Chapter 8: Deadlocks
Operating System Concepts – 9th Edition Silberschatz, Galvin and Gagne ©2013
Chapter 8: Deadlocks
■ System Model
■ Deadlock Characterization
■ Methods for Handling Deadlocks
■ Deadlock Prevention
■ Deadlock Avoidance
■ Deadlock Detection
■ Recovery from Deadlock
Operating System Concepts – 9th Edition 7.2 Silberschatz, Galvin and Gagne ©2013
Chapter Objectives
■ To develop a description of deadlocks, which prevent sets of concurrent processes from completing their tasks
■ To present a number of different methods for preventing or avoiding deadlocks in a computer system
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System Model
■ System consists of resources
■ 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
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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 for a resource that is held by P0.
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Deadlock with Mutex Locks
■ Deadlocks can occur via system calls, locking, etc.
■ See example box in text page 318 for mutex deadlock
Operating System Concepts – 9th Edition 7.6 Silberschatz, Galvin and Gagne ©2013
Resource-Allocation Graph
A set of vertices V and a set of edges E.
■ V is partitioned into two types:
● P={P1,P2,…,Pn},thesetconsistingofalltheprocesses in the system
● R={R1,R2,…,Rm},thesetconsistingofallresource types in the system
■ request edge – directed edge Pi → Rj
■ assignment edge – directed edge Rj → Pi
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Resource-Allocation Graph (Cont.)
■ Process
■ Resource Type with 4 instances
■ Pi requests instance of Rj
Pi
■ Pi is holding an instance of Rj Pi
Rj
Rj
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Example of a Resource Allocation Graph
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Resource Allocation Graph With A Deadlock
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Graph With A Cycle But No Deadlock
Operating System Concepts – 9th Edition 7.11 Silberschatz, Galvin and Gagne ©2013
Basic Facts
■ If graph contains no cycles ⇒ no deadlock
■ If graph contains a cycle ⇒
● ifonlyoneinstanceperresourcetype,thendeadlock ● ifseveralinstancesperresourcetype,possibilityof
deadlock
Operating System Concepts – 9th Edition 7.12 Silberschatz, Galvin and Gagne ©2013
Methods for Handling Deadlocks
■ Ensure that the system will never enter a deadlock state:
● Deadlockprevention
● Deadlockavoidence
■ 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
● System hangs and you reboot.
! And lose the last 3 hours of work
Operating System Concepts – 9th Edition 7.13 Silberschatz, Galvin and Gagne ©2013
Deadlock Prevention
Restrain the ways request can be made
■ Mutual Exclusion – not required for sharable resources (e.g., read-only files); must hold for non-sharable resources
■ Hold and Wait – must guarantee that whenever a process requests a resource, it does not hold any other resources
● Requireprocesstorequestandbeallocatedallits resources before it begins execution, or allow process to request resources only when the process has none allocated to it.
● Lowresourceutilization;starvationpossible
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Deadlock Prevention (Cont.)
■ No Preemption –
● Ifaprocessthatisholdingsomeresourcesrequests another resource that cannot be immediately allocated to it, then all resources currently being held are released
● Preemptedresourcesareaddedtothelistofresources for which the process is waiting
● Processwillberestartedonlywhenitcanregainitsold 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
Operating System Concepts – 9th Edition 7.15 Silberschatz, Galvin and Gagne ©2013
Deadlock Example
/* thread one runs in this function */
void *do_work_one(void *param) {
pthread_mutex_lock(&first_mutex);
pthread_mutex_lock(&second_mutex);
/** * Do some work */
pthread_mutex_unlock(&second_mutex);
pthread_mutex_unlock(&first_mutex);
pthread_exit(0);
}
/* thread two runs in this function */
void *do_work_two(void *param) {
pthread_mutex_lock(&second_mutex);
pthread_mutex_lock(&first_mutex);
/** * Do some work */
pthread_mutex_unlock(&first_mutex);
pthread_mutex_unlock(&second_mutex);
pthread_exit(0);
}
Operating System Concepts – 9th Edition 7.16 Silberschatz, Galvin and Gagne ©2013
Deadlock Example with Lock Ordering
void transaction(Account from, Account to, double amount)
{
mutex lock1, lock2;
lock1 = get_lock(from);
lock2 = get_lock(to);
acquire(lock1);
acquire(lock2);
withdraw(from, amount);
deposit(to, amount);
release(lock2);
release(lock1);
}
Transactions 1 and 2 execute concurrently. Transaction 1 transfers $25 from account A to account B, and Transaction 2 transfers $50 from account B to account A
Operating System Concepts – 9th Edition 7.17 Silberschatz, Galvin and Gagne ©2013
Deadlock Avoidance
Requires that the system has some additional a priori information
available
■ Simplest and most useful model requires that each process declare the maximum number of resources of each type that it may need
■ The deadlock-avoidance algorithm dynamically examines the resource-allocation state to ensure that there can never be a circular-wait condition
■ Resource-allocation state is defined by the number of available and allocated resources, and the maximum demands of the processes
Operating System Concepts – 9th Edition 7.18 Silberschatz, Galvin and Gagne ©2013
Safe State
■ 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
Operating System Concepts – 9th Edition 7.32 Silberschatz, Galvin and Gagne ©2013
Example: P1 Request (1,0,2)
■ Check that Request ≤ Available (that is, (1,0,2) ≤ (3,3,2) ⇒ true Allocation Need Available
ABC ABC ABC P0 010 743 230 P1 302 020
P2 302 600
P3 211 011
P4 002 431
■ Executing safety algorithm shows that sequence < P1, P3, P4, P0, P2> satisfies safety requirement
■ Can request for (3,3,0) by P4 be granted?
■ Can request for (0,2,0) by P0 be granted?
Operating System Concepts – 9th Edition 7.33 Silberschatz, Galvin and Gagne ©2013
Deadlock Detection
■ Allow system to enter deadlock state
■ Detection algorithm
■ Recovery scheme
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Single Instance of Each Resource Type
■ Maintain wait-for graph
● Nodesareprocesses
● Pi →Pj ifPi iswaitingforPj
■ 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 an order of n2 operations, where n is the number of vertices in the graph
Operating System Concepts – 9th Edition 7.35 Silberschatz, Galvin and Gagne ©2013
Resource-Allocation Graph and Wait-for Graph
Resource-Allocation Graph Corresponding wait-for graph
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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 [i][j] = k, then process Pi is requesting k more instances of resource type Rj.
Operating System Concepts – 9th Edition 7.37 Silberschatz, Galvin and Gagne ©2013
Detection Algorithm
1. LetWorkandFinishbevectorsoflengthmandn,respectively Initialize:
(a) Work = Available
(b) For i = 1,2, …, n, if Allocationi ≠ 0, then
Finish[i] = false; otherwise, Finish[i] = true
2. Findanindexisuchthatboth: (a) Finish[i] == false
(b) Requesti ≤ Work
If no such i exists, go to step 4
Operating System Concepts – 9th Edition 7.38 Silberschatz, Galvin and Gagne ©2013
Detection Algorithm (Cont.)
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
Algorithm requires an order of O(m x n2) operations to detect whether the system is in deadlocked state
Operating System Concepts – 9th Edition 7.39 Silberschatz, Galvin and Gagne ©2013
Example of Detection Algorithm
■ Five processes P0 through P4; three resource types
A (7 instances), B (2 instances), and C (6 instances)
■ Snapshot at time T0:
Allocation Request
Available ABC ABC ABC
P0 010 000 000 P1 200 202
P2 303 000
P3 211 100
P4 002 002
■ Sequence
Operating System Concepts – 9th Edition 7.40 Silberschatz, Galvin and Gagne ©2013
Example (Cont.)
■ P2 requests an additional instance of type C Request
ABC P0 000 P1 202 P2 001
P3 100 P4 002
■ State of system?
● CanreclaimresourcesheldbyprocessP0,butinsufficient
resources to fulfill other processes; requests
● Deadlock exists, consisting of processes P1, P2, P3, and P4
Operating System Concepts – 9th Edition 7.41 Silberschatz, Galvin and Gagne ©2013
Detection-Algorithm Usage
■ When, and how often, to invoke depends on:
● Howoftenadeadlockislikelytooccur?
● Howmanyprocesseswillneedtoberolledback?
! 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.
Operating System Concepts – 9th Edition 7.42 Silberschatz, Galvin and Gagne ©2013
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?
1. Priority of the process
2. How long process has computed, and how much longer to completion
3. Resources the process has used
4. Resources process needs to complete
5. How many processes will need to be terminated
6. Is process interactive or batch?
Operating System Concepts – 9th Edition 7.43 Silberschatz, Galvin and Gagne ©2013
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
Operating System Concepts – 9th Edition 7.44 Silberschatz, Galvin and Gagne ©2013
End of Chapter 8
Operating System Concepts – 9th Edition Silberschatz, Galvin and Gagne ©2013