Deadlock

NAME:- SHIVANGI
DEPT:- COMPUTER SCIENCE & ENGINEERING
ROLL NO:- ‘9’
University roll:-’10500112023’
GROUP:- ”X”
TOPIC NAME:- “Deadlocks”
SUBMITTED TO:- tapas sangiri

The Deadlock Problem
System Model
Deadlock Characterization
Methods for Handling Deadlocks
Deadlock Prevention
Deadlock Avoidance
Recovery from Deadlock
CONTENTS

The Deadlock Problem
EXAMPLES:
 "It takes money to make money".
 You can't get a job without experience; you can't get
experience without a job.
BACKGROUND:
The cause of deadlocks: Each process needing what another
process has. This results from sharing resources such as
memory devices, links.
Under normal operation, a resource allocations proceed like
this:
 Request a resource (suspend until available if necessary.
 Use the resource.

Deadlock Characterization
Deadlock can arise if four conditions hold simultaneously.
 Mutual exclusion: only one process at a time can use a
resource
 Hold and wait: a process holding at least one resource is
waiting to acquire additional resources held by other
processes
 No preemption: 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

A visual ( mathematical ) way to determine if a deadlock has, or may occur.
G = ( V, E ) The graph contains nodes and edges
V Nodes consist of processes = { P1, P2, P3, ...} and
resource types
{ R1, R2, ...}
E Edges are ( Pi, Rj ) or ( Ri, Pj )
An arrow from the process to resource indicates the process is
requesting the resource. An arrow from resource to process shows an
instance of the resource has been allocated to the process.
Process is a circle, resource type is square; dots represent number of
instances of resource in type. Request points to square, assignment
comes from dot.
RESOURCE ALLOCATION
GRAPH
Pi
Rj
Pi
Rj
Pi

Example of a
Resource
Allocation Graph
Resource
Allocation
Graph With A
Deadlock
Graph With A
Cycle But No
Deadlock
RESOURCE ALLOCATION GRAP

Methods for Handling
Deadlocks
 Ensure that the system will never enter a
deadlock state – deadlock prevention
 Allow the system to enter a deadlock state
and then recover
 Ignore the problem and pretend that
deadlocks never occur in the system; used
by most operating systems, including UNIX

Do not allow one of the four conditions to occur.
Mutual exclusion:
a) Automatically holds for printers and other non-
sharables.
b) Shared entities (read only files) don't need mutual
exclusion (and aren’t susceptible to deadlock.)
c) Prevention not possible, since some devices are
intrinsically non-sharable.
Hold and wait:
a) Collect all resources before execution.
b) A particular resource can only be requested when no
others are being held. A sequence of resources is
always collected beginning with the same one.
c) Utilization is low, starvation possible.
Deadlock
Prevention

Deadlock Prevention (Cont.)
 No Preemption –
 If a process that is holding some resources
requests another resource that cannot be
immediately allocated to it, then all resources
currently being held are released
 Preempted resources are added to the list of
resources for which the process is waiting
 Process will be restarted only when it can regain
its old resources, as well as the new ones that it is
requesting
 Circular Wait – impose a total ordering of all
resource types, and require that each process
requests resources in an increasing order of
enumeration

NOTE: All deadlocks are unsafe, but all unsafes
are NOT deadlocks.
SAFE
DEADLOCK
UNSAFE
Only with luck will
processes avoid
deadlock.
O.S. can avoid
deadlock.
Deadlock
Avoidance

 Single instance of a resource type
Use a resource-allocation graph
 Multiple instances of a resource
type
 Use the banker’s algorithm
Avoidance
algorithms

Safety Algorithm
1. Let Work and Finish be vectors of length m and n,
respectively. Initialize:
Work = Available
Finish [i] = false for i = 0, 1, …, n- 1
2. Find an i such that both:
(a) Finish [i] = false
(b) Needi  Work
If no such i exists, go to step 4
3. Work = Work + Allocationi
Finish[i] = true
go to step 2

Example of Banker’s Algorithm
 5 processes P0 through P4;
3 resource types:
A (10 instances), B (5instances), and C (7
instances)
Snapshot at time T0:
Allocation Max Available
A B C A B C A B C
P0 0 1 0 7 5 3 3 3 2
P1 2 0 0 3 2 2
P2 3 0 2 9 0 2
P3 2 1 1 2 2 2

Example (Cont.)
 The content of the matrix Need is defined to be Max
– Allocation
Need
A B C
P0 7 4 3
P1 1 2 2
P2 6 0 0
P3 0 1 1
P4 4 3 1
 The system is in a safe state since the sequence <
P1, P3, P4, P2, P0> satisfies safety criteria

Recovery from Deadlock:
Process Termination
 Abort all deadlocked processes
 Abort one process at a time until the deadlock cycle is
eliminated
 In which order should we choose to abort?
 Priority of the process
 How long process has computed, and how much
longer to completion
 Resources the process has used
 Resources process needs to complete
 How many processes will need to be terminated
 Is process interactive or batch?

Recovery from Deadlock:
Resource Preemption
 Selecting a victim – minimize cost
 Rollback – return to some safe state,
restart process for that state
 Problem: starvation – same process may
always be picked as victim, include number
of rollback in cost factor

Deadlock

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