RFR: 8213565: Crash in DependencyContext::remove_dependent_nmethod

Robbin Ehn robbin.ehn at oracle.com
Fri Nov 23 15:26:55 UTC 2018

Looks good, thanks!


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

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