Talk:COMP 3000 Essay 1 2010 Question 6

Hey guys, this is Munther. I'm one of the members of the group assigned to this question. Before we start, let me just say that since this is a collective piece of work thats supposed to include contributions from each member of the group, let us all assume the role of the editor. So we will all contribute and help edit the final version of the article.

Regarding our question. As a starting point, I figured it would be appropriate to start defining what mutual exclusion (mutex) and race conditions mean. Lets start with race conditions, since mutual exclusion basically came to life because of the need to control race conditions.

Race conditions: situations where one or more processes are trying to write, read or access the same piece of data, and the final result depends on who runs precisely when. Look at the text book in pages 117-118 for a detailed example of that.

Mutual exclusion (mutex): the idea of making sure that processes access data in a serialized way. Meaning that, if process A for instance, happens to be executing or using a particular data structure (called a critical section), then no other process like B would be allowed to execute or use that very same data structure (critical section) until process A finishes executing or decides to leave the data structure. Common algorithms and techniques used in mutual exclusion include: locks, semaphores and monitors.

Our question asks for examples of systems that have failed due to flawed efforts. For starters, this is a wiki-programming page (Rosetta code) that examines race conditions and offers an example from the Unix/Linux operating systems, whether the example mentioned here is considered a "failure" we should check with the prof. Anyways, its a good starting point. http://rosettacode.org/wiki/Race_condition

Heres also a paper that goes back to 1992, which basically examines the excessive amount of expenses and resources used in older versions of the Unix system when implementing mutual exclusion. The paper goes to explain the problem and offers a better solution. Its pretty easy to follow and understand, worth reading as well. http://www.usenix.org/publications/library/proceedings/sa92/moran.pdf

-- Munther

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Hey Andrew here another member of this group. Those are some good starting points. The Wikipedia page on race conditions have references to a few good examples http://en.wikipedia.org/wiki/Race_condition

Couple notable ones:

The Therac-25 x-ray machine which killed a bunch of people http://courses.cs.vt.edu/~cs3604/lib/Therac_25/Side_bar_1.html

A blackout in 2003 was caused by a race condition in one of the power company's alarm systems http://www.securityfocus.com/news/8412 (really awful block of text)

--Andrew

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Alright, so the things that the prof mentioned in our last lecture proved to be super helpful. Basically, what he means by "systems", is any device based operating system. It doesn't necessarily has to be a PC-based operating system (Windows, Linux, etc.). So the Therac-25 story mentioned by Andrew in the above post is a prime example of the type of things we might be looking for.

Other notable examples:

1. The Opportunity Mars-Rover 1116 incident. (A rover is basically a space exploration vehicle designed to navigate the surface of a planet in order to gather images, samples or any possible information about that particular surface.). The rover experienced a rare unexpected error due to a race-conditions fault. For some reason, this seems to be a fairly common problem for those Mars-Rovers, since the same kind of error was experienced on the Spirit Mars-Rover as well.

Heres an overview of the Opportunity 1116 incident from MarsToday : http://www.marstoday.com/news/viewsr.html?pid=23772

Heres a paper that examines the race conditions experienced on those rovers, discuses the Spirit Rover incident and even goes to explain the underlying architecture of the rover hardware: http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/39897/1/06-0922.pdf

2. A file-system based type of race condition involves an older version of the Unix operating system, in which the user-mode can actually be bypassed, allowing the user to access the entire system. I can see this being considered an error or a case of failure as well. This actually may be a bit more approachable, as far as understanding the Unix kernel and stuff like that, I'm sure we can find a lot of resources for this.

A small article exploring the issue: http://www.osdata.com/holistic/security/attacks/racecond.html

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Heres also a paper that examines Race Conditions in depth, talks about the importance of mutual exclusion and provides a number of solutions : http://www.google.ca/url?sa=t&source=web&cd=4&ved=0CCoQFjAD&url=http%3A%2F%2Fciteseerx.ist.psu.edu%2Fviewdoc%2Fdownload%3Fdoi%3D10.1.1.1.5897%26rep%3Drep1%26type%3Dpdf&rct=j&q=race%20conditions%20case%20study%20steve%20carr%2010.1.1.1&ei=FTCtTOzRN8mVnAeL-OThDA&usg=AFQjCNHdyHdeFSpES0nMjzb7lPkFxKwC2g&sig2=u2Qo9kdemxdCWAlH10GNeQ

Heres another paper from the ACM Portal: http://portal.acm.org/citation.cfm?id=130616.130623&coll=Portal&dl=GUIDE&CFID=104720795&CFTOKEN=13393160

If anyone can't access the pdf files on the ACM Portal or even CiteSeer for that matter, you need to log in to the netwrk using your Carleton library account. Go to the following: http://portal.acm.org.proxy.library.carleton.ca You will be prompted to enter your Student ID card barcode number, thats the number below your name on your student ID. And the password is your CarletonCentral password.

I think so far we have managed to gather a handful amount of cases. In the next couple of days, we should probably delve deeper into some of those cases.

-- Munther