RFR(XXS): 8227338: templateInterpreter.cpp: copy_table() needs to be safer

[coleen.phillimore at oracle.com]

On 7/9/19 9:46 AM, Daniel D. Daugherty wrote:
> On 7/9/19 9:13 AM, Daniel D. Daugherty wrote:
>> Hi Kim,
>>
>> Thanks for the review.
>>
>>
>> On 7/8/19 7:00 PM, Kim Barrett wrote:
>>>> On Jul 7, 2019, at 8:08 PM, David Holmes <david.holmes at oracle.com> 
>>>> wrote:
>>>>
>>>> On 7/07/2019 6:48 pm, Erik Osterlund wrote:
>>>>> The real danger is SPARC though and its BIS instructions. I don’t 
>>>>> have the code in front of me, but I really hope not to see that 
>>>>> switch statement and non-volatile loop in that 
>>>>> pd_disjoint_words_atomic() function.
>>>> sparc uses the same loop.
>>>>
>>>> Let's face it, almost no body expects the compiler to do these 
>>>> kinds of transformations. :(
>>> See JDK-8131330 and JDK-8142368, where we saw exactly this sort of 
>>> transformation from a fill-loop
>>> to memset (which may use BIS, and indeed empirically does in some 
>>> cases).  The loops in question
>>> seem trivially convertible to memcpy/memmove.
>>
>> Very interesting reads. Thanks for pointing those out.
>>
>> src/hotspot/share/interpreter/templateInterpreter.cpp:
>>
>> DispatchTable TemplateInterpreter::_active_table;
>> DispatchTable TemplateInterpreter::_normal_table;
>> DispatchTable TemplateInterpreter::_safept_table;
>>
>> So it seems like changing _active_table to:
>>
>> volatile DispatchTable TemplateInterpreter::_active_table;
>>
>> might be a good idea... Do you concur?
>
> This change would require a bunch of additional changes so I'm
> not planning to make it (way too intrusive).

Can you file an additional RFE to examine the uses of dispatch tables 
for when we only have handshakes for safepoints?  And capture this idea 
of making the tables volatile?

thanks,
Coleen
>
> Dan
>
>
>>
>>
>>> Also see JDK-8142349.
>>>
>>>>> And I agree that the atomic copying API should be used when we 
>>>>> need atomic copying. And if it turns out the implementation of 
>>>>> that API is not atomic, it should be fixed in that atomic copying 
>>>>> API.
>>>> I agree to some extent, but we assume atomic load/stores of words 
>>>> all over the place - and rightly so. The issue here is that we need 
>>>> to hide the loop inside an API that we can somehow prevent the C++ 
>>>> compiler from screwing up. It's hardly intuitive or obvious when 
>>>> this is needed e.g if I simply copy three adjacent words without a 
>>>> loop could the compiler convert that to a block move that is 
>>>> non-atomic?
>>>>
>>>>> So I think this change looks good. But it looks like we are not 
>>>>> done yet. :c
>>>> I agree that changing the current code to use the atomic copy API 
>>>> to convey intent is fine.
>>> I’ve been reserving Atomic::load/store for cases where the location 
>>> “ought” to be declared std::atomic<T> if
>>> we were using C++11 atomics (or alternatively some homebrew 
>>> equivalent).  Not all places where we do
>>> stuff “atomically” is appropriate for that though (consider card 
>>> tables, being arrays of bytes, where using an
>>> atomic<T> type might impose alignment constraints that are 
>>> undesirable here).  I *think* just using volatile
>>> here would likely be sufficient, e.g. we should have
>>>
>>>      Copy::disjoint_words_atomic(const HeapWord* from,volatile 
>>> HeapWord* to, size_t count)
>>
>> I think this part should be taken up in the follow bug that I filed:
>>
>>     JDK-8227369 pd_disjoint_words_atomic() needs to be atomic
>>     https://bugs.openjdk.java.net/browse/JDK-8227369
>>
>> Thanks for chiming in on the review!
>>
>> Dan
>>
>>
>