Deadlocks

In: Computers and Technology

Submitted By uche88
Words 1230
Pages 5
COS 318: Operating Systems
Deadlocks

Today’s Topics
Conditions for a deadlock
 Strategies to deal with deadlocks




Announcement


Last year’s midterm and solution is on the course web page

2

Definitions
Use processes and threads interchangeably
 Resources







Use a resource




Request, Use, Release

Starvation




Preemptable: CPU (can be taken away)
Non-preemptable: Disk, files, mutex, ... (can’t be taken away)

Processes wait indefinitely

Deadlocks


A set of processes have a deadlock if each process is waiting for an event that only another process in the set can cause

3

Resource Allocation Graph


Process A is holding resource R

A



R

Process B requests resource S

B

S



A cycle in resource allocation graph ⇒ deadlock



If A requests for S while holding R, and B requests for
R while holding S, then

A

S

R

B

How do you deal with multiple instances of a resource?
4

An Example


A utility program





A

Resources






Copy a file from tape to disk
Print the file to printer
Tape
Disk
Printer

Tape

A deadlock



A holds tape and disk, then requests for a printer
B holds printer, then requests for tape and disk

B

5

Conditions for Deadlock


Mutual exclusion condition




Hold and Wait




Resources cannot be taken away

Circular chain of requests




Processes holding resources can request new resources

No preemption




Each resource is assigned to exactly one process

One process waits for another in a circular fashion

Question


Are all conditions necessary?

6

Eliminate Competition for Resources?






If running A to completion and then running B, there will be no deadlock Generalize this idea for all processes? Is it a good idea to develop a
CPU scheduling algorithm that causes no deadlock?

A

S

R

B

Previous example

7

Strategies
…...

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