understand, and goes back to my 
>>>>>> initial question. Why are you doing these operations on from-space 
>>>>>> objects? I'm thinking you should be in a position in the GC to 
>>>>>> make sure this can never happen. If you need to do that in the GC 
>>>>>> (which is fine), then the GC could apply a "resolve" function to 
>>>>>> get the to-space object, and call size() (or whatever) on that 
>>>>>> object. This shouldn't have to leak out of the GC, right?
>>>>>
>>>>> It is a problem when we are about to evacuate an object. Then we 
>>>>> need to know its size in order to allocate and copy an appropriate 
>>>>> chunk. The problem is that this part is racy: two threads (e.g. two 
>>>>> Java threads via barrier, or one Java thread vs one GC thread) 
>>>>> might compete over this: both would create a copy of the object, 
>>>>> but ultimately only one would succeed (by CASing the fwd pointer). 
>>>>> Therefore, getting hold of the object size is racy, by design, and 
>>>>> this requires to resolve the _klass. Now, we can do that ahead of 
>>>>> time, and call oopDesc::size_given_klass() and all would be good, 
>>>>> except that size_given_klass() asserts that the object is indeed of 
>>>>> the given klass, and hence fetches _klass again, which, at this 
>>>>> point, is racy. Solving this inside the GC would require to 
>>>>> basically copy all the machinery to get hold of object size into 
>>>>> the GC. Are you asking me to do that?
>>>>
>>>> Other GCs store forwarding pointers in the mark word. See 
>>>> oopDesc::forward_to and friends. Could you do the same and get rid 
>>>> of this problem?
>>>>
>>>
>>> No. Other GCs store the fwd pointer there, but only during a pause, 
>>> and while possibly stashing the mark word somewhere else in the 
>>> meantime.
>>>
>>> We need to do it outside of GC pauses, plus we need a way (bit) to 
>>> indicate what it actually is (fwd pointer or Klass*). The mark word 
>>> is already badly overloaded and also accessed much more often and in 
>>> critical paths (e.g. locking), while the Klass* is basically 
>>> immutable, and has the lowest 3 bits free (when running with 
>>> -UseCompressedClassPointers, which would be enforced by Shenandoah). 
>>> Using the Klass* slot is therefore the simplest and most efficient 
>>> place to keep the fwd pointer. Attempting to use the mark word would 
>>> require much more barriers and cause more overhead to manage it.
>>
>> Are you sure? Remember, the object is in from-space and no thread is 
>> allowed to change it, except the threads that are copying out of the 
>> from-space.
> 
> Right. But we still need one bit in it to differentiate between fwd ptr 
> and regular mark word. And installing the fwd pointer concurrently with 

We already have such bits, and oopDesc::forward_to_atomic() will set 
them for you. Aren't they enough?

> locking seems more of a horror story than dealing with an otherwise 
> immutable Klass*.

With a to-space invariant, I can't see how this can happen concurrently 
with locking.

cheers,
Per

> 
> In any case, I guess we could do without any GC interface changes by 
> dropping the asserts in size_given_klass() if you think that is 
> reasonable, also avoid the extra asserts in klass() and friends (even 
> though I would prefer to have some way to assert sanity there...), plus 
> the changes for JDK-8222537 which are arguably an improvement in any case.
> 
> Roman