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

Roman Kennke rkennke at redhat.com
Mon Mar 19 23:32:45 UTC 2018

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.

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

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