IJ30884: SEGV WHEN USING HASHMAP$HASHITERATOR.NEXTNODE() IN MULTIPLE THREADS CONCURRENTLY

APAR status

• Closed as program error.

Error description

• Error Message: SEGV in the JIT library.
  .
  Stack Trace: 0x090000000308b638
  {libj9jit29.so}{fast_jitCheckCast} <OSB>0x100363b7af0<CSB>
  0x0900000003073334 {libj9jit29.so}{jitCheckCast}
  <OSB>0x100363b7af0<CSB>
  .
  The problem can result in many different symptoms, but the most
  likely is a crash in JIT'ed code or a JIT helper (in the JIT
  library). It's also possible for a crash in the interpreter or
  in the the garbage collector. It's unlikely, but even unexpected
  behaviour is possible.
  The problem can occur on any platform and operating system
  running the J9 JVM starting with version 8.0.5.0
  

Local fix

• The problem can only occur when running a JIT compiled version
  of HashMap$HashIterator.nextNode(), so we can prevent the
  problem by preventing this method from being compiled by using
  the following java command line argument.
  -Xjit:dontInline={*HashIterator.nextNode*},exclude={*HashIterato
  r.nextNode*}
  This workaround should have a minor impact on performance (less
  then 1%), unless the application spends an extraordinary amount
  of time using this method.
  

Problem summary

• The JIT was assuming that it was impossible for an array overrun
  to occur when using the HashMap$HashIterator.nextNode() method.
  This is true unless a race condition is encountered which allows
  two index increments to occur concurrently. This would cause one
  thread to increment beyond the end of the HashMap table array
  bounds and access data passed the end of the array.
  

Problem conclusion

• The JIT was modified so that it does not remove array bound
  checks when compiling the HashMap$HashIterator.nextNode()
  method.
  .
  This APAR will be fixed in the following Java Releases:
     8    SR6 FP26  (8.0.6.26)
  .
  Contact your IBM Product's Service Team for these Service
  Refreshes and Fix Packs.
  For those running stand-alone, information about the available
  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