Homework 15: Threads

Due: 6/1 12:30pm

Name & Perm #:
Homework buddy (leave blank if you worked alone):

Reading: Chapters 25, 26, and 28.1 of Operating Systems: Three Easy Pieces (you can download them from the web page). The book uses a slightly different API for threads (POSIX threads rather than C++ thread API), and you don't need to learn the details of that API besides the main points the book explains.

1. (2 pts) What is the advantage of using threads over processes?

2. \( \)

Arpaci-Dusseaus give an example program in Figure 26.6, and discuss that program in detail throughout section 3.

(1 pt) Their example has the same issue as the first version of the bank account program we looked at in the lecture. What is the name of the specific problem both programs share?
(2 pts) What is the root cause of this problem?
(2 pts) How do locks solve the problem?

3. \( \)

Suppose we have a mutex lock m and a piece of code like the following:

m.lock(); // (*)
// ...
m.unlock(); // (**)

Also assume that there are two threads in the program: T1 and T2. Suppose T1 starts running first and reaches the code marked with ... (so it has already acquired the lock).

(1 pt) Suppose T2 starts running the line marked with (*) while T1 is running the code marked with .... What happens when T2 runs the first line?
(1 pt) After T2 runs the first line of code, T1 reaches the last line (marked with (**)). What will happen with respect to the execution of T2?