CSS430_Deadlock.pptx

CSS430
Deadlocks
Textbook Chapter 7
Instructor: Stephen G. Dame
e-‐mail: sdame@uw.edu
V0.2 CSS430 Deadlocks 1
These slides were adapted from the OSC textbook slides (Silberschatz, Galvin, and Gagne),
Professor Munehiro Fukuda and the instructor’s class materials.

WKP 17
“The competent programmer is fully
aware of the strictly limited size of his
own skull; therefore he approaches the
programming task in full humility, and
among other things he avoids clever
tricks like the plague.”
-
‐
-
Edsger Dijkstra

Deadlock Examples 1
Kansas Legislature: “when two trains approach each other
at a crossing, both shall come to a full stop and neither
shall start up again until the other has gone.”
Two processes exchange a long message with each other, but their socket buffer
is smaller than the message.
Process
B
Socket
buﬀer
message message
Process
A
Socket
buﬀer

Deadlock Examples 2
Bridge crossing example: traffic only in one direction
where two cars are driving from the opposite direction.
A deadlock is not resolved unless one gets back up.
Two processes try to go into the same nested critical section in a different order.
P0
wait (A);
wait (B);
P1
wait(B)
wait(A)

System Model
 Resource types R1, R2, . . ., Rm
CPU cycles, memory space, I/O devices
 Each resource type Ri has Wi instances
 Each process utilizes a resource as follows:
• request
• use
• release

Deadlock Characterization
Deadlock can arise if four conditions hold simultaneously:
① Mutual exclusion: only one process at a time can use a
resource.
② Hold and wait: a process holding resource(s) is waiting to
acquire additional resources held by other processes.
③ No preemption: a resource can be released only voluntarily
by the process holding it upon its task completion.
④ Circular wait: there exists a set {P0, P1, …, P0} 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.

Resource-Allocation Graph
 P = {P1, P2, …, Pn}, the set consisting of all the
processes in the system
 R = {R1, R2, …, Rm}, the set consisting of all resource
types in the system
 request edge – directed edge Pi  Rj
 assignment edge – directed edge Rj  Pi
A set of vertices V and a set of edges E
 V is partitioned into two types:
Pi
Rj

 Process
 Resource Type with 4 instances
 Pi requests instance of Rj
 Pi is holding an instance of Rj
 Pi releases an instance of Rj
Rj
Sequence of
process resource
utilization
Request edge
Assignment edge
Pi
Pi
Pi
Pi

P1 P2 P3
R1 R3
R2
R4
Is deadlock possible?
YES!
CYCLE 1

P1 P2
R1 R3
R2
R4
YES!
0
P3
CYCLE 2

Resource Allocation Graph With A
Cycle But No Deadlock
P1
P2
P3
 If graph contains no
cycles  no deadlock.
1
 If graph contains a cycle

• if only one instance per
resource type, then
deadlock.
• if several instances per
resource type, possibility
of deadlock.
P3

Methods for Handling Deadlocks
Ensure that the system will never enter a
deadlock state. (Prevention and Avoidance)
Allow the system to enter a deadlock state
and then recover. (Detection and Recovery)
Ignore the problem and pretend that
deadlocks never occur in the system; used by
most operating systems, including UNIX.
2

Deadlock Prevention
Restrain one of the following four conditions:
it does not hold any other resources.
3
execution: Low resource utilization
① Mutual Exclusion – not required for sharable resources. (but not work always.)
② Hold and Wait – must guarantee that whenever a process requests a resource,
• Require a process to request and be allocated all its resources before its
• Allow process to request resources only when the process has none:
starvation possible.
③ No Preemption –
• If a process holding some resources requests another resource that cannot
be immediately allocated to it, all resources currently being held are released.
• If a process P1 requests a resource R1 that is allocated to some other
process P2 waiting for additional resource R2, R1 is allocated to P1.
④ Circular Wait – impose a total ordering of all resource types, and require that
each process requests resources in an increasing order of enumeration.

Deadlock Prevention
Circular Wait
P1 P2 P3
P4
tape disk printer
1 2 scanner 3 4
Each process can request resources only in an
increasing order of enumeration.
4
Not allowed Order(tape)=1 < Order(printer)=4

 When a process requests an available resource, system must
decide if immediate allocation leaves the system in a safe state.
 System is in safe state if there exists a sequence
<P1, P2, …, Pn>
of ALL the processes in the systems such that for each Pi, the
resources that Pi can still request can be satisfied by currently
available resources + resources held by all the Pj, with j < i
 That is:
 If Pi resource needs are not immediately available, then Pi can wait
until all Pj have finished.
 When Pj is finished, Pi can obtain needed resources, execute,
return allocated resources, and terminate.
 When Pi terminates, Pi +1 can obtain its needed resources, and so
on.
Safe State
5
A system is in a safe state only if there
exists a only if there exists a safe sequence.

 If a system is in safe state  no deadlocks
 If a system is in unsafe state  possibility of deadlock
 Avoidance  ensure that a system will never enter an
unsafe state.
Basic Facts
6

Safe, Unsafe , Deadlock State
7

 Single instance of a resource type
• Use a resource-allocation graph
 Multiple instances of a resource type
• Use the banker’s algorithm
Avoidance algorithms
8

Claim edge Pi  Rj indicated that process Pj may request
resource Rj; represented by a dashed line
 Claim edge converts to request edge when a process
requests a resource
 Request edge converted to an assignment edge when the
resource is allocated to the process
 When a resource is released by a process, assignment edge
reconverts to a claim edge
 Resources must be claimed a priori in the system
Resource-Allocation Graph Scheme
9

P1
R1
P2
R2
0

Unsafe State In Resource-
Allocation Graph
P1
R1
P2
R2
1

 Suppose that process Pi requests a resource Rj
 The request can be granted only if converting the request
edge to an assignment edge does not result in the formation
of a cycle in the resource allocation graph
Algorithm
2

V0.2 CSS430 Deadlocks
CSS430
Deadlocks
23
Deadlock Avoidance
Cycle possibly formed (unsafe state),
thus P2 has to wait for a safe state
Resource-Allocation Algorithm
Processes supply OS with
future resource requests
Claim edge (future request)
Works only with single
instance resource types.

wait
 Multiple resource instances
 Each process must a priori claim maximum use
 When a process requests a resource it may have to
 When a process gets all its resources it must return
them in a ﬁnite amount of time
Deadlock Avoidance
Banker’s Algorithm - Definitions
24

Deadlock Avoidance
25
Banker’s Algorithm - Definitions
Let n = number of processes, and m = number of resources types.
 Available – Vector of length m indicates the number of available
resources of each type. If Available[ j] = k, then k instances of
resource type Rj are available.
 Max : n × m matrix. Deﬁnes the maximum demand of each process.
If Max[i,j] = k, then process Pi may request at most k instances of
resource type Rj.
 Allocation : n × m matrix. Deﬁnes the number of resources of each
type currently allocated to each process. If Allocation[i,j] = k, then
process Pi is currently allocated k instances of resource type Rj.
 Need : n × m matrix. Indicates the remaining resource need of each
process. If Need[i,j] = k, then process Pi may need k more instances of
resource type Rj to complete its task. Note that
Need[i,j] = Max[i,j] − Allocation[i,j]

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 + Allocation
Finish[i] = true
go to step 2
4. If Finish [i] == true for all i, then the system is in a safe state
26

Request = request vector for process Pi. If Requesti [j] = k then process Pi
wants k instances of resource type Rj
1. If Requesti  Needi go to step 2. Otherwise, raise error condition, since
process has exceeded its maximum claim
2. If Requesti  Available, go to step 3. Otherwise Pi must wait, since
resources are not available
3. Pretend to allocate requested resources to Pi by modifying the state
as follows:
Available = Available – Request;
Allocationi = Allocationi + Requesti;
• Needi = Needi – Requesti
If safe  the resources are allocated to Pi
• If unsafe  Pi must wait, and the old resource-‐allocation state is
restored
Resource-Request Algorithm for Process Pi
27

Deadlock Avoidance
Banker’s Algorithm
 5 processes P0 through P4
 Each process must claim Max use in advance.
 Resource Types: A (10 instances), B (5instances), and C (7 instances)
28
Process
Allocation Max Need Initial Avail
A B C A B C A B C A B C A B C
P0 0 1 0 7 5 3 7 4 3 10 5 7 3 3 2
P1 2 0 0 3 2 2 1 2 2
P2 3 0 2 9 0 2 6 0 0
P3 2 1 1 2 2 2 0 1 1
P4 0 0 2 4 3 3 4 3 1
Snapshot at time T0:

Deadlock Avoidance
Banker’s Algorithm – P1 Request (1 0 2)
 Check that Request  Available (that is, (1 0 2)  (3 3 2)  true
 Execute safety algorithm shows that sequence < P1, P3, P4, P0, P2>
satisfies safety requirement
29
Process
P0 0 1 0 7 5 3 7 4 3 10 5 7 2 3 0
P1 3 0 2 3 2 2 1 2 2
P2 3 0 2 9 0 2 6 0 0
P3 2 1 1 2 2 2 0 1 1
P4 0 0 2 4 3 3 4 3 1

Deadlock Avoidance
 Check that Request  Available (that is, (3 3 0)  (3 3 2)  true
30
Process
P0 0 1 0 7 5 3 7 4 3 10 5 7 0 0 2
P1 2 0 0 3 2 2 1 2 2
P2 3 0 2 9 0 2 6 0 0
P3 2 1 1 2 2 2 0 1 1
P4 3 3 2 4 3 3 4 3 1

Deadlock Avoidance
 Check that Request  Available (that is, (4 3 0)  (3 3 2)  false
31
Process
P0 0 1 0 7 5 3 7 4 3 10 5 7 3 3 2
P1 2 0 0 3 2 2 1 2 2
P2 3 0 2 9 0 2 6 0 0
P3 2 1 1 2 2 2 0 1 1
P4 0 0 2 4 3 3 4 3 1

 Allow system to enter deadlock state
 Detection algorithm
 Recovery scheme
Deadlock Detection
32

Single Instance of Each Resource Type
 Maintain wait-for graph
• Nodes are processes
• Pi  Pj if Pi is waiting for Pj
 Periodically invoke an algorithm that searches for
a cycle in the graph. If there is a cycle, there
exists a deadlock
 An algorithm to detect a cycle in a graph requires
33
an order of n2 operations, where n is the number
of vertices in the graph

Resource-Allocation Graph and
Wait-for Graph
34
Corresponding wait-for graph

Several Instances of a
Resource Type
Available: A vector of length m indicates the number of
available resources of each type.
Allocation: An n x m matrix defines the number of
resources of each type currently allocated to each process.
Request: An n x m matrix indicates the current request of
each process. If Request [ij] = k, then process Pi is
requesting k more instances of resource type. Rj.
35

Detection Algorithm
1. Let Work and Finish be vectors of length m and n,
respectively Initialize:
(a) Work = Available
(b) For i = 1,2, …, n, if Allocationi  0, then
Finish[i] = false;otherwise, Finish[i] = true
2. Find an index i such that both:
(a) Finish[i] == false
(b) Requesti  Work
If no such i exists, go to step 4
3. Work = Work + Allocationi
Finish[i] = true
go to step 2
4. If Finish[i] == false, for some i, 1  i  n, then the system is in
deadlock state. Moreover, if Finish[i] == false, then Pi is
deadlocked
This algorithm requires an order of O(m x n2) operations to detect
whether the system is in deadlocked state.

Detection Algorithm Example
 Five processes P0 through P4;
 Three resources types A (7 instances), B (2 instances), and C (6
instances)
 Sequence <P0, P2, P3, P1, P4> will result in Finish[ i ] = true for all i
37
Process
Allocation Request Available
A B C A B C A B C
P0 0 1 0 0 0 0 0 0 0
P1 2 0 0 2 0 2
P2 3 0 3 0 0 0
P3 2 1 1 1 0 0
P4 0 0 2 0 0 2
Snapshot at time T0

Example (Cont.)
 P2 requests an additional instance of type C
38
 State of system?
 Can reclaim resources held by process P0, but insufficient
resources to fulfill other processes; requests
 Deadlock exists, consisting of processes P1, P2, P3, and P4
Process Request
A B C
P0 0 0 0
P1 2 0 1
P2 0 0 1
P3 1 0 0
P4 0 0 2

Detection-Algorithm Usage
 When, and how often, to invoke depends on:
 How often a deadlock is likely to occur?
 How many processes will need to be rolled back?
 one for each disjoint cycle
If detection algorithm is invoked arbitrarily, there may be
many cycles in the resource graph and so we would not be
able to tell which of the many deadlocked processes
“caused” the deadlock.

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
 Starvation – same process may always be picked as
victim, include number of rollback in cost factor

Exercises (No turn-in)
1. Why aren’t deadlock detection and recovery so
attractive?
2. Solve Exercise 7.3, 7.6, 7.9, 7.10, 7.14, and 7.19
3. Can the Java code in the next slide cause a
deadlock? If so, write a resource allocation graph
with a deadlock.
42

CSS430 Deadlocks
public class Deadlock {
public Deadlock( ) {
Mutex mutex[] = new Mutex[4];
for ( int i = 0; i < 4; i++ )
mutex[i] = new Mutex( );
A threadA = new A( mutex );
B threadB = new B( mutex );
C threadC = new C( mutex );
threadA.start( );
threadB.start( );
threadC.start( );
}
public static void main( String arg[] ) {
Deadlock d = new Deadlock( );
}
class Mutex{ }
private class A extends Thread
{
private Mutex[] resource;
public A( Mutex[] m ) {
resource = m;
}
public void run( ) {
System.out.println( "A started" );
synchronized ( resource[1] ) {
System.out.println( "A got rsc 1" );
System.out.println( "A got rsc 0" );
}
}
System.out.println( "A finished" );
}
}
private class B extends Thread
{
public B( Mutex[] m ) {
resource = m;
}
System.out.println( "B started" );
System.out.println( "B got rsc 3" );
}
}
}
System.out.println( "B finished" );
}
}
43
private class C extends Thread
{
public C( Mutex[] m ) {
resource = m;
}
System.out.println( "C started" );
System.out.println( "C got rsc 2" );
System.out.println( "C got rsc 1" );
}
}
System.out.println( "C finished" );
}
}
}

CSS430_Deadlock.pptx

More Related Content

Similar to CSS430_Deadlock.pptx (20)

More from amadayshwan (10)

Recently uploaded (20)

CSS430_Deadlock.pptx