Deadlock Considerations

If you are locking more than one RIN at a time in your program, there is a chance that you can cause a deadlock between two or more processes. Deadlock occurs when two or more processes are mutually blocked, waiting for each other to release a needed resource.

Example 2-6 illustrates how a deadlock can occur between two processes, Process A and Process B. Assume that both processes in the example are executing concurrently, line by line.

{    PROCESS A EXECUTING     }       {    PROCESS B EXECUTING     } 

               .                                   . 

               .                                   . 

               .                                   . 

        LOCKGLORIN (1);                     LOCKGLORIN (2); 

        LOCKGLORIN (2);                     LOCKGLORIN (1); 

                                                               

  PROCESS A REMAINS BLOCKED      PROCESS B REMAINS BLOCKED     

  UNTIL RIN 2 IS UNLOCKED        UNTIL RIN 1 IS UNLOCKED       

  BY PROCESS B. PROCESS A        BY PROCESS A. PROCESS B       

  CANNOT UNLOCK RIN 1 WHILE      CANNOT UNLOCK RIN 2 WHILE     

  BEING BLOCKED                  BEING BLOCKED                 

                                                               

In Example 2-6, Process A successfully locks global RIN 1, then attempts to lock global RIN 2 (already locked by Process B). Process A is blocked until it can successfully lock global RIN 2. While Process A is blocked, it cannot unlock global RIN 1, thus making global RIN 1 unavailable for locking by Process B.

Process B, meanwhile, has locked global RIN 2 and has been blocked attempting to lock global RIN 1 (locked, or course, by Process A). Global RIN 2 remains unavailable to Process A.

Both Process A and Process B find themselves mutually blocked and in the state of deadlock. Even if subsequent code in both program files unlocks one or both global RINs, neither process can execute that code.

One way to avoid deadlocks is by ranking the RINs used by cooperating processes. In Example 2-5, cooperating processes agree to first attempt to lock RIN 1 before attempting to lock RIN2. In addition, the processes that have successfully locked both RINs agree to unlock the two RINs in the reverse order, first RIN 2, then RIN 1. Because cooperating processes must lock RINs in ascending order and unlock them in descending order, deadlock cannot occur.

If you have only standard user capabilities and deadlock occurs between two or more processes in your program's process structure, you must abort your program to resolve the deadlock.

If you have Multiple RIN (MR) Capability and deadlock occurs between your program and processes located in different jobs/sessions, you must immediately contact your System Manager to resolve the deadlock.