What do you mean by deadlock? Give two examples of deadlock prevention.
Deadlock is a potential problem in any operating system. It occurs when a group of processes each have been granted exclusive access to some resources, and each one wants yet another resource that belongs to another process in the group. All of them are blocked and none will ever run again.
Deadlock can be defined as: “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.”
Examples:
- Two processes each want to record a scanned document on a CD. Process A requests permission to use the scanner and is granted it. Process B is programmed differently and request the CD recorder first and is also granted it. Now A asks for the CD recorder, but the request is denied until B releases it. Unfortunately, instead of releasing the CD recorder B asks for the scanner. All this point both processes are blocked and will remain so forever.
- In a database system, for example, a program may have to lock several records it is using, to avoid race conditions. If process A locks record R1 and Process B locks record R2, and then each process tries to lock the other one’s record, we also have a deadlock
Thus deadlocks can occur on hardware resources or on software resources.
Condition for deadlock:
- Mutual exclusive condition: only one process at a time can use a resource
- Hold and wait: a process holding at least one resource is waiting to acquire additional resources held by other processes.
- No preemption condition: a resource can be released only voluntarily by the process holding it, after that process has completed its task.
- Circular wait: there exists a set {P0, P1,P2,P3} of waiting processes such that P0 is waiting. For a resource that is held by P1, P1 is waiting for a resource that is held by P2, P2 is waiting for a resource held by P3, P3 is waiting for a resource that is held by P0.