Processing the requests

The control program constructs a unique list for each qname, rname, and scope combination it receives. When a task makes a request, the control program searches the existing lists for a matching qname, rname, and scope. If it finds a match, the control program adds the task's request to the end of the existing list; the list is not ordered by the priority of the tasks on it. If the control program does not find a match, it creates a new list, and adds the task's request as the first (and only) element. The task gets control of the resource based on the following:

The position of the task's request on the list
Whether or not the request was for exclusive or shared control

The best way to describe how the control program processes the list of requests for a resource is through an example. Figure 1 shows the status of a list built for a qname, rname, and scope combination. The S or E next to the entry indicates that the request was for either shared or exclusive control. The task represented by the first entry on the list always gets control of the resource, so the task represented by ENTRY1 (Figure 1, Step 1) is assigned the resource. The request that established ENTRY2 (Figure 1, Step 1) was for exclusive control, so the corresponding task is placed in the wait condition, along with the tasks represented by all the other entries in the list.

Figure 1. ISGENQ Macro Processing

Eventually, the task represented by ENTRY1 releases control of the resource, and the ENTRY1 is removed from the list. As shown in Figure 1, Step 2, ENTRY2 is now first on the list, and the corresponding task is assigned control of the resource. Because the request that established ENTRY2 was for exclusive control, the tasks represented by all the other entries in the list remain in the wait condition.

Figure 1, Step 3, shows the status of the list after the task represented by ENTRY2 releases the resource. Because ENTRY3 is now at the top of the list, the task represented by ENTRY3 gets control of the resource. ENTRY3 indicates that the resource can be shared, and, because ENTRY4 also indicates that the resource can be shared, ENTRY4 also gets control of the resource. In this case, the task represented by ENTRY5 does not get control of the resource until both the tasks represented by ENTRY3 and ENTRY4 release control because ENTRY5 indicates exclusive use.

The control program uses the following general rules in manipulating the lists:

The task represented by the first entry in the list always gets control of the resource.
If the request is for exclusive control, the task is not given control of the resource until its request is the first entry in the list.
If the request is for shared control, the task is given control either when its request is first in the list or when all the entries before it in the list also indicate a shared request.
If the request is for several resources, the task is given control when all of the entries requesting exclusive control are first in their respective lists and all the entries requesting shared control are either first in their respective lists or are preceded only by entries requesting shared control.