Approaches to deal with deadlock
- Deadlock avoidance
- Deadlock detection and recovery
- Deadlock avoidance
- Deadlock prevention
Deadlock prevention
Deadlock can be prevented by attacking the one of the four conditions that leads to deadlock.
Attacking the Mutual Exclusion Condition
- No deadlock if no resource is ever assigned exclusively to a single process.
- Some devices can be spooled such as printer, by spooling printer output; several processes can generate output at the same time.
- Only the printer daemon process uses physical printer.
- Thus deadlock for printer can be eliminated.
- Not all devices can be spooled.
- Principle:
- Avoid assigning a resource when that is not absolutely necessary.
- Try to make sure that as few processes as possible actually claim the resource.
Attacking the Hold and Wait Condition
- Require processes to request all their resources before starting execution.
- A process is allowed to run if all resources it needed is available. Otherwise nothing will be allocated and it will just wait.
- Problem with this strategy is that a process may not know required resources at start of run.
- Resource will not be used optimally.
- It also ties up resources other processes could be using.
- Variation: A process must give up all resources before making a new request. Process is then granted all prior resources as well as the new ones only if all required resources are available.
- Problem: what if someone grabs the resources in the meantime how can the processes save its state?
Attacking the No Preemption Condition
- This is not a possible option.
- When a process P0 request some resource R which is held by another process P1 then resource R is forcibly taken away from the process P1 and allocated to P0.
- Consider a process holds the printer, halfway through its job; taking the printer away from this process without having any ill effect is not possible.
Attacking the Circular Wait Condition
- To provide a global numbering of all the resources.
- Now the rule is this: processes can request resources whenever they want to, but all requests must be made in numerical order.
- A process need not acquire them all at once.
- Circular wait is prevented if a process holding resource n cannot wait for resource m, if m > n.
- No way to complete a cycle.
