RFR (S) 8197844: JVMTI GetLoadedClasses should use the Access API

Roman Kennke rkennke at redhat.com
Wed Mar 21 10:05:10 UTC 2018

> 2) I wonder if it would be any trouble to provide an opt-out for the
> keep alive stuff, so that people that explicitly do not want to keep
> things alive, can opt out.

Sounds like this would amount to adding extra API to the closures, e.g.
keep_alive_cld() that would be queried in the loop? To be honest, this
sounds like adding extra complexity that is right now not used/needed.
I'd rather add this facility if an actual need arises.


> Thanks,
> /Erik
> On 2018-03-20 22:49, coleen.phillimore at oracle.com wrote:
>> Kim and I had some offline discussions about this and he recommended
>> that the ClassLoaderData::ensure_loader_alive function return the oop
>> to handleize while the closure runs on that ClassLoaderData. This is
>> that change.
>> Tested with jvmti, runtime and jfr tests that use
>> ClassLoaderData::*classes/modules/package iterations.
>> http://cr.openjdk.java.net/~coleenp/8197844.03/webrev/index.html
>> thanks,
>> Coleen
>> On 3/19/18 7:38 PM, coleen.phillimore at oracle.com wrote:
>>> On 3/19/18 7:32 PM, Roman Kennke wrote:
>>>> The change looks good to me.
>>>> I have also verified that Shenandoah does not call into any of the
>>>> iterators that might resurrect CLDs by mistake.
>>> That's good!  Thank you for verifying this.  These iterators should
>>> now be only runtime iterators.
>>> thanks,
>>> Coleen
>>>> Thanks, Roman
>>>>> After reading my own reply and Erik's, I want to reconsider my change
>>>>> for this bug.
>>>>> There are many places that iterate over ClassLoaderDataGraph, and I've
>>>>> found another that is buggy in the same way that JVMTI
>>>>> GetLoadedClasses
>>>>> should have a G1 barrier.  I've also spent more time trying to find if
>>>>> other uses are buggy, with dissatisfying inconclusion.
>>>>> The safest thing to do is mark loaders that we are iterating over as
>>>>> alive.  This alive barrier doesn't affect oops_do, cld_do or
>>>>> do_unloading, which are the cases that would be walked by the GC.   It
>>>>> also can't include verify, which I've removed from the original
>>>>> change.
>>>>> I can't assert that the thread isn't a GC thread because
>>>>> VM_GC_HeapInspection builds a KlassInfoTable as a GC thread.
>>>>> I think a klass walk of the ClassLoaderDataGraph when done for
>>>>> profiling
>>>>> and assorted runtime code, must necessarily keep the classes alive
>>>>> during the interval between GCs.   The class can be unloaded in a
>>>>> further GC.
>>>>> open webrev at http://cr.openjdk.java.net/~coleenp/8197844.02/webrev
>>>>> This is the same patch with the ensure_loader_alive removed from
>>>>> ClassLoaderDataGraph::verify.
>>>>> thanks,
>>>>> Coleen
>>>>> On 3/16/18 11:29 AM, Erik Österlund wrote:
>>>>>> Hi Coleen,
>>>>>> I for one would like to see your version of the fix, plus some
>>>>>> suitable assert to catch bad use of these APIs.
>>>>>> The motivation is that it seems unfortunate to expose APIs for
>>>>>> getting
>>>>>> Metadata that when retrieved is unstable and unsafe to use unless you
>>>>>> have deep knowledge about GC races and class unloading, and hope all
>>>>>> callsites know that they can't just use the things they retrieved
>>>>>> because they are dangerously explosive. The only problem with the
>>>>>> more
>>>>>> conservative API is if the GC itself calls into the code and
>>>>>> accidentally keeps everything alive preventing class unloading. But I
>>>>>> think a suitable assert could catch that.
>>>>>> I'm not 100% sure what the assert condition should be. But I'm
>>>>>> thinking something along the lines of:
>>>>>> Thread::current()->is_Java_thread() ||
>>>>>> Thread::current()->is_VM_thread() &&
>>>>>> (!SafepointSynchronize::is_at_safepoint() || !HeapLock->is_locked())
>>>>>> Other suggestions for a good condition are welcome - it is just an
>>>>>> example that may or may not work.
>>>>>> Thanks,
>>>>>> /Erik
>>>>>> On 2018-03-16 16:01, coleen.phillimore at oracle.com wrote:
>>>>>>> Hi Roman, thank you for your comments.
>>>>>>> On 3/15/18 9:46 AM, Roman Kennke wrote:
>>>>>>>> Am 14.03.2018 um 18:37 schrieb coleen.phillimore at oracle.com:
>>>>>>>>> Summary: Make sure the holder of a class loader is accessed during
>>>>>>>>> iteration of CLDG
>>>>>>>>> This is where we should have put the GC barrier. This can be
>>>>>>>>> cleaned
>>>>>>>>> somewhat when we have a WeakHandle holder in the
>>>>>>>>> ClassLoaderData, then
>>>>>>>>> the code in ensure_loader_alive() becomes _holder.resolve();
>>>>>>>>> Tested with tier1-5.
>>>>>>>>> open webrev at
>>>>>>>>> http://cr.openjdk.java.net/~coleenp/8197844.01/webrev
>>>>>>>>> bug link https://bugs.openjdk.java.net/browse/JDK-8197844
>>>>>>>>> Thanks,
>>>>>>>>> Coleen
>>>>>>>> I am not sure at all that this wouldn't have adverse effects.
>>>>>>>> Suppose a
>>>>>>>> GC is iterating all CLDs and marks all of them alive, we'd probably
>>>>>>>> never unload it? I probably wouldn't do the keep-alive stuff
>>>>>>>> wholesale
>>>>>>>> in the iterator methods.
>>>>>>>> I've made (and later withdraw) a (IMO) more straightforward
>>>>>>>> patch to
>>>>>>>> address the same:
>>>>>>>> http://cr.openjdk.java.net/~rkennke/8199612/webrev.00/
>>>>>>>> It has the advantage that it doesn't to a bogus load, just to keep
>>>>>>>> something alive. It loads the mirror and at the same time
>>>>>>>> communicates
>>>>>>>> the GC to keep it alive. Maybe better?
>>>>>>> So when I made this change, I was being conservative. Anytime we
>>>>>>> iterate over the CLDG, and take out oops to put somewhere else, we
>>>>>>> need to mark that the CLD needs to stay alive for that GC, until the
>>>>>>> somewhere else is walked.  We also have to worry about these classes
>>>>>>> being unloaded if we are still holding metadata after walking the
>>>>>>> CLDG.
>>>>>>> I've gone through all the CLDG::classes_do, loaded_classes_do,
>>>>>>> dictionary_all_entries_do, methods_do, modules_do and packages_do at
>>>>>>> least 100 times.  (also oops_do variants and cld_do). All these
>>>>>>> calls do something with metadata, and it appears that these places
>>>>>>> are unaffected by GC or class unloading, except the instance in
>>>>>>> jvmtiGetLoadedClasses that Poonam found.
>>>>>>> Also if the closure or function passed to the CLDG iterator
>>>>>>> functions
>>>>>>> stop for a safepoint and unload, the walk will be broken. I didn't
>>>>>>> find any instances of this in the code on my 100 traversals.  My
>>>>>>> change is trying to make it so I don't have to do this analysis
>>>>>>> anymore.
>>>>>>> Poonams, Erik's change and your change puts the responsibility on
>>>>>>> the
>>>>>>> caller of ClassLoaderDataGraph to keep the class alive that it is
>>>>>>> processing through the walk.   I think I can accept that as a fix,
>>>>>>> and withdraw mine, since we seem to have no instances of bugs in
>>>>>>> other walks.  If it turns out to be something that becomes buggy, I
>>>>>>> will suggest my fix again as an alternative.
>>>>>>> This fix doesn't affect the walks done during GC.  As of this
>>>>>>> morning, there are no KlassClosures in GC anymore. Your concern is
>>>>>>> well placed though because it could help us introduce bugs which
>>>>>>> could negatively affect class unloading.
>>>>>>> So on the balance, there's risk in both versions of the fix, so I'll
>>>>>>> withdraw mine.  I think the best fix is yours:
>>>>>>> http://cr.openjdk.java.net/~rkennke/8199612/webrev.00/src/hotspot/share/prims/jvmtiGetLoadedClasses.cpp.udiff.html
>>>>>>> with Erik's function java_mirror_phantom() that does the
>>>>>>> RootAccess load.
>>>>>>> http://cr.openjdk.java.net/~eosterlund/8197844/webrev.00/src/hotspot/share/oops/klass.cpp.udiff.html
>>>>>>> but not have it mark the class_loader, only do below.
>>>>>>> +oop Klass::java_mirror_phantom() {
>>>>>>> + // Loading the klass_holder as a phantom oop ref keeps the
>>>>>>> class alive.
>>>>>>> + // After the holder has been kept alive, it is safe to return the
>>>>>>> mirror.
>>>>>>> + oop mirror =
>>>>>>> RootAccess<ON_PHANTOM_OOP_REF>::oop_load(k->java_mirror_handle().ptr_raw());
>>>>>>> + return mirror;
>>>>>>> +}
>>>>>>> I didn't like that Klass was a friend of ClassLoaderData and could
>>>>>>> get to the class_loader field (in case I move it).
>>>>>>> Your change here is covered by Kim's work to accessorize jni.
>>>>>>> http://cr.openjdk.java.net/~rkennke/8199612/webrev.00/src/hotspot/share/runtime/jniHandles.cpp.udiff.html
>>>>>>> So I withdraw and I'll unassign myself and you and/or Erik can take
>>>>>>> this bug back again.
>>>>>>> Thanks!
>>>>>>> Coleen
>>>>>>>> Roman

More information about the hotspot-runtime-dev mailing list