IV67555: JAVA JIT - UNSYNCHRONIZED USED OF ARRAYLIST LEADS TO A CRASH

APAR status

• Closed as program error.

Error description

• Error Message: Crash in JIT compiled code, GC or the interpreter
  due to a bad object reference in the backing array of an
  ArrayList.
  .
  Stack Trace: The stack trace for a failure of this type is not
  consistent. It could fail in several ways with different stack
  traces.
  .
  The problem can occur on any platform
  

Local fix

• -Xjit:disableArrayCopyOpts
  
  This option will have a small (likely unmeasurable) performance
  impact, but a more pronounced effect could be seen in extreme
  cases where arraycopy() calls account for the majority of the
  execution time.
  

Problem summary

• The JIT attempts to optimize arraycopy() calls made by ArrayList
  methods knowing that the code is written such that the
  arraycopy() is never asked to copy more then the number of
  elements in the backing array. This is only true if uses of the
  ArrayList are protected by a lock or only used by a single
  thread. Unsynchronized multi-threaded uses of an ArrayList could
  result in race conditions that allows for the ArrayList counters
  and the backing array to become out of sync. This allows the
  possibility for arraycopy() to be asked to copy more elements
  then exist in the backing array.
  

Problem conclusion

• The JIT was modified so that it no longer assumes that the
  ArrayList methods will never call arraycopy() with arguments
  that cause it to copy beyond the end of the backing array. This
  means that rather then copying random garbage past the end of
  the array, it will throw an exception in the cases where
  unsynchronized multi-threaded access to an ArrayList results in
  internal inconsistencies within the ArrayList.
  .
  This APAR will be fixed in the following Java Releases:
     6 R1 SR8 FP3   (6.1.8.3)
     7 R1 SR3         (7.1.3.0)
     6    SR16 FP3  (6.0.16.3)
     7    SR8 FP10  (7.0.8.10)
  .
  Contact your IBM Product's Service Team for these Service
  Refreshes and Fix Packs.
  For those running stand-alone, information about the Service
  Refreshes and Fix Packs can be found at:
             https://www.ibm.com/developerworks/java/jdk/
  

Temporary fix

Comments

APAR Information

• APAR is sysrouted FROM one or more of the following:

• APAR is sysrouted TO one or more of the following:

Fix information

• Fixed component name

  JIT

• Fixed component ID

  620700124

Applicable component levels

• R260 PSY

     UP

• R130 PSY

     UP

• R600 PSY

     UP