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08 Deadlock

Types of Resources

Preemptable Resource Non-Preemptable Resource
Can be removed from process without causing computation fail Once allotted to a process, it cannot be removed from a process unless the process relinquishes the resource by itself
Example Primary Memory can be swapped out Printers

Necessary conditions for Deadlocks

Condition Meaning
Mutual-exclusion Only one process can use a resource at a time
Hold & Wait A process holding at least one resource is waiting to acquire additional resources held by other processes (hence causing a wait)
Non-Preemptive resources A resource can be released only voluntarily by the process holding it, after that process has completed its task.
Circular Wait There exists a set {P0, P1, …, Pn} of waiting processes such that P0 is waiting for a resource that is held by P1, P1 is waiting for a resource that is held by P2, …, Pn–1 is waiting for a resource that is held by Pn, and Pn is waiting for a resource that is held by P0.

RAG

Resource Allocation Graph

Directed graph that how processes and resources are related

Symbol Meaning
Circle Process
Rectangle with circles Resource Type with instances
Edges Request Edge (process \(P_i\) requests resource \(R_j\)) \(P_i \to R_j\)
Assignment Edge (resource \(R_j\) is alloted to \(P_i\)) \(R_j \to P_i\)

Checking deadlock

Cycle exists? Instances of each resource type \(\implies\) Deadlock exists?
❌ N/A ❌
âś… 1 âś…
âś… Multiple Inconclusive

Disadvantage

Inconclusive for the 3rd case

Banker’s Algo for Deadlock Detection

It is an algorithm which is implemented as a system process in the OS

In this section, processes are referring to user processes

Let

  • \(n =\) No of processes
  • \(m =\) No of resources
Dimension Meaning Initial value
Max
Matrix		 \(n \times m\) Maximum resource requirement of each type by every process
Allocation
Matrix		 \(n \times m\) Total number of resources of each type that is currently allocated to each process
Need
Matrix		 \(n \times m\) Denotes the number of resources of each resource type that are yet to be allocated to a process
Available
Vector		 \(m\) Total no of instances of each resource type that is available
Work
Vector		 \(m\) No of instances of each resource type that is currently available Work = Available
Finish
Vector		 \(n\) Denotes the completion status of each process False
Request
Vector for process \(i\)		 \(m\) No of instances of each type of resource that process \(i\) requests
\[ \text{Need}[i][j] = \text{Max}[i][j] - \text{Allocation}[i][j] \\ i \in [1, n],\\ j \in [1, m] \]

Vector Comparison

Let \(X, Y\) be 2 vectors

\[ X \le Y \iff X[i] \le Y[i], \\ \forall i \in \text{len}(X) = \text{len}(Y) \\ (0, 0, 0) \le (0, 0, 1) \\(0, 1, 0) \not \le (0, 0, 1) \]

Every element of \(X\) should be smaller than/equal to every corresponding element of \(Y\)

Algorithm

  1. Initialize all vectors

  2. Find process \(i\) with

  3. Finish[i] = False

  4. Need[i] \(\le\) Work

If we can’t find, go to step 4

    • Work = Work + Allocation[i]
    • Finish[i] = True
    • Go to step 2

  2. If finish[i]==True \(\forall i\), the system is in a safe state

Safe Sequence

You may get multiple valid safe sequences for the same list of process

Resource-Request Algorithm

Consider a Request[i] vector for process \(P_i\)

  1. Check if Request[i] \(\le\) Need[i]

    • if true, got to step 2
    • else, raise error condition that \(P_i\) has requested more than it needs

  2. Check if Request[i] \(\le\) Available[i]

  3. if true, go to step 3

  4. else, \(P_i\) must wait, as resources are not available

  5. Pretend to allocate requested resources to \(P_i\), by modifying the states as follows

Available -= Request[i]
Allocation[i] += Request[i]
Need[i] -= Request[i]

  1. Run the safety algo to check if the system is in a safe state

    • If safe, resources are allocated to \(P_i\)
    • else, \(P_i\) must wait and the old resource-allocated state is restored

Deadlock Handling
Deadlock Avoidance
Deadlock Prevention
Deadlock Detection & Recovery

Deadlock Detection

Almost exactly the same as Banker’s; just that this has request instead of need.

  1. Initialization

  2. Let work = available

  3. Check if \(\text{allocation}[i] \ne 0\)

    1. true \(\implies\) set finish[i] = false
    2. false \(\implies\) set finish[i] = true

  4. Find \(i\) such that

  5. Finish[i] = false

  6. request[i] \(\le\) work

If not found, go to step 4

  1. Work = Work + Allocation Finish[i] = true Go to step 2

  2. If finish[i] == false, for some \(i \implies\) system is in deadlock state or, in other words, there is no deadlock if

    • all the finish[i] = false \(\forall i\)
    • we are able to derive a safe sequence with all the processes, then there is no deadlock
Last Updated: 2023-01-25 ; Contributors: AhmedThahir

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