Deadlocks

1
Deadlocks

• Chapter 3

3.1. Resource 3.2. Introduction
to deadlocks 3.3. The ostrich algorithm
3.4. Deadlock detection and recovery
3.5. Deadlock avoidance 3.6.
Deadlock prevention 3.7. Other issues
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• What is a deadlock?
• e.g., two processes A, B want to record a
  scanned document on a FD.

B
A
2
2
1
1
FD
Scanner
Deadlock!!

• Deadlock can also occur across machines.
• More complicated situations can cause deadlocks
  involving three or more devices and users.

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3.1 Resources

• What is a resource?
• A resource is anything that can be used by
  only a single process at any instant of time.
• Active resource
• provide as service, e.g., processor and
  network adapter.
• Passive resource
• system capabilities that is required by
  active resources, e.g., memory.
• Exclusive resource
• only one process at a time can use it. e.g.,
  speaker, processor.
• Shared Resource
• can be used by multiple processes, e.g.,
  memory.
• Single resource
• exists only one in the system, e.g., speaker.
• Multiple resource
• e.g., processors in a multi-processor system.

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3.1.1 Preemptable and Nonpreemptable Resources

• Preemptable resource
• can be taken away from a process owning it with
  no ill effect, e.g. memory (when a process
  exceeds its time quantum and is swapped out)
• Nonpreemptable resource
• cannot be taken away from its current owner
  without causing the computation to fail. e.g. CD
  recorder (If a process has begun to burn a CD,
  taking the CD recorder away from it will result
  in a garbled CD)
• Sequence of events required to use a resource
• request the resource
• use the resource
• release the resource

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3.2 Introduction to Deadlocks

• Deadlock can be defined Formally as follows A
  set of processes is deadlocked if each process in
  the set is waiting for an event that only another
  process in the set can cause.
• All processes are waiting.
• None of them will ever cause any of the events
  that could wake up any of the other members of
  the set.
• All processes continue waiting for ever.
• In general, the event that each process is
  waiting for is the release of some resource
  currently possessed by another member of the set.

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3.2.1 Conditions for Deadlock

• Coffman et al. (1971)
• 1. Mutual exclusion condition. Each resource is
  either currently assigned to exactly one process
  or is available.
• 2. Hold and wait condition. Processes currently
  holding resources granted earlier can request new
  resources.
• 3. No preemption condition. Resources previously
  granted cannot be forcibly taken away from a
  process. They must be explicitly released by the
  process holding them.
• 4. Circular wait condition. There must be a
  circular chain of two or more processes, each of
  which is waiting for a resource held by the next
  member of the chain.
• All 4 conditions must hold for a deadlock to
  occur. If one of them is absent, no deadlock is
  possible.

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3.2.3 Deadlock Modeling

• Modeled with directed graphs

Process
Resource

• resource R assigned to process A
• process B is requesting/waiting for resource S
• process C and D are in deadlock over resources T
  and U

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How deadlock occurs? e.g. Process A requests R
S, B requests S, T, C requests T,Rscheduling
round robin
Deadlock occurred!!
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How deadlock can be avoided?If granting a
particular request might lead to deadlock, the
operating system can simply suspend the process
without granting request.
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Strategies for dealing with Deadlocks

• Just ignore the problem altogether. Maybe if you
  ignore it, it will ignore you.
• Detection and recovery. Let deadlocks occurs,
  detect them, and take action.
• Dynamic avoidance by careful resource allocation.
• Prevention, by structurally negating one of the
  four conditions necessary to cause a deadlock.

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3.3 The Ostrich Algorithm

• Pretend there is no problem
• It is reasonable if
• deadlocks occur very rarely
• cost of prevention is high
• It is a trade-off between
• convenience
• correctness

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3.4 Deadlock Detection and Recovery
General task of detection Is the system
deadlocked? If so which processes are
involved? 3.4.1 Deadlock detection with One
Resource of Each Type only one resource of each
type exists. e.g. a system might have one
scanner, one CD recorder, one tape drive.
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A system has 7 processes (A-G) and 6 resources
(R-W) Process A holds R and wants S Process B
holds nothing and wants T Process C holds nothing
and wants S Process D holds U and wants S and
T Process E holds T and wants V Process F holds
W and wants S Process G holds V and wants U. Is
the system deadlocked? If so which processes are
involved?
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• Cycle detection in resource graph with
  depth-first search algorithm
• For each node, N in the graph, perform the
  following 5 steps with N as the starting node.
• Initialize L to the empty list, and designate all
  the arcs as unmarked.
• Add the current node to the end of L and check to
  see if the node now appears in L two times. If it
  does, the graph contains a cycle (listed in L)
  and the algrithm terminates.
• From the given node, see if there are any
  unmarked outgoing arcs. If so, goto step 5 if
  not, go to step 6.
• Pick an unmarked outgoing arc at random and mark
  it. Then follow it to the new current node and go
  to step 3.
• We have reached a dead end. Go back to the
  previous node, that is, the one that was current
  just before this one, make that one the current
  node, and go to step 3. If this node is the
  initial node, the graph does not contain any
  cycles and algorithm terminates.

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• What this algorithm does?
• Takes each node in turn as the root and a
  depth-first search on it.
• If it ever comes back to a node it has already
  encountered , then it has found a cycle.
• If it exhausts all the arcs from any given node,
  it backtracks to the previous node.
• If it backtracks to the root and cannot go
  further, the subgraph reachable from the current
  node does not contain any cycles.
• If this property holds for all node, no
  deadlock!!

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3.4.2 Detection with Multiple Resource of Each
Type

• using vectors and matrices multiple resources

request
hold
P1 P2

• Deadlock detection algorithm is based on
  comparing vectors.
• For vectors U, V, U?V holds if and only if Ui?Vi,
  for 1? i ?m.

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Deadlock detection algorithm

• Initialize all processes as unmarked.
• Look for an unmarked process, Pi, for which the
  i-th row of R is less than or equal to A.
• If such a process is found, add the i-th row of C
  to A, mark the process, and go back to step 1.
• If no such process exists, the algorithm
  terminates.
• When the algorithm finishes, all unmarked
  processes, if any, are deadlocked.
• What the algorithm does in step 1 is looking for
  a process that can be run to completion.
• Such a process is characterized as having
  resources demands that can be met by the
  currently available resources.
• The selected process run till completion and
  releases the resources
• If all processes are able to run, no deadlock.
• If some can never run, deadlock!!

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Example A system has three processes, P1, P2,
and P3, four resource classes, tape drives (4),
plotters (2), scanners (3), and CR-ROMs (1).
Current allocation matrix is shown by C, and
request matrix is shown by R. Will this this
system meet any deadlock?

• P1 cannot run, because there is no CD-ROM
  available.
• P2 cannot run too, because there is no scanner
  available.
• P3 can run. When it finishes, it return all its
  resource, giving A(2, 2, 2, 0).
• At this point P2 can run. When it finishes, it
  return all its resource, giving A(4, 2, 2, 1).
• Then, P1 can run. NO DEADLOCK!!

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3.4.3 Recovery from Deadlock

• Recovery through preemption
• take a resource from some other process
• depends on nature of the resource
• e.g. printer
• Recovery through rollback
• Using checkpoints. A process is checked
  periodically. The checkpoints contain the memory
  image, the resource state. New checkpoints should
  be written to new files.
• When a deadlock is detected, it is easy to
  identify which resources are used by which
  process, a process that owns the needed resource
  is rolled back to a point in time before it
  acquired the resource.

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• Recovery through killing processes
• crudest but simplest way to break a deadlock
• kill one of the processes in the deadlock cycle
• the other processes get its resources
• choose process carefully that can be rerun from
  the beginning
• e.g., killing a process that is updating
  database may not be always run safely.

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3.5 Deadlock Avoidance3.5.1 Resource Trajectories

• Two process resource trajectories

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3.5.2 Safe and Unsafe States

• A state is said to be safe if it is not
  deadlocked and there is some scheduling table in
  which every process can run to completion even if
  all of them suddenly request their maximum number
  of resources immediately.
• Total number of resources 10
• Run B exclusively, until it asked for and got two
  more resources (see (b)).
• When B completes (see (c)), the scheduler run C
  (see (d)).
• Run C NO DEADLOCK!!

• Demonstration that the state in (a) is safe

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• Suppose A requests and gets another resource (see
  (b)). Can we find a sequence that is guaranteed
  to work?
• Run B exclusively, until it asked for and got two
  more resources (see (c)).
• When B completes, the state is shown in (d).
• 
  DEADLOCK!!

(a) (b)
(c)
(d)

• Demonstration that the sate in (b) is not safe

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The Banker's Algorithm for a Single Resource
(a)
(b)
(c)

• Three resource allocation states
• safe
• safe
• unsafe

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Banker's Algorithm for Multiple Resources

• Example of banker's algorithm with multiple
  resources

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3.6 Deadlock Prevention

• Conditions for Deadlocks
• Mutual exclusion condition
• Hold and wait condition
• No preemption condition
• Circular wait condition
• 3.6.1 Attacking the Mutual Exclusion Condition
• Dont assign resource exclusively to a process.
• Feasible spooling printer output
• Not feasible spooling of process table
• Avoid assigning a resource if not absolutely
  necessary
• As few processes as possible actually claim the
  resources

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• 3.6.2 Attacking the Hold and Wait Condition
• Prevent processes that hold resources from
  waiting for more resources
• Only one way processes request all resources
  before starting execution.
• If all resources are available, a process can
  start otherwise it must wait.
• Problems
• Processes often do not know how many resources
  they will need until they have started running.
• Resources will not be optimally used in this
  method.
• Variation (different way to break the
  hold-and-wait condition)
• A process must release all resources it holds,
  then request everything it needs all at a time.

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• 3.6.3 Attacking the No Preemption Condition
• Forcibly taking away the resource that a process
  is holding and assign to another.
• Some resources are nonpreemptable, e.g., printer.
• 3.6.4 Attacking the Circular wait condition
• To have a rule that a process is entitled only
  to a single resource at any moment. If it needs a
  second one, it must release the first one.
  Unacceptable, e.g., copy a huge file from tape to
  printer.
• Another way global numbering all resources.
  Processes can request resources whenever they
  want to, but all requests must be made in
  numerical order. It might be impossible to find
  an ordering that satisfies everyone.

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(a)
(b)
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• Summary of approaches to deadlock prevention

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• Example. 1 The following shows a resource graph.
  Do illegal graphs exist, that is graphs that
  structurally violate the model introduced in
  section 3.2 ? If so, mark them with X.

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• Example. 2 Process A holds resource W and
  requests for resource U, Process B holds resource
  U and requests for resource V, Process C holds
  nothing and request for V, Process D holds
  nothing and requests for W, Process E holds
  resource V and requests for resource W. Draw the
  resource graph and identify whether the system is
  deadlocked or not. And if deadlocked, which
  processes are involved?

The system is deadlocked. Processes A, B, and E
are deadlocked over resources W, U, and V.
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Example. 3 A system has 7 processes (A-G) and 6
resources (R-W). Process A holds R and wants S
Process B holds nothing and wants T Process C
holds nothing and wants S Process D holds U and
wants S and T, Process E holds T and wants W
Process F holds W and wants S, Process G holds V
and wants U. Draw the resource graph. Is the
system deadlocked? If so which processes are
involved?
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• Example. 4 A system has three processes, P1, P2,
  and P3, and four resource classes, which are 4
  tape drives, 2 plotters, 3 scanners, and 1
  CD-ROM, respectively.
• Find the existing resource vector, E.
• At an instant of time, 1 scanner is assigned to
  P1, 2 tape drives and 1 CD-ROM are assigned to
  P2, and 1 plotter and 2 scanners are assigned to
  P3. Find the current allocation matrix, C, and
  available resource vector, A.
• After a quantum of time passed, P1 further
  requests 2 tape drives and 1 CD-ROM, P2 requests
  for 1 tape drive and 1 scanner, P3 request for 2
  tape drives and 1plotters. Find the request
  matrix, R.
• In above situation, is the entire system
  deadlocked? If yes, what resources are involved?
  If not, what is the possible sequence to run
  these processes?

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• Example. 5 A system has four processes, P1, ,
  P4, and five allocatable resources. The current
  allocation and maximum needs are as follows
• Allocated Maximum Available resource vector
• P1 1 0 2 1 1 1 1 2 1 3 A(0 0 x 1 1)
• P2 2 0 1 1 0 2 2 2 1 0
• P3 1 1 0 1 0 2 1 3 1 0
• P4 1 1 1 1 0 1 1 2 2 1
• What is the smallest value of x for which this is
  a safe state?

If x0, the system encounters a deadlock
immediately. If x1, P4 can run. When it
finishes, A(1 1 2 2 1). In the situation, no
other processes can run, the system is
deadlocked. If x2, P4 can run. When it finishes,
A(1 1 3 2 1). Then P3 can run. When P3 finishes,
A(2 2 3 3 1). Then P2 can run, and the P1
run. Therefore, x2
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• Example. 6 A computer system has six tape drives,
  with n processes competing for them. Each process
  may need two drives. For which values of n is the
  system deadlock free?

Ans. If n3, each process can have 2 drives, no
deadlock. If n4, the distribution of drives can
be (2 2 1 1). The first two processes can run
first, and the last two can run. No deadlock. If
n5, the distribution of drives can be (2 1 1 1
1). The first process can run, then others can
run. No deadlock. If n6, the distribution of
drives can be (1 1 1 1 1 1). When a process
request for another, the system is
deadlocked. Therefore, when n lt 6, the system is
deadlock free.
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• Example. 7 Two processes, A and B, each needs
  three records, a, b, and c, in a database. If A
  asks for them in the order a, b, and c, and B
  asks for them in the same order, deadlock is not
  possible. However, if B asks for them in the
  order c, b, and a, then the deadlock is possible.
  What fraction of all the combinations are
  guaranteed to be deadlock free?

Ans. There are six cases for process B to ask for
them as follows. a b c deadlock free a c b
deadlock free b a c possible deadlock b
c a possible deadlock c a b possible
deadlock c b a possible deadlock Chance of
avoiding a deadlock 2/61/3 Chance of getting a
deadlock 4/62/3