I'll file a RFE for that.
Great, thanks!
/Robbin
Thanks,
David
Thanks, Robbin
On 4/24/19 9:12 AM, David Holmes wrote:
Bug: https://bugs.openjdk.java.net/browse/JDK-8222518
webrev: http://cr.openjdk.java.net/~dholmes/8222518/webrev/
The original implementation of Unsafe.unpark simply extracted the JavaThread
reference from the java.lang.Thread oop and if non-null extracted the Parker
instance from it and invoked unpark. This was racy however as the native
JavaThread could terminate at any time and deallocate the Parker.
That logic was fixed by JDK-6271298 which used of combination of
type-stable-memory "event" objects for the Parker, along with use of the
Threads_lock to obtain the initial reference to the Parker (from a JavaThread
guaranteed to be alive), together with caching the native Parker pointer in a
field of java.lang.Thread. Even though the native thread may have terminated
the Parker was still valid (even if associated with a different thread) and
the unpark at worst was a spurious wakeup for that other thread.
When JDK-8167108 introduced Thread Safe-Memory-Reclaimation (SMR) the logic
was updated to always use the safe mechanism - we grab a ThreadsListHandle
then check the cached field, else lookup the native thread to see if it is
alive and locate the Parker instance that way.
With SMR the caching of the Parker pointer no longer serves any purpose - we
no longer have a lock-free use-the-cache path versus a lock-using
populate-the-cache path. With SMR we've already"paid" for the ability to
ensure the native thread can't terminate regardless of whether we lookup the
field from the java.lang.Thread or the JavaThread. So we can simplify the
code and save a little footprint by removing the cache from java.lang.Thread:
/*
* JVM-private state that persists after native thread termination.
*/
private long nativeParkEventPointer;
and the supporting code from unsafe.cpp and javaClass.*pp in the JVM.
I considered restoring the fast-path use of the cache without recourse to
Thread-SMR but performance measurements failed to show any benefit in doing.
See bug report for details.
Thanks,
David