Deadlock Prevention

Difference from avoidance is that here, the system itself is build in such a way that there are no deadlocks.

Make sure at least one of the 4 deadlock conditions is never satisfied.

This may however be even more conservative than deadlock avoidance strategy.

Attacking Mutual condition
- never grant exclusive access. but this may not be possible for several resources.
Attacking pre-emption
- not something you want to do.
Attacking hold and wait condition
- make a process hold at the most 1 resource at a time.
- make all the requests at the beginning. All or nothing policy. If you feel, retry. eg. 2-phase locking
Attacking circular wait
- Order all the resources.
- Make sure that the requests are issued in the correct order so that there are no cycles present in the resource graph.
  
  Resources numbered 1 ... n. Resources can be requested only in increasing order. ie. you cannot request a resource whose no is less than any you may be holding.

Deadlock Avoidance

Avoid actions that may lead to a deadlock.

Think of it as a state machine moving from 1 state to another as each instruction is executed.

Safe State

Safe state is one where

It is not a deadlocked state
There is some sequence by which all requests can be satisfied.

To avoid deadlocks, we try to make only those transitions that will take you from one safe state to another. We avoid transitions to unsafe state (a state that is not deadlocked, and is not safe)

 e.g.

Total  of instances of resource = 12

(Max, Allocated, Still Needs)

P0 (10, 5, 5)  P1 (4, 2, 2)             P2 (9, 2, 7)     Free = 3    - Safe

The sequence  is a reducible sequence, the first state is safe.

 What if P2 requests 1 more and is allocated 1 more instance?

- Results in Unsafe state

 So do not allow P2's request to be satisfied.