Deadlocks

Deadlocks

In a multi programming environment several processes may compete for finite number of resources.

In normal operation a process must request a resource before using it, and release it when it is done, in the following sequence:

1. Request - If the request cannot be immediately granted, then the process must wait until the resource(s) it needs become available. For example the system calls open( ), malloc( ), new( ), and request( ).
2. Use - The process uses the resource, e.g. prints to the printer or reads from the file.
3. Release - The process relinquishes the resource. so that it becomes available for other processes. For example, close( ), free( ), delete( ), and release( ).

A set of processes is deadlocked when every process in the set is waiting for a resource that is currently allocated to another process in the set ( and which can only be released when that other waiting process makes progress. )

Deadlock Characterization

1. Necessary Conditions

There are four conditions that are necessary for a deadlock:

1. Mutual Exclusion - At least one resource must be held in a non-sharable mode; If any other process requests this resource, then that process must wait for the resource to be released. (Only one process at a time can use the resource)
2. Hold and Wait - A process must be simultaneously holding at least one resource and waiting for at least one resource that is currently being held by some other process.
3. No preemption - Once a process is holding a resource ( i.e. once its request has been granted ), then that resource cannot be taken away from that process until the process voluntarily releases it.
4. Circular Wait - A set of processes { P0, P1, P2, . . ., PN } must exist such that every P[ i ] is waiting for P[ ( i + 1 ) % ( N + 1 ) ]. (Hold and wait)

2. Resource Allocation Graph

Deadlocks can be understood more clearly through the use of Resource-Allocation Graphs, having the following properties

• A set of resource categories, { R1, R2, R3, . . ., RN }, which appear as square nodes on the graph. Dots inside the resource nodes indicate specific instances of the resource.
• A set of processes, { P1, P2, P3, . . ., PN }
• Request Edges - A set of directed arcs from Pi to Rj, indicating that process Pi has requested Rj, and is currently waiting for that resource to become available.
• Assignment Edges - A set of directed arcs from Rj to Pi indicating that resource Rj has been allocated to process Pi, and that Pi is currently holding resource Rj.

• If a resource-allocation graph contains no cycles, then the system is not deadlocked.
• If a resource-allocation graph does contain cycles AND each resource category contains only a single instance, then a deadlock exists.
• If a resource category contains more than one instance, then the presence of a cycle in the resource-allocation graph indicates the possibility of a deadlock, but does not guarantee one.

Methods for Handling Deadlocks

Generally there are three ways to handle Deadlocks:

1. Deadlock prevention or avoidance - Do not allow the system to get into a deadlocked state.
2. Deadlock detection and recovery - Abort a process or preempt some resources when deadlocks are detected.
3. Ignore the problem all together - This is the approach that both Windows and UNIX take. Pretends that deadlocks never occur. 

If deadlocks are neither prevented nor detected, then when a deadlock occurs the system will gradually slow down, as more and more processes become stuck waiting for resources currently held by the deadlock and by other waiting processes.