RFR: 8213565: Crash in DependencyContext::remove_dependent_nmethod

Erik Österlund erik.osterlund at oracle.com
Fri Nov 23 15:16:01 UTC 2018


Here is an updated version with some minor adjustments based on feedback 
from Robbin Ehn.




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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