RFR (S) 8216136: Don't take Compile_lock for SystemDictionary::_modification_counter

[Erik Österlund]

Hi Coleen,

Looks good. Ship it!

Thanks,
/Erik

On 2019-01-24 14:01, coleen.phillimore at oracle.com wrote:
> Thanks Dean and thank you for the consulation on this RFE.
> Coleen
>
> On 1/24/19 12:17 AM, dean.long at oracle.com wrote:
>> Looks good.
>>
>> dl
>>
>> On 1/23/19 6:20 AM, coleen.phillimore at oracle.com wrote:
>>>
>>> After some internal discussion, Dean convinced me that removing the 
>>> Compile_lock here might be too dangerous.   So for these asserts and 
>>> the error condition, the compiler thread goes to VM from native to 
>>> check the SystemDictionary::modification_counter under the 
>>> Compile_lock, with safepoint checking always.
>>>
>>> Tested with tier1,2,6 and 8.
>>>
>>> open webrev at http://cr.openjdk.java.net/~coleenp/8216136.02/webrev
>>> bug link https://bugs.openjdk.java.net/browse/JDK-8216136
>>>
>>> Thanks,
>>> Coleen
>>>
>>> On 1/17/19 7:15 AM, coleen.phillimore at oracle.com wrote:
>>>>
>>>>
>>>> On 1/16/19 10:53 PM, dean.long at oracle.com wrote:
>>>>> Hi Coleen.  You still can't safely call notice_modification() 
>>>>> outside of Compile_lock, (at least not without other changes), so 
>>>>> this:
>>>>>
>>>>> - static inline void notice_modification() { 
>>>>> assert_locked_or_safepoint(Compile_lock); 
>>>>> ++_number_of_modifications; }
>>>>> + static inline void notice_modification() { 
>>>>> Atomic::inc(&_number_of_modifications); }
>>>>>
>>>>> should be:
>>>>>
>>>>> static inline void notice_modification() { 
>>>>> assert_locked_or_safepoint(Compile_lock); 
>>>>> Atomic::inc(&_number_of_modifications); }
>>>>>
>>>>>
>>>>> Are you trying to eventually remove Compile_lock completely? If 
>>>>> so, then notice_modification() would have to be called *before* the
>>>>> class hierarchy is changed, not after, and probably other changes 
>>>>> would be needed as well.
>>>> Dean, Thank you for looking at this and your comments.
>>>>
>>>> No, I'm not trying to remove Compile_lock entirely and I can assert 
>>>> that notice_modification has the Compile_lock as above. The class 
>>>> hierarchy code has been changed to be lock free rather than 
>>>> requiring the Compile_lock, although I think the Compile_lock still 
>>>> protects some of this code.
>>>>
>>>> There are also some Compile_lock free ways of getting to 
>>>> dependencies, because putting notice_modification after 
>>>> flush_dependencies caused bugs that I'll ask to you offline about.
>>>>
>>>> Thanks for your help.  I was just trying to peel off one place 
>>>> where Compile_lock seemed wrong.
>>>>
>>>> Thanks,
>>>> Coleen
>>>>>
>>>>> dl
>>>>>
>>>>>
>>>>> On 1/16/19 8:43 AM, coleen.phillimore at oracle.com wrote:
>>>>>> Summary: make SystemDictionary::modification_counter atomic so 
>>>>>> not to require Compile_lock.
>>>>>>
>>>>>> I moved updating the modification counter when the class is 
>>>>>> defined and added to the hierarchy.  I didn't remove the 
>>>>>> Compile_lock completely because there may be other code currently 
>>>>>> under the lock that needs it (flush_dependencies). Can someone 
>>>>>> from the compiler area also review this?
>>>>>>
>>>>>> Made Compile_lock an always safepointing lock.
>>>>>>
>>>>>> Tested with mach5 tier1-6.
>>>>>>
>>>>>> open webrev at http://cr.openjdk.java.net/~coleenp/8216136.01/webrev
>>>>>> bug link https://bugs.openjdk.java.net/browse/JDK-8216136
>>>>>>
>>>>>> Thanks,
>>>>>> Coleen
>>>>>
>>>>
>>>
>>
>