RFR (S) 8213092: Add more runtime locks for concurrent class unloading

coleen.phillimore at oracle.com coleen.phillimore at oracle.com
Tue Oct 30 17:13:22 UTC 2018

Hi Martin,

I see that now:
find _enclosing_space is called by find_enclosing_virtual_space is 
called by is_range_in_committed is called by oopDesc::is_valid and 
Klass::is_valid which are both used for error reporting.

I'm going to revert the metaspace.cpp and virtualSpaceList.cpp changes 
and let Erik find out why he wanted them :)

Is the rest reviewed?


On 10/30/18 10:09 AM, Doerr, Martin wrote:
> Hi Coleen,
> I think the acquisition of MetaspaceExpand_lock should get moved to VirtualSpaceList::contains because find_enclosing_space is used during error reporting. If the VM crashes while the lock is held, error reporting will hang.
> Best regards,
> Martin
> -----Original Message-----
> From: hotspot-runtime-dev <hotspot-runtime-dev-bounces at openjdk.java.net> On Behalf Of coleen.phillimore at oracle.com
> Sent: Dienstag, 30. Oktober 2018 14:21
> To: hotspot-runtime-dev at openjdk.java.net
> Subject: Re: RFR (S) 8213092: Add more runtime locks for concurrent class unloading
> On 10/30/18 9:16 AM, coleen.phillimore at oracle.com wrote:
>> On 10/30/18 8:36 AM, David Holmes wrote:
>>> On 30/10/2018 10:24 PM, coleen.phillimore at oracle.com wrote:
>>>> On 10/30/18 12:17 AM, David Holmes wrote:
>>>>> Hi Coleen,
>>>>> On 30/10/2018 1:45 PM, coleen.phillimore at oracle.com wrote:
>>>>>> Summary: Add locks for calling CLDG::purge concurrently as well
>>>>>> and for calling SystemDictionary::do_unloading concurrently.
>>>>>> Ran linux-x64 tier1-6 through mach5 and hotspot/jtreg/runtime
>>>>>> tests, which include the module tests.
>>>>>> open webrev at http://cr.openjdk.java.net/~coleenp/8213092.01/webrev
>>>>>> bug link https://bugs.openjdk.java.net/browse/JDK-8213092
>>>>> So ... all the locks covered by an assert_locked_or_safepoint, or
>>>>> which are acquired by the VMThread at a safepoint, must never be
>>>>> held by a JavaThread if it could reach a safepoint whilst that lock
>>>>> is held - else we could deadlock. So can we check that with
>>>>> NoSafepointVerifiers?
>>>> Actually I think this is not possible to add NSV.   You can acquire
>>>> the ClassLoaderDataGraph_lock and then the Module_lock. The latter
>>>> would check for a safepoint also for a Java thread. This is
>>>> currently done for Jvmti and JFR, but not in other code that I can
>>>> see.  I don't actually know how to fix this problem.
>>> This seems risky. If a JavaThread can hold the CLDG_lock while
>>> blocked at a safepoint (acquiring the Module_lock), then what is to
>>> stop the VMThread from hitting one of these sections of code
>>> protected by locked_or_safepoint and then proceeding into what is
>>> effectively a critical section (by virtue of there being a safepoint)
>>> when the JavaThread is itself in the midst of a critical section? Do
>>> we actively take steps to prevent this somehow, or to make it safe
>>> for the VMThread to proceed?
> I might not have answered your question about this lock in particular.
> There is only the linking and unlinking (in a safepoint except at ZGC)
> that are protected by CLDG_lock, and these are not interrupted by a
> safepoint.  So this is safe.
> Coleen
>> No we don't.  I think we have this problem today (not introduced or
>> made worse by this patch).  I'll file an bug to fix it and hopefully
>> add detection for this.  I think we don't need to take CLDG_lock in a
>> safepoint and should prevent doing so, but things like this are more
>> reliable to do with computers than visual inspection. Can you add your
>> suggestions to my RFE? https://bugs.openjdk.java.net/browse/JDK-8213150
>> The CLDG lock can be shared by non-java threads and java threads,
>> which is the point.  There may be other locks though.
>>>> The locks added in this patch set though are for the NonJavaThreads,
>>>> who do not do safepoint checks.  The NonJavaThreads that acquire
>>>> these locks use the STS joiner mechanism which disallows safepoints
>>>> while being held (and since they are non Java threads, do not check
>>>> for safepoints themselves).
>>>> This is how it's going to look for the ZGC caller:
>>>> {
>>>> SuspendibleThreadSetJoiner sts_joiner;
>>>> // Unlink the classes.
>>>> MutexLockerEx ml(ClassLoaderDataGraph_lock);
>>>> unloading_occurred =
>>>> SystemDictionary::do_unloading(ZStatPhase::timer(),
>>>> true /* do_cleaning */););
>>>> }
>>> Somehow I missed the creation/invention of the STS joiner mechanism.
>> Me too!   It's in gc/shared but it's really runtime code, except
>> people in runtime didn't know about it because it's used by GC threads.
>> Let me know if you have more questions and can review this code.
>> Thanks,
>> Coleen
>>> David
>>>>> Further, are these locks acquired by non-JavaThreads such that the
>>>>> VMThread may be delayed whilst a safepoint is active?
>>>> Yes, theoretically they could delay the VMThread from getting to a
>>>> safepoint or doing its work while in a safepoint but the threads
>>>> that take out these locks only hold them for short durations.
>>>> Thanks,
>>>> Coleen
>>>>> Thanks,
>>>>> David
>>>>>> Thanks,
>>>>>> Coleen

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