RFR(S): 8222518: Remove unnecessary caching of Parker object in java.lang.Thread

[David Holmes]

Thanks Dan! Extraneous ; culled.

David

On 25/04/2019 1:16 am, Daniel D. Daugherty wrote:
> On 4/24/19 3:12 AM, David Holmes wrote:
>> Bug: https://bugs.openjdk.java.net/browse/JDK-8222518
>> webrev: http://cr.openjdk.java.net/~dholmes/8222518/webrev/
> 
> src/hotspot/share/classfile/javaClasses.cpp
>      L1629:   macro(_park_blocker_offset,  k, "parkBlocker", 
> object_signature, false);
>          Line ends with a ';' and the previous last line did not. When the
>          THREAD_FIELDS_DO macro is called, it is already followed by a ';':
> 
>          L1635:   THREAD_FIELDS_DO(FIELD_COMPUTE_OFFSET);
>          L1640:   THREAD_FIELDS_DO(FIELD_SERIALIZE_OFFSET);
> 
> src/hotspot/share/classfile/javaClasses.hpp
>      No comments.
> 
> src/hotspot/share/prims/unsafe.cpp
>      No comments.
> 
> src/java.base/share/classes/java/lang/Thread.java
>      No comments.
> 
> Thumbs up.  I don't need to see another webrev if you choose to remove
> the ';' on L1629.
> 
> Dan
> 
>>
>> 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
>