• Preemptable and Nonpreemptable Resources
• Sequence of events required to use a resource:
  • Request the resource.
  • Use the resource.
  • Release the resource.

• Using a semaphore to protect resources.

• (a) One resource. (b) Two resources.

• (a) Deadlock-free code. (b) Code with a potential deadlock.

Deadlock can be defined formally as follows:

• A set of processes is deadlocked if each process in the set is waiting for an event that only another process in the set can cause.

Conditions for Resource Deadlocks

1. Mutual exclusion condition
2. Hold and wait condition.
3. No preemption condition.
4. Circular wait condition.

Resource allocation graphs.

• (a) Holding a resource. (b) Requesting a resource. (c) Deadlock.

An example of how deadlock occurs and how it can be avoided.

• Just ignore the problem.
• Detection and recovery. Let deadlocks occur, detect them, take action.
• Dynamic avoidance by careful resource allocation.
• Prevention, by structurally negating one of the four required conditions.

• Deadlock Detection with One Resource of Each Type

• (a) A resource graph. (b) A cycle extracted from (a).

Deadlock Detection with Multiple Resources of Each Type

• The four data structures needed by the deadlock detection algorithm.

An example for the deadlock detection algorithm.

• Recovery through preemption
• Recovery through rollback
• Recovery through killing processes

• Two process resource trajectories.

• Demonstration that the state in (a) is safe.

• Demonstration that the state in (b) is not safe.

• The Banker’s Algorithm for a Single Resource

• Three resource allocation states: (a) Safe. (b) Safe. (c) Unsafe.

• The banker’s algorithm with multiple resources.

• Attacking the mutual exclusion condition
• Attacking the hold and wait condition
• Attacking the no preemption condition
• Attacking the circular wait condition

• Chapter 6: Deadlocks, Modern Operating Systems . Forth Edition, Andrew S. Tanenbaum