CHAPTER 2: MEMORY AND PROCESS MANAGEMENT

2.3 Deadlock

Definition

• A situation that occurs when two programs or devices
are each waiting for a response from the other before
continuing.

• In operating systems, a situation in which two or
more processes are prevented from continuing while
each waits for resources to be freed by the
continuation of the other

• A process requests resources; if the resources are
not available at that time, the process enters a
waiting state. Sometimes, a waiting process is never
again able to change state, because the resources it
has requested are held by other waiting processes.
This situation is called a deadlock

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2.3 Deadlock

Resource Deadlock

• This system is deadlocked because each process holds
a resource being requested by the other process and
neither process is willing to release the resource it
holds

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2.3 Deadlock

Deadlock Conditions

4 Types of Deadlock

1. Mutual exclusion :
only one process at a
time can use a resource

Process A
Process B

t0 t3 t6

2. Hold and wait : a process holding at least one resource
while waiting for another resources.

Holds Holds
Currently I’m Currently I’m using
using a printer a scanner
Process B
Process A
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2.3 Deadlock

Deadlock Conditions

3. No preemption : a process hold a resource for a long
time. Resource can be released only voluntarily by the
process holding it, after that process has completed its
task

Yes! I’m
conquered
this
printer!

Process A

Process B

I need to --wait-- --wait-- --wait-- --wait-- --wait-- --wait-- --wait-- --wait----wait-- --wait-- --wait-- -- t200
wait for a t0 wait--
long time!

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2.3 Deadlock

Deadlock Conditions

4. Circular wait : processes are waiting for resources held
by other processes, which are waiting for other resources

Waiting for

Process A Process B

Waiting for Waiting for

Process D Process C

Waiting for

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2.3 Deadlock

Deadlock Solutions

3 Various Of Deadlock Solutions

a. Prevention and avoidance of deadlock
 Ensure deadlock does not happen
 Use information about resource requests to
dynamically avoid unsafe situations

b. Allow deadlock, detect deadlock and recover
deadlock
 Allow deadlocks, but detect when occur
 Recover and continue

c. Ignore deadlock

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