Deadlock Prevention

• The strategy of deadlock prevention is to design a system in such a way that the possibility of deadlock is excluded a priori
• Methods for preventing deadlock are of two classes:
  • indirect methods prevent the occurrence of one of the necessary conditions listed earlier.
  • direct methods prevent the occurrence of a circular wait condition.
• Deadlock prevention strategies are very conservative; they solve the problem of deadlock by limiting access to resources and by imposing restrictions on processes
• Make it impossible that one of the four conditions for deadlock arise
• Mutual exclusion
  • In general, this condition cannot be disallowed
  • we can avoid assigning resources when not absolutely necessary
  • as few processes as possible should claim the resource
• Hold-and-wait
  • The hold and-wait condition can be prevented by requiring that a process request all its required resources at one time, and blocking the process until all requests can be granted simultaneously
  • Can we require processes to request all resources at once?
  • Most processes do not statically know about the resources they need
  • Wasteful, but works
• No preemption
  • One solution is that if a process holding certain resources is denied a further request, that process must release its unused resources and request them again, together with the additional resource
  • Preemption is feasible for some resources (e.g., processor and memory), but not for others (state must be saved and restored)
• Circular Wait
  • The circular wait condition can be prevented by defining a linear ordering of resource types. If a process has been allocated resources of type R, then it may subsequently request only those resources of types following R in the ordering
  • order resources by an index: $R_1,R_2,\ldots$
  • requires that resources are always requested in order
  • $P_1$ holds $R_i$ and requests $R_j$, and $P_2$ holds $R_j$ and requests $R_i$ is impossible
  • sometimes a feasible strategy, but not generally efficient