RFR: 8213565: Crash in DependencyContext::remove_dependent_nmethod

[Erik Österlund]

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
>>>
>>