COMP 3000 Essay 1 2010 Question 6

Question

What are some examples of notable systems that have failed due to flawed efforts at mutual exclusion and/or race conditions? How significant was the failure in each case?

Answer

Check the discussion tab. -- Munther

Introduction Paragraph: Introduces the question and gives some general background etc.

Paragraph 1: Gives first example in detail

Paragraph 2: Gives second example in detail

Paragraph 3: Gives third example in detail

Paragraph 4: Gives fourth example in detail

Paragraph 5: Gives fifth example in detail

Conclusion: Relates it all back together or something (never been good with conclusions)

Therac-25

(This is still very rough and needs work. Thought I would lay it out there as a starting point)

The Therac-25 was an x-ray machine developed in Canada by Atomic Energy of Canada Limited (AECL). The machine was used to treat people using radiation therapy. Between 1985 and 1987 six patients were given overdoses of radiation by the machine. Half these patients died due to the accident. The cause of the incidents has been traced back to a programming bug which caused a race-condition. The Therac-25 software was written by a single programmer in PDP-11 assembly language. Portions of code were reused from software in the previous Therac-6 and Therac-20 machines. The main portion of the code runs a function called “Treat” this function determins which of the programs 8 main subroutines it should be executing. The Keyboard handler task ran concurrently with “Treat”

The 8 main subroutines were:

Reset

Datent

Set Up Done

Set Up Test

Patient Treatment

Pause Treatment

Terminate Treatment

Date, Time, ID Changes

The Datent subroutine communicated with the keyboard hander task through a shared variable which signaled if the operator was finished entering the necessary data. Once the Datent subroutine sets the flag signifying the operator has entered the necessary information it allows the main program to move onto the next subroutine. If the flag was not set the “Treat” task reschedules itself in turn rescheduling the Datent subroutine. This continues until the shared data entry flag is set.

The Datent subroutine was also responsible for preparing the x-ray to administer the correct radiation dosage. The subroutine was setup so that before returning to “Treat” instructions to move on to the next of its 8 subroutines it would first call the “Magnet” subroutine. This subroutine parsed the operators input and moved the x-ray machines magnets into position to administer the prescribed radiation. This magnet subroutine took approximately 8 seconds to complete and while it ran the keyboard handler was also running. If the operator modified the data before the “magnet” subroutine returned their changes would not be register and the x-ray strength would already be set to its prior value ignoring the operator’s changes.

Hypothetical example situation:

-Operator types up data, presses return

-(Magnet subroutine is initiated)

-Operator realizes there is an extra 0 in the radiation intensity field -Operator moves cursor up and fixes the error and presses return again.

-Magnets are set to previous power level .subroutine returns

-Program moves on to next subroutine without registering changes

-Patient is administered a lethal overdose of radiation