mpiu_object_release_ref.tex

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\startmanpage
\mantitle{MPIU{\tt \char`\_}Object{\tt \char`\_}release{\tt \char`\_}ref}{tex}{10/7/2002}
\manname{MPIU{\tt \char`\_}Object{\tt \char`\_}release{\tt \char`\_}ref}
--- Decrement the reference count for an MPI object 
\subhead{Synopsis}
\startvb\begin{verbatim}
int MPIU_Object_release_ref( MPIU_Object_head *ptr, int *newval_ptr )

\end{verbatim}
\endvb

\subhead{Input Parameter}
\startarg{objptr }{Pointer to the object.
}
\par
\subhead{Output Parameter}
\startarg{newval{\tt \char`\_}ptr }{Pointer to the value of the reference count after decrementing.
This value is either zero or non-zero. See below for details.
}
\subhead{Notes}
In an unthreaded implementation, this function will usually be implemented
as a single-statement macro.  In an {\tt MPI{\tt \char`\_}THREAD{\tt \char`\_}MULTIPLE} implementation,
this routine must implement an atomic decrement operation, using, for
example, a lock on datatypes or special assembly code such as
\begin{verbatim}
   try-again:
      load-link          refcount-address to r2
      sub                1 to r2
      store-conditional  r2 to refcount-address
      if failed branch to try-again:
      store              r2 to newval_ptr
\end{verbatim}

on RISC architectures or
\begin{verbatim}
      lock
      dec                   refcount-address 
      if zf store 0 to newval_ptr else store 1 to newval_ptr
\end{verbatim}

on IA32; "lock" is a special opcode prefix that forces atomicity.  This
is not a separate instruction; however, the GNU assembler expects opcode
prefixes on a separate line.  {\tt zf} is the zero flag; this is set if the
result of the operation is zero.  Implementing a full decrement-and-fetch
would require more code and the compare and swap instruction.
\par
Once the reference count is decremented to zero, it is an error to
change it.  A correct MPI program will never do that, but an incorrect one
(particularly a multithreaded program with a race condition) might.
\par
The following code is {\em invalid}:
\begin{verbatim}
   MPID_Object_release_ref( datatype_ptr );
   if (datatype_ptr->ref_count == 0) MPID_Datatype_free( datatype_ptr );
\end{verbatim}

In a multi-threaded implementation, the value of {\tt datatype{\tt \char`\_}ptr-$>$ref{\tt \char`\_}count
}may have been changed by another thread, resulting in both threads calling
{\tt MPID{\tt \char`\_}Datatype{\tt \char`\_}free}.  Instead, use
\begin{verbatim}
   if (MPID_Object_release_ref( datatype_ptr ) == 0) 
       MPID_Datatype_free( datatype_ptr );
\end{verbatim}

\par
\subhead{Module}
MPID{\tt \char`\_}CORE
\endmanpage