What are the four necessary conditions of deadlock prevention?      

Ans:  Four essential conditions for deadlock prevention:

a. The meaning of removing the mutual exclusion condition is that no process may have exclusive access to a resource. This confirms impossible for resources that cannot be spooled, and even along with spooled resources deadlock could still occur. Algorithms which avoid mutual exclusion are called non-blocking synchronization algorithms.

b. The "hold and wait" conditions may be eliminated by requiring processes to request all the resources they will need previous to starting up. Another way is to need processes to release all their resources previous to requesting all the resources they will need.

c. A "no preemption" (lockout) condition may as well be difficult or impossible to evade as a process has to be able to have a resource for  a specific amount of time, or the processing outcome may be inconsistent or thrashing may occur. Though, inability to enforce preemption may interfere with a priority algorithm. Algorithms that permit preemption include lock-free and wait-free algorithms and optimistic concurrency control.

d. The circular wait condition: Algorithms which avoid circular waits contain "disable interrupts during critical sections", and "use a hierarchy to define a partial ordering of resources" and Dijkstra's solution.