[IOS] Ch 7 Deadlocks

Ch 7 Deadlocks

• The Deadlock Problem
• System Model
• Deadlock Characterization
• Methods for Handling Deadlocks
• Deadlock Prevention
• Deadlock Avoidance
• Deadlock Detection
• Recovery from Deadlock
• 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.
  

Deadlock problem:

• A set of blocked processes each holding a resource and waiting to acquire a resource held by another process in the set
• Example:
  
  • System has 2 disk drives
  • P1 and P2 each hold one disk drive and each needs another one.
  • semaphores A and B, initialized to 1

    P0          P1
  wait(A);    wait(B);
  wait(B);    wait(A);

Bridge Crossing Example:

• Traffic only in one direction
• Each section of a bridge can be viewed as a resource
• If a deadlock occurs, it can be resolved if one car backs up (preempt resources and rollback)
• Several cars may have to be backed up if a deadlock occurs
• Starvation is possible
• Note – Most OSes do not prevent or deal with deadlocks

Ch 7.1 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: The process requests the resource. If the request cannot be granted immediately (for example, if the resource is being used by another process), then the requesting process must wait until it can acquire the resource.
  • Use: The process can operate on the resource (for example, if the resource is a printer, the process can print on the printer).
  • Release: The process releases the resource.

Ch 7.2 Deadlock Characterization

7.2.1 Necessary Conditions

Deadlock can arise if four conditions hold simultaneously:

• Mutual exclusion: only one process at a time can use a resource
  Mutex deadlock example:

    /* Create and initialize the mutex locks */
    pthread_mutex_t first_mutex;
    pthread_mutex_t second_mutex;

    pthread_mutex_init(&first_mutex, null);
    pthread_mutex_init(&second_mutex, null);

    /* 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);
    }

• 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

7.2.2 Resource-Allocation Graph

A set of vertices V and a set of edges E.

• V is partitioned into two types:
  
  • 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

Example of a Resource Allocation Graph:



Resource Allocation Graph With A Deadlock:



Graph With A Cycle But No Deadlock:


Basic Facts:

• If graph contains no cycles –> no deadlock
• If graph contains a cycle –>
  
  • if only one instance per resource type, then deadlock
  • if several instances per resource type, possibility of deadlock

Ch 7.3 Methods for Handling Deadlocks

• Ensure that the system will never enter a deadlock state
• 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

Ch 7.4 Deadlock Prevention

Restrain the ways request can be made:

• Mutual Exclusion – not required for sharable resources; must hold for nonsharable resources
• Hold and Wait – must guarantee that whenever a process requests a resource, it does not hold any other resources.
  
  • Require process to request and be allocated all its resources before it begins execution, or allow process to request resources only when the process has none
  • Low resource utilization; starvation possible
• 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.

Ch 7.5 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.

7.5.1 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 of ALL the processes in the systems such that for each P i , the resources that Pi can still request can be satisfied by currently available resources and resources held by all the Pj , with j
• 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
• 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.

7.5.2 Resource-Allocation Graph Scheme

• Single instance of a resource type.
• Claim edge Pi - - -> Rj indicated that process Pi may request resource Rj; (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.
• Example for safe state:

Since it won’t be a deadlock because P2 doesn’t hold the R2.

• Example for unsafe state in resource-allocation graph:
  

P2 hold the R2, when P1 request R2, deadlock may occur.

• 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

7.5.3 Banker’s Algorithm

• Multiple instances
• Each process must a priori claim maximum use
• When a process requests a resource it may have to wait
• When a process gets all its resources it must return them in a finite amount of time

Data Structures for the Banker’s Algorithm:
Let n = number of processes, and m = number of resources types.

• Available: Vector of length m.
  
  • If available[j] = k, there are k instances of resource type Rj available
• Max: n x m matrix.
  
  • If Max [i,j] = k, then process Pi may request at most k instances of resource type Rj
• Allocation: n x m matrix.
  
  • If Allocation[i,j] = k, then Pi is currently allocated k instances of Rj
• Need: n x m matrix.
  
  • If Need[i,j] = k, then Pi may need k more instances of Rj to complete its task
  • Need [i,j] = Max[i,j] – Allocation[i,j]

7.5.3.1 Safety Algorithm

Step 1: Let Work and Finish be vectors of length m and n, respectively.

• Initialize:

    Work = Available // Available[j] = k, there are k instances of Rj available 
    Finish[i] = false for i = 0, 1, ..., n- 1

Step 2: Find an i such that both:

    Finish[i] = false
    Need[i][j] <= Work[j] for j = 0, 1, ..., m-1 //m: numbers of resource type

• If no such i exists, go to step 4

Step 3:

    Work[j] = Work[j] + Allocation[i,j] for j = 0, 1, ..., m-1
    Finish[i] = true

• go to step 2

Step 4:

 if (Finish[i] == true) for all i
    // then the system is in a safe state

7.5.3.2 Resource-Request Algorithm

Request = request vector for process Pi
If Request[i][j] = k then process Pi wants k instances of resource type Rj.

Step 1.

• If Request[i][j] <=Need[i][j] for j = 0, 1, …, m-1, go to step 2.
• Otherwise, raise error condition, since process has exceeded its maximum claim

Step 2.

• If Request[i][j] <= Available[j] for j = 0, 1, …, m-1, go to step 3.
• Otherwise Pi must wait, since resources are not available

Step 3.

• Pretend to allocate requested resources to Pi by modifying the state as follows:

    Available[j] = Available[j] – Request[i][j] for j = 0, 1, ..., m-1;
    Allocation[i][j] = Allocation[i][j] + Request[i][j] for j = 0, 1, ..., m-1;
    Need[i][j] = Need[i][j] – Request[i][j] for j = 0, 1, ..., m-1;

• If safe, the resources are allocated to Pi
• If unsafe, Pi must wait, and the old resource-allocation state is restored.

7.5.3.3 An Illustrative Example

• 5 processes: P0 through P4;
• 3 resource types:
  
  • A (10 instances)
  • B (5instances)
  • C (7 instances)
• Snapshot at time T0:
  Need = Maz - Allocation

	Allocation	Max	Avaliable	Need
	ABC	ABC	ABC	ABC
P0	010	753	332	743
P1	200	322		122
P2	302	902		600
P3	211	222		011
P4	002	433		431

• The system is in a safe state since the sequence < P1, P3, P4, P2, P0 > satisfies safety criteria. (Since Need will always <= Avaliable in this sequence)
• If P1 request (1, 0, 2):
  
  • Check that Request <= Available (since (1,0,2) <= (3,3,2) it’s true)

	Allocation	Max	Avaliable	Need
	ABC	ABC	ABC	ABC
P0	010	753	230	743
P1	302	322		020
P2	302	902		600
P3	211	222		011
P4	002	433		431

• Executing safety algorithm shows that sequence < P1, P3, P4, P0, P2 > satisfies safety requirement

Ch7.6 Deadlock Detection

• Allow system to enter deadlock state
• Detection algorithm
• Recovery scheme

7.6.1 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 an order of n² operations, where n is the number of vertices in the graph.

7.6.2 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.

7.6.3 Detection-Algorithm Usage

Step 1.

• Let Work and Finish be vectors of length m and n, respectively Initialize:

    Work = Available;

    for (i = 1,2, ..., n)
        if (Allocation[i] != 0)
            Finish[i] = false;
        else 
            Finish[i] = true;

Step 2. Find an index i such that both:

    Finish[i] == false
    Request[i] <= Work

• If no such i exists, go to step 4

Step 3.

    Work = Work + Allocation[i];
    Finish[i] = true;

• go to step 2

Step 4.

    if Finish[i] == false, for some i, 1 <= i <= n
        // then the system is in deadlock state. 

    if Finish[i] == false
        // then Pi is deadlocked

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.

Ch 7.7 Recovery from Deadlock

7.7.1 Process Termination

• Abort all deadlocked processes
  
  • Great expense.
• Abort one process at a time until the deadlock cycle is eliminated
  
  • Incurs considerable overhead. (deadlock-detection algorithm)
• 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?

7.7.2 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

Reference: Operating System Concepts 8th, by Silberschatz, Galvin, Gagne