Deadlock Prevention

1
Deadlock Prevention

• A conservative approach based on Under committing
  resources .
• Indirect method Prevent one of the three
  necessary conditions from happening.
• Direct Method Prevent the circular waiting
  scenario i.e. breaking the Cycle in the Cyclic
  Graph by making it acyclic.

2
Deadlock Prevention Indirect Strategy - 1

• Violate Mutual Exclusion Not viable since most
  of the peripheral resources like Tape Drives,
  CD-WRITERS happen to be intrinsically non
  sharable. Hence any process presenty occupying
  any of these resources must have exclusive access
  to that resource.

3
Deadlock Prevention Indirect Strategy 2

• Violate Hold Wait Not easily accomplished.
  Following two possible ways can be explored.
• A. Any process may be required to request
  all of its required resources at one time and
  blocking that process unless all its requested
  resources can be granted simultaneously.
• Problems
• 1) A process may have to wait for long
  before getting all its requested resources when
  it could have proceeded with some of its
  allocated resources.
• 2) Some of the resources allocated to the
  process may remain unused for long thereby
  denying other processes.
• 3) Impractical to obtain all resource
  requirements a priori.
• 4) Not suitable for consumable resources .
  

4
Deadlock Prevention Indirect Strategy 2
Contd.

• Violate Hold Wait Second method.
• B. Before making any fresh request for any
  resource, a process may be required to release
  all the resources it is holding at that moment.
  This may stall the process for long.

5
Deadlock Prevention Indirect Strategy 2
contd.

• Violate Hold Wait ( Example) Consider the
  following scenario for any typical process Pi.
• Sl. No . Activity Required
  Resource
• 1. Copy Data from Tape Tape Drive, Disk
  Drive
• to Disk .
• 2. Sort the Disk File. Disk
  Drive.
• 3. Print the Sorted File. Disk
  Drive, Printer.

6
Deadlock Prevention Indirect Strategy 2
contd.

• Violate Hold Wait ( Example)
• Policy 1 Allocates all 3 resources i.e. Tape
  Drive , Disk Drive as well as Printer to the
  concerned process Pi at the start resulting in
  underutilization of resources also some other
  processes may starve.
• Policy 2 As soon as the process requests for
  printer , it is forced to release all other
  resources it is holding i.e. the Tape Drive as
  well as the Disk Drive causing a Halt in the
  printing of the sorted disk file. This inevitably
  causes starvation.

7
Deadlock Prevention Indirect Strategy 3

• Violate No Pre Emption The general policy
• Process Pi which is currently holding some
  resources asks / requests for a new resource
  Incremental Request .
• Policy A If the resource(s) requested by the
  process Pi cannot be granted then preempt / take
  away all the resources currently being held by
  the process Pi and are marked as free ( Easier
  said than done). However the current state of all
  the occupied resources needs to be saved.
• Restart / Resume the process Pi later when it
  can have all its requested resources including
  the previously held ones (after restoring the
  saved status of resources).

8
Deadlock Prevention Indirect Strategy 3
Contd.

• Policy B
• If the resource Rj being requested by the
  process Pi cannot be granted then check whether
  any of the other processes Pk currently holds
  that resource Rj while waiting for some other
  resources .
• If that is the case then preempt the process
  Pk from the resource Rj and allocate that to the
  requesting process Pi. However the current state
  of that occupied resource Rj w.r.t the process
  Pk needs to be saved.
• The Preempted process Pk may be restarted
  later when it can have all its requested
  resources including the previously held ones
  (after restoring the saved status of preempted
  resources).

9
Deadlock Prevention Indirect Strategy 3
Contd.

• Violating Pre Emption Associated problems.
• For most of the resources like tape drive CD
  writer , saving current status with possible
  resumption later is not at all feasible.
• Not applicable for consumable resources.
• Ensuring that the process being preempted
  possesses a lower priority than the process being
  favored presents an additional overhead.

10
Deadlock Prevention Direct Strategy

• Prevent Circular Waiting ? Prevent formation of
  Cyclic Resource Allocation Request Graph.
• Represents the most effective way of Deadlock
  prevention.
• Can be done in two ways
• a) By enforcing an ordering on all the
  resources and allowing request grant based on
  that order.
• b) Reduce number of processes thereby
  ensuring that some resource instance is always
  free (helps to prevent starvation) difficult to
  implement.

11
Deadlock Prevention Direct Strategy Contd.

• Imposing Resource Ordering Mostly applicable to
  reusable resources.
• The strategy
• Consider each resource Rj along with all its
  instances as a single entity.
• Specify a one to one mapping such that
• F Rj ? N where
• 1) N is a natural Number.
• 2) F (Rj) gt F(Ri) i.e. the resource Rj is to be
  allocated / used after the resource Ri.

12
Deadlock Prevention Direct Strategy Contd.

• Resource Ordering The policy
• Any request for any instance of any particular
  resource Rj by any process Pi , is treated as a
  request for F(Rj) ( the mapped natural number ).
• Any process Pi can request for a new resource
  Rk after holding other resources R0..Rj iff
• F (Rk) gt F(Ri) i 0 .. j . Otherwise that
  request is not granted i.e. that process Pi is
  not allowed to progress. It is also forced to
  release ALL the concurrently non sharable
  resources currently being held by it.

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Deadlock Prevention Direct Strategy Application
- 1

• Process Dining_Philosopher Pi
• / begin Philosopher /
• repeat / begin repeat /
• Think
• N1ForkI Wait (N1)
• Grab (N1) N2 Fork (I1) 5
• if (N2 ltN1) / Ordering Violation/
• /begin THEN 1/
• print(Illegal Request)
• / Go To Release Fork/
• putdown (N1) Signal (N1)
• /end THEN 1/

14
Deadlock Prevention Direct Strategy Application
1(Contd.)

• else / Valid Request /
• / begin ELSE 1 Grab Fork Eat/
• Wait (N2 ) Grab (N2) Eat
• / Release All Forks /
• Putdown ( Fork (I1) 5 )
• Signal ( Fork (I1) 5 )
• Putdown ( Fork I )
• Signal ( Fork I )
• / End ELSE 1/
• forever / end repeat /
• / end philosopher /

15
Deadlock Prevention Direct Strategy Application
- 2

• Process Dining_Philosopher Pi
• / begin Philosopher /
• repeat / begin repeat /
• Think if (Pi gt 3)
• /begin THEN 1/
• En_Queue (Room) / Only 3
  philosophers are allowed/
• else / begin ELSE 1/
• N1ForkI Wait (N1)
• Grab (N1) N2 Fork (I1) 5
• if (N2 ltN1) / Ordering Violation/
• /begin THEN 2/
• print(Illegal Request)
• / Go To Release Fork/
• putdown (N1) Signal (N1)
• / end THEN 2 /

16
Deadlock Prevention Direct Strategy Application
1(Contd.)

• else / Valid Request /
• / begin ELSE 2 Grab Fork Eat/
• Wait (N2 ) Grab (N2) Eat
• / Release All Forks /
• Putdown ( Fork (I1) 5 )
• Signal ( Fork (I1) 5 )
• Putdown ( Fork I )
• Signal ( Fork I )
• / end ELSE 2/
• De_Queue(Room) / Allow one more
  Philosopher from Room/
• /end ELSE 1/
• forever / end repeat /
• / end philosopher /