7 Deadlocks

Deadlocks
 System Model
 Deadlock Characterization
 Methods for Handling Deadlocks
Deadlock Prevention
 Deadlock Avoidance
 Deadlock Detection
 Recovery from Deadlock

The 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.
• Example
– semaphores A and B, initialized to 1
P0 P1
wait (A); wait(B)
wait (B); wait(A)

Loganathan R, CSE, HKBKCE 2

1. System Model
• A system consists of a finite number of resources to be distributed among a
number of competing processes
• Resource types R1, R2, . . ., Rm
CPU cycles, memory space, files, I/O devices(Printers,DVDs,
• Each resource type Ri may have Wi instances.
• Each process utilizes a resource in only the following sequence
1. Request : If the request cannot be granted immediately, then the
requesting process must wait until it can acquire the resource
2. Use :The process can operate on the resource
3. Release :The process releases the resource
• The request and release of resources are system calls, for example
– request () and release() device,
– open() and close () file,
– allocate () and free () memory system calls
• Multithreaded programs are good candidates for deadlock because multiple
threads can compete for shared resources


2. Deadlock Characterization
2.1 Necessary Conditions
• 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 at least one resource 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, after that process has completed its
task.
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 P0 is waiting for a resource that is held by P0.

2. Deadlock Characterization Contd…

2.2 Resource-Allocation Graph
• Deadlocks can be described more precisely in terms of a directed graph
called a system resource-allocation graph
• Consists of 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
• Symbols Used in the graph
Pi
Rj
Process Pi requests instance of Rj

Pi
Rj
Resource Type with 4 instances Pi is holding an instance of Rj


Example of a Resource Allocation Graph
• The sets P, R, and £ :
P={P1,P2,P3}
R= {R1,R2,R3,R4}
£ = {P1→R1, P2 →R3, R1 →P2, R2 →P2,R2→P1, R3 →P3}
• Resource instances:
• One instance of resource type R1
• Two instances of resource type R2
• One instance of resource type R3
• Three instances of resource type R4
• Process states:
• Process P1 is holding an instance of resource type
R2 and is waiting for an instance of resource type R1
• Process P2 is holding an instance of R1 and R2 and is waiting for an instance of R3
• Process P3 is holding an instance of R3.
• If the graph contains no cycles, then no process in the system is deadlocked
• If the graph does contain a cycle, then a deadlock may exist
if only one instance per resource type, then deadlock.
if several instances per resource type, possibility of deadlock.


Graph With Deadlock
Cycles: P1 → R1 → P2 → R3 → P3 → R2 → P1 Graph With A Cycle But No Deadlock
P2 → R3 → P3 → R2 → P2
Cycle : P1→R1 →P3 →R2 →P1
Processes P1, P2, and P3 are deadlocked. Process
P2 is waiting for resource R3, which is held by P3.
P4 May Release R2 So No Deadlock
Process P3 is waiting for P1 or P2 to release R2
Process P1 is waiting for process P2 to release
resource R1


3. Methods for Handling Deadlocks
• Three Ways to Deal deadlock problem
1. Use a protocol to prevent or avoid deadlocks, ensuring
that the system will never enter a deadlock state.
– Deadlock prevention provides a set of methods for ensuring
that at least one of the necessary cannot hold
– Deadlock avoidance requires the information of which
resources a process will request and use during its lifetime, the
resources currently available, the resources currently allocated
to each process, the future requests and releases of each
process then it can decide for each request whether or not the
process should wait
2. Allow the system to enter a deadlock state, detect it and
recover.
3. Ignore the problem and pretend that deadlocks never
occur in the system; used by most operating systems,
including UNIX.


4. Deadlock Prevention
Restrain the ways request can be made
• Mutual Exclusion – not required for sharable resources; 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.
– 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.


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.
5.1 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.


5. Deadlock Avoidance Contd…

• 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.
• Example
A system with 12 magnetic tape drives and three processes: P0,P1 and P2
Process Max Needs Current Needs
P0 10 5
P1 4 2
P2 9 2
At time t0 (3 remaining) the system with
sequence <P1, P0, P2> is in a safe state
At time t0,P2 requests 1 more drive and
its allocated ( remaining 2), system can
go from a safe state to an unsafe state
For deadlock avoidance :
1. Single instance of a resource type use a
resource-allocation graph
2. Multiple instances of a resource type.
Use the banker’s algorithm
Safe, unsafe, and deadlock state spaces


5.2 Resource-Allocation Graph Algorithm
• 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.
• The request can be granted only if converting the request edge Pi  Rj to an
assignment edge Rj —> Pi does not result in the formation of a cycle
• Example
An unsafe state in a
resource-allocation
graph

Resource-allocation
graph for deadlock
avoidance


5.3 Banker’s Algorithm
• Multiple instances of a resource
• 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.
• Let n = number of processes, and m = number of resources types.
• Data structures
– 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].


5.3.1 Safety Algorithm
• To find whether or not a system is in a safe state
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 and 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.
4. If Finish [i] == true for all i, then the system is in a safe state.



5.3.2 Resource-Request Algorithm
• Determines if requests can be safely granted
• Let Requesti is request vector for process Pi.
• If Requesti [j] = k then process Pi wants k instances of resource
type Rj then following actions are taken
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


• Example : 5 processes P0 through P4
• 3 resource types: A (10 instances), B (5instances), and C (7 instances).
• Snapshot at time T0:
Allocation Max Available
ABC ABC ABC
P0 010 753 332
P1 200 322
P2 302 902
P3 211 222
P4 002 433
• The content of the matrix Need is defined to be Max – Allocation.
Need
ABC
P0 743
P1 122
P2 600
P3 011
P4 431
• The system is in a safe state since the sequence < P1, P3, P4, P2, P0> satisfies
safety criteria. Loganathan R, CSE, HKBKCE 16


Can request for (1,0,2) by P1 be granted?
• Check that Request  Available i.e. (1,0,2)  (3,3,2)  true.
Allocation Need Available
ABC ABC ABC
P0 010 743 230
P1 302 020
P2 301 600
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?
P4 cannot be granted, since the resources are not available
• Can request for (0,2,0) by P0 be granted?
P0 cannot be granted, since the resulting state is unsafe


6.Deadlock Detection
6.1 Single Instance of Each Resource Type
• All resources have only a single instance
• 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.

Corresponding wait-for graph
Resource-Allocation Graph


6.Deadlock Detection Contd…

6.2 Several Instances of a Resource Type
• Deadlock detection algorithm employs several time-varying data structures
– 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.
• 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, 0  i  n, then the system is in
deadlock state. Moreover, if Finish[i] == false, then Pi is deadlocked.
• Example : 5 processes P0 through P4
– 3 resource types A (7 instances), B (2 instances), and C (6 instances).
– Snapshot at time T0:
Allocation Request Available
ABC ABC ABC
P0 0 1 0 000 000
P1 2 0 0 202
P2 3 0 3 000
P3 2 1 1 100
P4 0 0 2 002
• Sequence <P0, P2, P3, P1, P4> will result in Finish[i] = true for all i.


• P2 requests an additional instance of type C.
Request
ABC
P0 000
P1 201
P2 001
P3 100
P4 002
• 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.
6.3 Detection-Algorithm Usage
• When to invoke depends on:
– How often a deadlock is likely to occur?
– How many processes will need to be rolled back?
• 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.

7. Recovery from Deadlock
There is 2 methods
Process Termination and Resource Preemption
7.1 Process Termination
• Abort all deadlocked processes
– Expensive- Computed for a long time and results of partial computations
• Abort one process at a time until the deadlock cycle is
eliminated
– After each process is aborted, a deadlock-detection algorithm must be
invoked
– 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?

7. Recovery from Deadlock
7.2 Resource Preemption
• Preempt some resources from processes and give
these resources to other processes until the deadlock
cycle is broken
• Issues
1. Selecting a victim
minimize cost - number of resources held and amount of
time consumed
2. Rollback – return to some safe state, restart process for
that state.
3. Starvation – same process may always be picked as victim,
include number of rollback in cost factor.


7 Deadlocks

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