HP 3000 Manuals HP 3000 Manuals
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
Figure 2.6
Example 2-6. Deadlock.
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.