# Summary ch7

1. Deadlock: Permanent blocking of a set of processes that either compete for system resources or communicate with each other.
2. Reusable Resources:   

   -Used by only one process at a time and not depleted by that use  

   -Processes obtain resources that they later release for reuse by other processes  

   -Processors, I/O channels, main and secondary memory, devices, and data structures such as files and databases  

   -Deadlock occurs if each process holds one resource and requests the other
3. Consumable Resources:  

   -Created (produced) and destroyed (consumed)  

   -Interrupts, signals, messages, and information in I/O buffers  
4. Conditions for Deadlock  

   -Mutual exclusion: Only one process may use a resource at a time   

   -Hold-and-wait: A process may hold allocated resources while awaiting assignment of others    

   -No preemption: No resource can be forcibly removed from a process holding it  

   -Circular wait: A closed chain of processes exists, such that each process holds at least one resource needed by the next process in the chain
5. Deadlock Avoidance  

   -A decision is made dynamically on whether the current resource allocation request will, if granted, potentially lead to a deadlock    

   -Requires knowledge of future process requests  

   -Resource-allocation-graph algorithm (Chap 7.5.2) is only applicable to systems with single instance of each resource type  

   -Do not grant an incremental resource request to a process if this allocation might lead to deadlock (Banker’s Algorithm)
6. Deadlock Avoidance  

   -Maximum resource requirement must be stated in advance  

   -Processes under consideration must be independent; their execution order must be unconstrained by any synchronization requirements  

   -There must be a fixed number of resources to allocate  

   -No process may exit while holding resources  
7. Strategies Once Deadlock Detected  

   -Abort all deadlocked processes  

   -Back up each deadlocked process to some previously defined checkpoint, and restart all processes  

   -Successively abort deadlocked processes until deadlock no longer exists  

   -Successively preempt resources until deadlock no longer exists