CONDITIONS FOR DEADLOCK

All four of these conditions must be present for deadlock to occur.  If one is not present, deadlock cannot occur.

1. Mutual exclusion of resources – situation where only one process can have the resource.  The resource is not shared.
2. Hold and Wait – a process with a resource holds on to that resource until it gets ALL of the resources it needs to complete the process.
   GREEDY.  Example:  If a process needs a CD burner and a tape drive to complete the process and the burner is available, the process cannot grab the burner and hold it until the tape drive is available.  This would keep other process that might be able to use the burner from using it.
3. Non-preemptable resource.  If you have a resource that you can’t give back until you are finished with it.
4. Circular wait – if you have a chain of processes waiting for the resource and no one can go because another process is holding the resource they need. “Must be a circular chain of two or more processes, each of which is waiting for a resource held by the next member of the chain.”  Think of the dining philosophers and the chopsticks.  If they are all sitting around the table holding one chopstick, no one will ever get their second chopstick.

Remember!!  Deadlock is FOREVER!

Click here for the study questions