Deadlock prevention is based on designing resource allocation policies, which make deadlocks impossible. Use of the deadlock prevention approach avoids the over- head of deadlock detection and resolution. However, it incurs two kinds of costs - overhead of using the resource allocation policy, and cost of resource idling due to the policy.
As described in earlier section, four conditions must hold for a resource deadlock to arise in a system:
¾ Non-shareable resources
¾ Hold-and-wait by processes
¾ No preemption of resources
¾ Circular waits.
Havender in his pioneering work showed that since all four of the conditions are necessary for deadlock to occur, it follows that deadlock might be prevented by denying any one of the conditions. Ensuring that one of these conditions cannot be satisfied prevents deadlocks. We first discuss how each of these conditions can be prevented and then discuss a couple of resource allocation policies based on the prevention approach.
3.2.6.1 Elimination of “Mutual Exclusion” Condition
The mutual exclusion condition must hold for non-sharable resources. That is, several processes cannot simultaneously share a single resource. This condition is difficult to eliminate because some resources, such as the tap drive and
printer, are inherently non-shareable. Note that shareable resources like read-only-file do not require mutually exclusive access and thus cannot be involved in deadlock.
Some resources can be made sharable. Spooling can make devices like printers or tape drives sharable. Spooling is storing the output on a shared medium, like disk, and using a single process to coordinate access to the shared resource.
Print spooling is the cannonical example. There are generally some resources that cannot be spooled - semaphores, for example. Removing mutual exclusion is not always an option.
3.2.6.2 Elimination of “Hold and Wait” Condition
There are two possibilities for elimination of the second condition. The first alternative is that a process request be granted all of the resources it needs at once, prior to execution. The second alternative is to disallow a process from requesting resources whenever it has previously allocated resources. This strategy requires that all of the resources a process will need must be requested at once. The system must grant resources on “all or none” basis. If the complete set of resources needed by a process is not currently available, then the process must wait until the complete set is available. While the process waits, however, it may not hold any resources. Thus the “wait for” condition is denied and deadlocks simply cannot occur. This strategy can lead to serious waste of resources. For example, a program requiring ten tap drives must request and receive all ten derives before it begins executing. If the program needs only one tap drive to begin execution and then does not need the remaining tap drives for several hours. Then substantial computer resources (9 tape drives) will sit idle for several hours. This strategy can cause indefinite postponement (starvation).
Since not all the required resources may become available at once.
3.2.6.3 Elimination of “No-preemption” Condition
The nonpreemption condition can be alleviated by forcing a process waiting for a resource that cannot immediately be allocated to relinquish all of its currently held resources, so that other processes may use them to finish. Suppose a system does allow processes to hold resources while requesting additional
resources. Consider what happens when a request cannot be satisfied. A process holds resources a second process may need in order to proceed while second process may hold the resources needed by the first process. This is a deadlock. This strategy requires that when a process that is holding some resources is denied a request for additional resources. The process must release its held resources and, if necessary, request them again together with additional resources. Implementation of this strategy denies the “no-preemptive” condition effectively.
The main drawback of this approach is high cost. When a process releases resources the process may lose all its work to that point. One serious consequence of this strategy is the possibility of indefinite postponement (starvation). A process might be held off indefinitely as it repeatedly requests and releases the same resources.
3.2.6.4 Elimination of “Circular Wait” Condition
Presence of a cycle in resource allocation graph indicates the “circular wait”
condition. The last condition, the circular wait, can be denied by imposing a total ordering on all of the resource types and than forcing, all processes to request the resources in numerical order (increasing or decreasing). With this rule, the resource allocation graph can never have a cycle.
For example, provide a global numbering of all the resources, as shown
1 Card Reader
2 Printer 3 Plotter 4 Tape Drive
5 Card Punch
Now the rule is this: processes can request resources whenever they want to, but all requests must be made in numerical order. A process may request first printer and then a tape drive (order: 2, 4), but it may not request first a plotter and then a printer (order: 3, 2). The problem with this strategy is that it may be impossible to find an ordering that satisfies everyone. The resource ranking policy works best when all processes require their resources in the order of increasing ranks.
However, difficulty arises when a process requires resources in some other order. Now processes may tend to circumvent such difficulties by acquiring lower ranking resources much before they are actually needed. In the worst case this policy may degenerate into the ‘all requests together’ policy of resource allocation. Anyway this policy is attractive due to its simplicity once resource ranks have been assigned.
“All requests together” is the simplest of all deadlock prevention policies. A process must make its resource requests together-typically, at the start of its execution. This restriction permits a process to make only one multiple request in its lifetime. Since resources requested in a multiple request are allocated together, a blocked process does not hold any resources. The hold-and-wait condition is satisfied. Hence paths of length larger than 1 cannot exist in the Resource Allocation Graph, a mutual wait-for relationships cannot develop in the system. Thus, deadlocks cannot arise.
Example: The most common method of preventing deadlock is to prevent the circular wait. A simple way to do this, when possible, is to order the resources and always acquire them in order. Because a process can’t be waiting on a lower numbered process while holding a higher numbered one, a cycle is impossible.
One can consider the Dining Philosophers to be a deadlock problem, and can apply deadlock prevention to it by numbering the forks and always acquiring the lowest numbered fork first.
#define N 5 /* Number of philosphers */
#define RIGHT(i) (((i)+1) %N)
#define LEFT(i) (((i)==N) ? 0 : (i)+1)
typedef enum { THINKING, HUNGRY, EATING } phil_state;
phil_state state[N];
semaphore mutex =1;
semaphore f[N]; /* one per fork, all 1*/
void get_forks (int i) {
int max, min;
if ( RIGHT(i) > LEFT(i) ) {
max = RIGHT(i); min = LEFT(i);
} else {
min = RIGHT(i); max = LEFT(i);
}
P (f [min]);
P (f [max]);
}
void put_forks (int i) {
V (f [LEFT (i)]);
V (f [RIGHT (i)]);
}
void philosopher (int process) {
While (1) {
think ();
get_forks (process);
eat ();
put_forks (process);
} }
This solution doesn’t get maximum parallelism, but it is an otherwise valid solution.