RFR (M): 8079205: CallSite dependency tracking is broken after sun.misc.Cleaner became automatically cleared
vladimir.x.ivanov at oracle.com
Fri May 8 17:16:38 UTC 2015
Recent change in sun.misc.Cleaner behavior broke CallSite context cleanup.
CallSite references context class through a Cleaner to avoid its
The problem is the following: to do a cleanup (invalidate all affected
nmethods) VM needs a pointer to a context class. Until Cleaner is
cleared (and it was a manual action, since Cleaner extends
PhantomReference isn't automatically cleared according to the docs), VM
can extract it from CallSite.context.referent field.
I experimented with moving cleanup logic into VM , but Peter Levart
came up with a clever idea and implemented FinalReference-based
cleaner-like Finalizator. Classes don't have finalizers, but Finalizator
allows to attach a finalization action to them. And it is guaranteed
that the referent is alive when finalization happens.
Also, Peter spotted another problem with Cleaner-based implementation.
Cleaner cleanup action is strongly referenced, since it is registered in
Cleaner class. CallSite context cleanup action keeps a reference to
CallSite class (it is passed to MHN.invalidateDependentNMethods). Users
are free to extend CallSite and many do so. If a context class and a
call site class are loaded by a custom class loader, such loader will
never be unloaded, causing a memory leak.
Finalizator doesn't suffer from that, since the action is referenced
only from Finalizator instance. The downside is that cleanup action can
be missed if Finalizator becomes unreachable. It's not a problem for
CallSite context, because a context is always referenced from some
CallSite and if a CallSite becomes unreachable, there's no need to
perform a cleanup.
Testing: jdk/test/java/lang/invoke, hotspot/test/compiler/jsr292
Contributed-by: plevart, vlivanov
PS: frankly speaking, I would move Finalizator from java.lang.ref to
java.lang.invoke and call it Context, if there were a way to extend
package-private FinalReference from another package :-)
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