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                      ObjectMonitor              T-deflate
    T-enter           +-----------------------+  ----------------------------------------
    ----------------  | owner=DEFLATER_MARKER |  cmpxchg(DEFLATER_MARKER, &owner, NULL)
    owner contended   | count=1               |  :
    atomic inc count  +-----------------------+  prev = cmpxchg(-max_jint, &count, 0)
    if (count > 0)                               if (prev != 0 ||
      do contended                                   owner != DEFLATER_MARKER)
        enter work                                 bailout on deflation (nothing to undo)

• • This diagram starts after "Racing Threads".
  • T-enter and T-deflate both observe a count field > 0.
  • T-enter has won the race and it proceeds with the normal contended enter work.
  • T-deflate detects that it has lost the race and bails out on deflating the ObjectMonitor.
  • In this example, T-deflate never reaches the DEFLATER_MARKER check and it has nothing to undo.

T-enter Wins By A-B-A

                                                ObjectMonitor                T-deflate
    T-enter                                     +-------------------------+  ----------------------------------------
    ------------------------------------------  | owner=DEFLATER_MARKER   |  cmpxchg(DEFLATER_MARKER, &owner, NULL)
    owner contended                             | count=1                 |  : <thread_stalls>
    atomic inc count                            +-------------------------+  :
    if (count > 0)                                          ||               :
      EnterI()                                              \/               :
        cmpxchg(Self, &owner, DEFLATER_MARKER)  +-------------------------+  :
      atomic dec count                          | owner=Self/T-enter      |  : <thread_resumes>
    }                                           | count=0                 |  prev = cmpxchg(-max_jint, &count, 0)
    // finished with enter                      +-------------------------+  if (prev != 0 ||)
    : <does app work>                                       ||                 bailout on owner != DEFLATER_MARKERdeflation (nothing to undo)
    exit() monitor                                          \/               else if bailout on deflationowner != DEFLATER_MARKER) {
      owner = NULL                              +-------------------------+    atomic add max_jint to count
                                                | owner=Self/T-enter|NULL |    bailout on deflation
                                                | count=-max_jint0                 |  }
                                                +-------------------------+

• • This diagram starts after "Racing Threads".
  • T-enter observes a count field > 0.
  • T-deflate stalls after setting the owner field to DEFLATER_MARKER.
  • T-enter has won the race and calls EnterI() to do the contended enter work.
    • EnterI() observes owner == DEFLATER_MARKER and uses cmpxchg() to set the owner field to Self/T-enter.
    • T-enter decrements the count field because it is no longer contending for the monitor; it owns the monitor.
  • The second ObjectMonitor box is showing the fields at this point.
  • T-deflate resumes, sets the count field to -max_jint, and passes the first part of the bailout expression because "prev == 0".
  • T-deflate observes that "owner != DEFLATE_MARKER" and bails out on deflation.
    • Depending on when T-deflate resumes after the stall, it will see "owner == T-enter" or "owner == NULL".
    • Both of those values will cause deflation to bailout, but in this case we have to undo setting count to -max_jint by atomically adding max_jint to count which will restore count to its proper value.
  • The third ObjectMonitor box is showing the fields at this point.

  • If the T-enter thread has managed to enter but not exit the monitor during the T-deflate stall, then our owner field A-B-A transition is:
        NULL → DEFLATE_MARKER → Self/T-enter
    so we really have A1-B-A2, but the A-B-A principal still holds.

  • If the T-enter thread has managed to enter and exit the monitor during the T-deflate stall, then our owner field A-B-A transition is:
        NULL → DEFLATE_MARKER → Self/T-enter  → NULL
    so we really have A-B1-B2-A, but the A-B-A principal still holds.

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