RFR: 8213565: Crash in DependencyContext::remove_dependent_nmethod

[Vladimir Kozlov]

Looks good to me too.

Thanks,
Vladimir

On 11/28/18 2:48 AM, Robbin Ehn wrote:
>> Incremental:
>> http://cr.openjdk.java.net/~eosterlund/8213565/webrev.01_02/
> 
> Still good!
> 
>>
>> Full:
>> http://cr.openjdk.java.net/~eosterlund/8213565/webrev.02/
>>
>> Still need one more reviewer for this change.
> 
> Also bump :)
> 
> /Robbin
> 
>>
>> /Erik
>>
>> On 2018-11-23 16:24, Erik Österlund wrote:
>>> Hi Robbin,
>>>
>>> Thanks for the review.
>>>
>>> /Erik
>>>
>>> On 2018-11-23 16:26, Robbin Ehn wrote:
>>>> Looks good, thanks!
>>>>
>>>> /Robbin
>>>>
>>>> On 2018-11-23 16:16, Erik Österlund wrote:
>>>>> Hi,
>>>>>
>>>>> Here is an updated version with some minor adjustments based on feedback from Robbin Ehn.
>>>>>
>>>>> Incremental:
>>>>> http://cr.openjdk.java.net/~eosterlund/8213565/webrev.00_01/
>>>>>
>>>>> Full:
>>>>> http://cr.openjdk.java.net/~eosterlund/8213565/webrev.01/
>>>>>
>>>>> Thanks,
>>>>> /Erik
>>>>>
>>>>> On 2018-11-20 16:10, Erik Österlund wrote:
>>>>>> Ping.
>>>>>>
>>>>>> /Erik
>>>>>>
>>>>>> On 2018-11-12 23:02, Erik Österlund wrote:
>>>>>>> Hi,
>>>>>>>
>>>>>>> In my change 8209189, I moved a lot of code around relating to code cache unloading. Unfortunately, I failed to 
>>>>>>> see that when making nmethods unloaded, and their dependents are flushed... they are not immediately removed. 
>>>>>>> Instead, they are tagged to have stale entries, that are later cleaned during weak metadata cleaning of 
>>>>>>> InstanceKlass. As for method handles, they leak instead, with some hacks to reduce the leaking by expunging 
>>>>>>> entries while adding and removing.
>>>>>>>
>>>>>>> Obviously, with InstanceKlass cleanup and code cache unloading now
>>>>>>> possibly running in parallel after 8209189, things can go wrong. These two phases used
>>>>>>> to be separated by a "wait barrier" for the worker threads preventing that parallelism.
>>>>>>>
>>>>>>> The reason dependency contexts were not cleaned when they were found
>>>>>>> during code cache cleaning, is because it was not thread safe when code
>>>>>>> cache unloading was made parallel instead of serial. But now that we are
>>>>>>> implementing concurrent class unloading, we are going to need concurrent
>>>>>>> cleanup of dependency contexts anyway. Therefore I will propose a bug fix that fixes the problem in a way that 
>>>>>>> works for both serial, parallel and concurrent class unloading.
>>>>>>>
>>>>>>> The solution is to during code cache unloading claim cleaning of
>>>>>>> encountered stale dependency contexts, and clean them straight away.
>>>>>>> Entries are unlinked in a lock-free fashion, and placed on a purge list that is walked and deleted during 
>>>>>>> ClassLoaderDataGraph::purge(). This follows the thinking of first unlinking, then syncing all threads, and then 
>>>>>>> purging.
>>>>>>>
>>>>>>> New accessors for the head and next links, hide is_unloading entries and helps unlinking them and putting them 
>>>>>>> into the purge list.
>>>>>>>
>>>>>>> Webrev:
>>>>>>> http://cr.openjdk.java.net/~eosterlund/8213565/webrev.00/
>>>>>>>
>>>>>>> Bug:
>>>>>>> https://bugs.openjdk.java.net/browse/JDK-8213565
>>>>>>>
>>>>>>> Thanks,
>>>>>>> /Erik
>>>>>>
>>>>>
>>>