Deadlocks

flashcards made from Deadlocks slides from week 9

Resources and Deadlocks

• Many resources in a computer system can only be used by one process at a time

– Printers, some backup media, system tables, database records, etc

• Operating systems need to grant exclusive access for these cases

• Some applications need to access several of these resources:

– E.g. Two different processes (A and B) need to save scanned documents to a DVD:

• However, A can request access to the scanner at the same as B requests access to the DVD, hence none can get their missing resource next: Deadlock!

• Deadlocks can also occur across machines or with more than just two processes/devices

Preemptable and Nonpreemptable Resources

• Preemptable resource:

– Can be taken away from the process that is using it without causing issues

– e.g. a process can be temporarily removed from memory

• Non-preemptable resource:

– Cannot be taken away from the process that is using it without potentially causing failure

– e.g. interrupting a record (burn) operation on a DVD will usually cause it to fail

• Normally the resources which are susceptible to be involved in deadlocks are non-preemptable

• In this context, the sequence of events required to use a resource is:

1. Request the resource

2. Use the resource

3. Release the resource

Resource Acquisition (1)

Resource Acquisition (2)