Deadlock

1
Deadlock

• COMP346 - Operating Systems
• Tutorial 5
• Edition 1.1, June 15, 2002

2
Topics

• Deadlock Debrief
• Simplest Example
• Necessary Deadlock Conditions
• Resource Allocation Graph
• Deadlock Handling
• Deadlock and Starvation with Semaphores
• Examples

3
Deadlock

• Simplest example
• Two processes require two resources to complete
  (and release the resources)
• There are only two instances of these resources
• If they acquire one resource each, they block
  indefinitely waiting for each other to release
  the other resource gt
• DEADLOCK, one of the biggest problems in
  multiprogramming.

4
Necessary Deadlock Conditions

• Recall which conditions must hold (p. 245)
• ME at least one process exclusively uses a
  resource
• Hold and wait a process possesses at least one
  resources and requires more, which are held by
  others
• No preemption resources are released only in
  voluntary manner by processes holding them
• Circular wait P1?P2 ? P3 ? ? PN ? P1

5
Resource-Allocation Graph

• An easy way to illustrate resource allocation and
  visually detect the deadlock situation(s)
• Example
• N1 resources
• N processes
• Every process needs 2 resources
• Upon acquiring 2 resources, a process releases
  them
• Is there a deadlock?

6
More Examples

• Example from Salsa Theory Review
• Resource Allocation Graph
• Is the following true or false?
• Deadlock cannot happen in a system with two
  processes.
• One CPU
• Two CPUs

7
Handling Deadlocks

• Deadlock Ignorance
• Deadlock Prevention
• Deadlock Avoidance
• Deadlock Detection and Recovery

8
Deadlock Ignorance

• As system designers we may pretend that deadlocks
  dont happen -).
• If they do happen, they dont happen very often.
• Most common approach because its cheap (in terms
  of human labor, complexity of the system and
  systems performance).
• Highest resource utilization.
• Sysadmin can simply kill deadlocked processes or
  restart the system if its not responding.

9
Deadlock Prevention

• Recall necessarily deadlock conditions
• Deadlock prevention algorithms assure at least
  one these conditions do not hold, thus preventing
  occurrence of the deadlock.
• Possible Disadvantages
• Low device utilization
• Reduced systems throughput
• 1 p. 250

10
Deadlock Avoidance

• Unlike in deadlock prevention, deadlock avoidance
  algorithms do not watch for necessary deadlock
  conditions, but use additional a priori info
  about future resource requests.
• This info will help the system to avoid entering
  the unsafe state.
• Disadvantages again, lower resource utilization
  (because resources may not be granted sometimes
  even if they are unused).
• 1 P. 253

11
Deadlock Detection and Recovery

• Instead of preventing or avoiding deadlocks we
  allow them to happen and also provide mechanisms
  to recover.
• Problems
• How often do we run detection algorithms?
• How do we recover?
• What is the cost of detection and recovery?
• 1 P. 260, p. 264

12
Deadlocks and Starvation with Semaphores

• Semaphores act like resources in this case, which
  can be acquired and (never) released.
• One example as book suggests
• 1 P. 204

P1 wait(sem1) wait(sem2) signal(sem1) signal(sem2) P2 wait(sem2) wait(sem1) signal(sem2) signal(sem1)
13
Deadlocks and Starvation with Semaphores (2)

• Starvation, or indefinite blocking, is when a
  process is waiting on a semaphore where nobody is
  ever going to release it.
• Another example of both deadlock and starvation

process A process B wait(mutex)
wait(mutex) ... ...
wait(synch) signal(synch) ...
... wait(synch) ... ...
... signal(mutex) signal
(mutex)
For this case assume mutex 1 synch
0 Try starting processes in any order and
see where they block.
14
Deadlocks and Starvation with Semaphores (3)

• Yet another example -)

a) semaphore S -2 P1 P2 P3
ltphase1gt ltphase1gt ltphase1gt V(S)
V(S) V(S) P(S) P(S) P(S)
ltphase2gt ltphase2gt ltphase2gt
b) semaphore S -2, S1 0, S2 0 P1 P2
P3 ltphase1gt ltphase1gt ltphase1gt
V(S) V(S) V(S) P(S) P(S) P(S)
V(S1) P(S1) P(S2) ltphase2gt V(S2)
ltphase2gt ltphase2gt
15
Deadlocks and Starvation with Semaphores (4)

• Things to note
• Assume the P2, P3, P1 order in the starvation
  example
• Assume the P1, P3, P2 order in the deadlock
  example
• Blocking means getting out from the running state
  to the waiting state, so a context switch to the
  next process in the queue happens
• In these example there are no multiple instances
  of these processes.

16
Deadlocks and Starvation with Semaphores (5)

• Starvation scenario (in b)
• 1) P2 ltphase1gt
• 2) V(S) (S -1)
• 3) P(S) (blocked-gtswitch to P3)
• 4) P3 ltphase1gt
• 5) V(S) (S 0)
• 6) P(S) (blocked-gtswitch to P1)
• 7) P1 ltphase1gt
• 8) V(S) (S 1)
• 9) P(S) (S 0)
• 10) V(S1) (S1 1)
• 11) ltphase2gt
• 12) terminates
• 13) ???

• Deadlock Scenario
• 1) P1 ltphase1gt
• 2) V(S) (S -1)
• 3) P(S) (blocked-gtswitch to P3)
• 4) P3 ltphase1gt
• 5) V(S) (S 0)
• 6) P(S) (blocked-gtswitch to P2)
• 7) P2 ltphase1gt
• 8) V(S) (S 1)
• 9) P(S1) (blocked)
• 10) ???