Deadlock in OS | Conditions for Deadlock

Deadlock in OS-

 

Deadlock is a situation where-

• The execution of two or more processes is blocked because each process holds some resource and waits for another resource held by some other process.

 

Example-

 

 

Here

• Process P1 holds resource R1 and waits for resource R2 which is held by process P2.
• Process P2 holds resource R2 and waits for resource R1 which is held by process P1.
• None of the two processes can complete and release their resource.
• Thus, both the processes keep waiting infinitely.

 

Conditions For Deadlock-

 

There are following 4 necessary conditions for the occurrence of deadlock-

1. Mutual Exclusion
2. Hold and Wait
3. No preemption
4. Circular wait

 

1. Mutual Exclusion-

 

By this condition,

• There must exist at least one resource in the system which can be used by only one process at a time.
• If there exists no such resource, then deadlock will never occur.
• Printer is an example of a resource that can be used by only one process at a time.

 

2. Hold and Wait-

 

By this condition,

• There must exist a process which holds some resource and waits for another resource held by some other process.

 

3. No Preemption-

 

By this condition,

• Once the resource has been allocated to the process, it can not be preempted.
• It means resource can not be snatched forcefully from one process and given to the other process.
• The process must release the resource voluntarily by itself.

 

4. Circular Wait-

 

By this condition,

• All the processes must wait for the resource in a cyclic manner where the last process waits for the resource held by the first process.

 

 

Here,

• Process P1 waits for a resource held by process P2.
• Process P2 waits for a resource held by process P3.
• Process P3 waits for a resource held by process P4.
• Process P4 waits for a resource held by process P1.

 

Important Note-

 

• All these 4 conditions must hold simultaneously for the occurrence of deadlock.
• If any of these conditions fail, then the system can be ensured deadlock free.

 

To gain better understanding about Deadlock in OS,

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Next Article- Deadlock Handling Strategies

 

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