Deadlocks

• When the process A get stuck because it requires a resource that is busy because it's being used by process B that requires process A to complete
• In this situation none of the process will ever continue: a deadlock
• Can happen either for hardware resources and for software resources

Conditions for a deadlock to occur

• A deadlock can arise when the following conditions hold simultaneously

• Mutual exclusion

  • A resource is non-sharable (only one process can use it at a time)
• Hold and wait
  • A process will hold the resources already acquired while it is waiting to acquire another busy resource
• No preemption
  • The OS (external scheduler) cannot interrupt the (stuck) process that holds a resource
  • The resource can only be released by the process itself
• Circular wait
  • A set of processes are waiting for each other in circular form

Code example

Thread A: Acquiring printer resource
Thread A: Using printer resource

Thread B: Acquiring camera resource
Thread B: Using camera resource

Thread A: Acquiring camera resource
Thread B: Acquiring printer resource

Handling deadlocks

If a deadlock has happened, the following techniques can be applied

1. Preemption

   • Forces the interrupt of one or more processes can are causing the deadlock and thus release the resource
   • It may cause problems since the resource is non-preemptive

2. Rollback

   • Roll everything back into a previous checkpoint state

3. Kill one or more processes

   • Similar to preemption, but in this case the process is killed forever