RFR: 8259068: Streamline class loader locking

[Ioi Lam]

On Wed, 13 Jan 2021 23:30:09 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

> The system_loader_lock_object is never actually acquired when loading a class with a parallelCapable class loader, which includes the bootloader (class_loader == NULL), except in one place before restore_unshareable_info is called.  In this case, the per-class lock (BuiltinLoader.getClassLoadingLock) is held or the placeholder for LOAD_INSTANCE is present which implements mutual exclusion.  Ioi and I separately verified this while chasing down another bug.
> This change removes the system_loader_lock_obj and extraneous code around compute_loader_lock_object to make it consistent.  This also removes the bool argument to ObjectLocker.  If the oop passed into ObjectLocker is null, we don't lock the object, which is consistent with the Mutex class.  This change also passes the class_loader to define_class to save an OopStorage.resolve() call.
> Tested with tier1-8.

Changes requested by iklam (Reviewer).

src/hotspot/share/classfile/systemDictionary.cpp line 208:

> 206:       == ObjectSynchronizer::owner_other) {
> 207:     ClassLoader::sync_nonSystemLoaderLockContentionRate()->inc();
> 208:   }

Should the systemLoaderLockContentionRate perf counter be removed as well? I don't know what kind of compatibility risk is there. Maybe we can doit in a separate RFE.

src/hotspot/share/classfile/systemDictionaryShared.cpp line 1042:

> 1040:       Handle lockObject = compute_loader_lock_object(THREAD, class_loader);
> 1041:       check_loader_lock_contention(THREAD, lockObject);
> 1042:       ObjectLocker ol(lockObject, THREAD);

I think we can replace the above 3 lines with something like
assert(!is_parallelCapable(class_loader), "ObjectLocker not required");

src/hotspot/share/classfile/systemDictionary.cpp line 1104:

> 1102:                                                      TRAPS) {
> 1103: 
> 1104:   assert(st != NULL, "invariant");

Is this necessary? We normally don't have asserts for non-null-ness.

src/hotspot/share/classfile/systemDictionary.cpp line 1635:

> 1633:   HandleMark hm(THREAD);
> 1634:   ClassLoaderData* loader_data = k->class_loader_data();
> 1635:   assert(loader_data->class_loader() == class_loader(), "they are the same");

What's the reason for adding a class_loader parameter? Is it to avoid creating a new Handle?
If there's not much performance difference, I would suggest keeping the old code to avoid code churn.

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PR: https://git.openjdk.java.net/jdk/pull/2071