Necessary conditions for deadlock

Conditions for Deadlock

Mutual exclusion: resources cannot be shared.
Hold and wait: processes request resources incrementally, and hold on to what they've got.
No preemption: resources cannot be forcibly taken from processes.
Circular wait: circular chain of waiting, in which each process is waiting for a resource held by the next process in the chain.

http://www1bpt.bridgeport.edu/sed/projects/cs503/Spring_2001/kode/os/deadlock.htm#conditions






four (4) conditions that must hold simultaneously for there to be a deadlock.

1. Mutual Exclusion Condition
The resources involved are non-shareable.
Explanation: At least one resource (thread) must be held in a non-shareable mode, that is, only one process at a time claims exclusive control of the resource. If another process requests that resource, the requesting process must be delayed until the resource has been released.

2. Hold and Wait Condition
Requesting process hold already, resources while waiting for requested resources.
Explanation: There must exist a process that is holding a resource already allocated to it while waiting for additional resource that are currently being held by other processes.

3. No-Preemptive Condition
Resources already allocated to a process cannot be preempted.
Explanation: Resources cannot be removed from the processes are used to completion or released voluntarily by the process holding it.

4. Circular Wait Condition
The processes in the system form a circular list or chain where each process in the list is waiting for a resource held by the next process in the list.

http://www.personal.kent.edu/~rmuhamma/OpSystems/Myos/deadlockCondition.htm







Necessary conditions for deadlock

A deadlock situation can arise if and only if the following four conditions hold simultaneously in a system-

Mutual Exclusion: At least one resource is held in a non-sharable mode that is only one process at a time can use the resource. If another process requests that resource, the requesting process must be delayed until the resource has been released.

Hold and Wait:There must exist a process that is holding at least one resource and is waiting to acquire additional resources that are currently being held by other processes.

No Preemption: Resouces cannot be preempted; that is, a resource can only be released voluntarily by the process holding it, after the process has completed its task.

Circular Wait: There must exist a set {p0, p1,.....pn} of waiting processes such that p0 is waiting for a resource which is held by p1, p1 is waiting for a resource which is held by p2,..., pn-1 is waiting for a resource which is held by pn and pn is waiting for a resource which is held by p0.


http://wikieducator.org/Necessary_conditions_for_deadlock