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Lab 2: Threads
Due date: May 10 2022, Tuesday.
Overview and Goal
In this lab, you will learn about threads in BLITZ, and gain familiarity writing programs involving
concurrency control. You will begin by studying the thread package, which implements multithreading,
and then make some modifications and additions to the existing code to solve some
traditional concurrency problems using the provided code in this package. You will also gain
familiarity programming in the KPL language while completing this lab.
Before you start this lab, it is required that you carefully read the entire document titled “The Thread
Scheduler and Concurrency Control Primitives,” by downloading it from the course website (one of
the documents in the archive).
Step 1: Setting up
In the Docker container image provided to you, the files needed for Lab 2 can be found in /lab2.
You should get the following files:
In this lab, you will only need to modify and submit the following three files:
You should be able to compile all the source code provided to you with the UNIX make command:
% make
The program executable we are building will be called “os”. You can run the program using the BLITZ
emulator by typing:
% blitz -g os
Feel free to modify other files besides Synch.h, Synch.c and Main.c, but the code you are
required to write and submit does not require any changes to the other files. For example, you may
wish to uncomment some of the print statements, to see what happens. However, your final versions
of Synch.h, Synch.c and Main.c must work with the other provided files, exactly as they are
distributed to you.
Do not reuse any of the files from Lab 1, as they are considered out of date.
Working in your Docker container image
You will need to install Docker, as shown in Lab 1. After you have successfully installed Docker on your
own computer, download the Docker image we provided to you for this lab from the course website
(under the section heading “Lab 2”), called lab2-docker.tar.gz. Please note: the provided
docker image is built on an Intel x86 architecture and does not support an M1 Mac. If you use an M1
Mac, you will need to find an Intel computer to work on labs in this course.
Load the Docker image as you did in Lab 1:
docker load -i lab2-docker.tar.gz
Then run the Docker image as a Docker container by using the following command:
docker run -it blitz
After the container is running, you will see a command prompt from within the Linux container. The
BLITZ tools have been preinstalled for you in /usr/local/blitz/bin, and files needed for Lab 2
can be found in /lab2. The source code for building BLITZ tools can be found in /blitz. Within
the container, the search path environment variable has already been set up for you to use BLITZ
command-line tools directly.
As you may have already tried in Lab 1, there are many other commands that may be useful for you to
work with Docker containers. For example, to remove all the Docker containers, you can use the
docker rm $(docker ps -a -q)
To remove all the Docker images (so that you can have a clean slate to start working with something
else), you may use the command:
docker rmi $(docker images -q)
To copy files from the Docker container to your host computer, use the command:
docker cp :/file/path/within/container /host/path/target
The container ID can be found in the command prompt while you are running the container.
The command-line editor vi has been pre-installed for you in the Docker container. To install other
editor alternatives (such as emacs) or any other packages, you can use the following command
within the container:
apt-get install -y
To learn more about Docker containers and images, refer to the Docker documentation. There was
also a mini-tutorial of other Docker commands that you may find useful, distributed to you on the
course website when Lab 1 was released.
Step 2: Study the Existing Code
The code you received in this lab provides the ability to create and run multiple threads in the kernel,
and to control concurrency through several synchronization methods.
Start by looking over the System package. Focus on the material toward the beginning of the file
System.c, namely the following functions:
Get familiar with these printing functions, as you may need to call them quite often in your code.
Some of these functions are implemented in assembly code, and some are implemented in KPL in the
System package.
The following functions are used to implement the heap in KPL:
Objects can be allocated on the heap and freed with the alloc and free statements. The HEAP
implementation is very rudimentary in this implementation. In your kernel, you may allocate objects
during start-up but after that, you should not allocate objects on the heap. This is because the heap
may fill up, and then the kernel may crash.
The following functions can be ignored since they are only related to aspects of the KPL language that
we will not be using in this lab:
The Runtime.s file contains a number of routines coded in assembly language. It contains the
program entry point and the interrupt vector in low memory. Read it carefully. Follow what happens
when program execution begins at location 0x00000000 (the label “_entry”). The code labeled
“_mainEntry” is included in the code the compiler produces. The “_mainEntry” code will call the
main function, which appears in the file Main.c.
In Runtime.s, follow what happens when a timer interrupt occurs. It makes an “up-call” to a
function called _P_Thread_TimerInterruptHandler. This name implies that it is “a function
called TimerInterruptHandler in a package called Thread.” (It is the name the compiler gives
to this function.)
All the code in this lab assumes that no other interrupt types (such as a DiskInterrupt) occur.
When reading Runtime.s, think about what would happen if another type of interrupt should ever
The KPL language will check for many error conditions, such as the use of a null pointer. Try changing
the program to make this error. Follow in Runtime.s to see what happens when this occurs.
Next, read the List package. First read the header file carefully. This package provides code that
implements a linked list. We will use linked lists in this lab. For example, the threads that are ready to
run (and waiting for time on the CPU) will be kept in a linked list called the “ready list.” Threads that
become BLOCKED will sit on other linked lists. Also read the code in List.c to check out how the
linked list is implemented in KPL.
The most important class in this lab is named Thread, and it is located in the Thread package along
with other code (see Thread.h, Thread.c). For each thread, there will be a single Thread object.
Thread is a subclass of Listable, which means that each Thread object contains a next pointer
and can be added to a linked list.
The Thread package in Thread.c is central and you should study this code thoroughly. This
package contains one class (called Thread) and several functions related to thread scheduling and
InitializeScheduler ()
IdleFunction (arg: int)
Run (nextThread: ptr to Thread)
PrintReadyList ()
ThreadStart ()
ThreadFinish ()
FatalError (errorMessage: ptr to array of char)
SetInterruptsTo (newStatus: int) returns int
TimerInterruptHandler ()
FatalError is the simplest function. We will call FatalError whenever we wish to print an error
message and abort the program. Typically, we will call FatalError after making some checks and
finding that things are not as we expected. FatalError will print the name of the thread invoking it,
print the message, and then shut down. It will throw us into the BLITZ emulator command line mode.
Normally, the next thing to do might be to type the “st” command (short for “stack”), to see which
functions and methods were active.
(Of course, when multiple threads were concurrently running, the information printed out by the
emulator will only pertain to the thread that invoked FatalError. The emulator does not know
about threads, and it is pretty much impossible to extract information about other threads by
examining bytes in memory.)
The next function to look at is SetInterruptsTo, which is used to change the “I” interrupt bit in the
CPU. We can use it to disable interrupts with code like this:
... = SetInterruptsTo (DISABLED)
and we can use it to enable interrupts:
... = SetInterruptsTo (ENABLED)
This function returns the previous status. This is very useful because we often want to DISABLE
interrupts (regardless of what they were before) and then later we want to return the interrupt status to
whatever it was before. In our kernel, we will often see code like:
var oldIntStat: int
oldIntStat = SetInterruptsTo (DISABLED)
oldIntStat = SetInterruptsTo (oldIntStat)
Next take a look at the Thread class. Here are the fields of Thread:
name: ptr to array of char
status: int
systemStack: array [SYSTEM_STACK_SIZE] of int
regs: array [13] of int
stackTop: ptr to void
Here are the operations (i.e., methods) you can do on a Thread:
Init(n: ptr to array of char)
Fork(fun: ptr to function (int), arg: int)
Each thread is in one of the following states: JUST_CREATED, READY, RUNNING, BLOCKED, and
UNUSED, and this is given in the status field. (The UNUSED status is given to a Thread after it has
terminated. We will need this in later labs.)
Each thread has a name. To create a thread, you will need a Thread variable. First, use Init to
initialize it, providing a name.
Each thread needs its own stack, and space for this stack is placed directly in the Thread object in
the field called systemStack. Currently, this is an array of 1000 words, which should be enough. (It
is conceivable our code could overflow this limit; so there exist code in the implementation to check
and make sure that we do not overflow this limited area.)
All threads in this lab are kernel threads, and will run in the Kernel (System) mode. The stack is
therefore called the “system stack.” In later labs, we will see that this stack is used only for kernel
routines. User programs will have their own stacks in their virtual address spaces in later labs.
The Thread object also has space to store the state of the CPU, namely the registers. Whenever a
thread switch occurs, the registers will be saved in the Thread object. These fields (regs and
stackTop) are used by the assembly code function named Switch.
After initializing a new Thread, we can start it running with the Fork method. This does not
immediately begin the thread execution; instead it makes the thread READY to run and places it on
the readyList. The readyList is a linked list of Threads, and is a global variable. All Threads
on the readyList have status READY. There is another global variable named currentThread,
which points to the currently executing Thread object; i.e., the Thread whose status is RUNNING.
The Yield method should only be invoked on the currently running thread. It will cause a switch to
some other thread.
Follow the code in Yield closely to see what happens when a context switch between threads
occurs. First, interrupts are disabled; we do not want any interference during a context switch. The
readyList and currentThread are shared variables and, while context switching between
threads, we want to be able to access and update them safely. Then Yield will find the next thread
from the readyList. (If there is no other thread, then Yield is effectively a nop.) After this Yield
will make the currently running process READY (i.e., no longer RUNNING) and it will add the current
thread to the tail end of the readyList. Finally, it will call the Run function to do the context switch.
The Run method will check for stack overflow on the current thread. It will then call Switch to do the
actual Switch.
Switch may be the most fascinating function you ever encounter. It is located in the assembly code
file Switch.s, which you should look at carefully. Switch does not return to the function that called
it. Instead, it switches to another thread. Then it returns. Therefore, the return happens to another
function in another thread!
The only place Switch is called is from the Run function, so Switch returns to some invocation of
the Run function in some other thread. That copy (i.e., invocation) of Run will then return to whoever
called it. This could have been some other call to Yield, so we will return to another Yield which
will return to whoever called it.
And this is exactly the desired functionality of Yield. A call to Yield should give up the processor
for a while, and eventually return after other threads have had a chance to execute.
Run is also called from Sleep, so we might be returning from a call to Sleep after a context switch.
How is everything set up when a thread is first created? How can we “return to a function” when we
have not ever called it? Take a look at function ThreadStart in file Thread.c and look at function
ThreadStartUp in file Switch.s. What happens when a thread is terminated? Take a look at
ThreadFinish in file Thread.c. Essentially, the thread is put to sleep with no hope of ever being
awakened. Our upcoming lectures will also cover more detailed information about these design
choices in the Thread package.
Next, take a look at what happens when a Timer interrupt occurs while some thread is executing. This
is an interrupt from hardware, so the CPU begins by interrupting the current routine’s execution and
pushing some state onto its system stack. Then it disables interrupts and jumps to the assembly
code routine called TimerInterruptHandler in Runtime.s, which just calls the
TimerInterruptHandler function in Thread.c.
In TimerInterruptHandler, we call Yield, which then switches to another thread. Later, we will
come back here, when this thread gets another chance to run. Then, we will return to the assembly
language routine which will execute a “reti” instruction. This will restore the state to exactly what it
was before and the interrupted routine (whatever it was) will get to continue.
Note that this code maintains a variable called currentInterruptStatus. This is because it is
rather difficult to query the “I” bit of the PSW (status register) in the CPU. It is easier to just change the
variable whenever a change to the interrupt status changes. We see this occurring in the
TimerInterruptHandler function. Clearly interrupts will be disabled immediately after the
interrupt occurs. And the Yield function will preserve the interrupt status. So when we return from
Yield, interrupts will still remain disabled. Before returning to the interrupted thread, we set the
currentInterruptStatus to ENABLED. (They must have been enabled before the interrupt
occurred — or else it could not have occurred — so after we execute the “reti” instruction, the status
will revert to what it was before, namely ENABLED.)
It now becomes apparent that you will be reading a lot of code provided to you, before you are ready to
start playing with and modifying the code. Please experiment with the code we have just discussed
as necessary to understand it better.
Step 3: Run the “SimpleThreadExample” Code
Execute and trace through the output of SimpleThreadExample in file Main.c.
In TimerInterruptHandler there is a statement
which is commented out. Try uncommenting it. Make sure you understand the output.
In TimerInterruptHandler, there is a call to Yield. Why is this there? Try commenting this
statement out, and see what happens. Make sure you understand how Yield works here.
Step 4: Run the “MoreThreadExamples” Code
Trace through the output. Try changing this code to see what happens.
Step 5: Implement the “Mutex” Class
In this part, you must implement the class Mutex. The class specification for Mutex is given to you in
class Mutex
superclass Object
Unlock ()
IsHeldByCurrentThread () returns bool
You will need to provide code for each of these methods. In Synch.c you will see a behavior
construct for Mutex. There are methods for Init, Lock, Unlock, and IsHeldByCurrentThread,
but these have dummy bodies. You will need to write the code for these four
methods. You will also need to add a couple of fields to the class specification of Mutex in Synch.h
to implement the desired functionality.
How can you implement the Mutex class? Take a close look at the Semaphore class that is
provided to you; your implementation of Mutex will be quite similar.
First consider the IsHeldByCurrentThread method, which may be invoked by any thread. The
code of this method will need to know which thread is holding a lock on the mutex; then it can
compare that to the currentThread to see if they are the same. So, you may consider adding a
field (perhaps called heldBy) to the Mutex class, which will be a pointer to the thread holding the
mutex. Of course, you will need to set it to the current thread whenever the mutex is locked. You
might use a null value in this field to indicate that no thread is holding a lock on the mutex.
When a lock is requested on the mutex, you will need to see if any thread already has a lock on this
mutex. If so, you will need to put the current process to sleep. For putting a thread to sleep, take a look
at the method Semaphore.Down. At any one time, there may be zero, one, or many threads waiting
to acquire a lock on the mutex; you will need to keep a list of these threads so that when an Unlock is
executed, you can wake up one of them. As in the case of Semaphores, you should use a FIFO
queue, waking up the thread that has been waiting the longest.
When a mutex lock is released (in the Unlock method), you will need to see if there are any threads
waiting to acquire a lock on the mutex. You can choose one and move it back onto the readyList.
Now the waiting thread will begin running when it gets a turn. The code in Semaphore.Up does
something similar.
It is also a good idea to add an error check in the Lock method to make sure that the current thread
asking to lock the mutex does not already hold a lock on the mutex. If it does, you can simply invoke
FatalError. (This would probably indicate a logical error in the code using the mutex. It would lead
to a deadlock, with a thread frozen forever, waiting for itself to release the lock.) Likewise, you should
also add a check in Unlock to make sure the current thread really does hold the lock and call
FatalError if it does not. You will be using your Mutex class later, so these checks will help your
debugging in later labs.
The function TestMutex in Main.c is provided to exercise your implementation of Mutex. It
creates 7 threads that uses the LockTester function to compete vigorously for a single mutex lock.
The file DesiredOutput1.pdf that is provided to you contains an example of the correct output
from running this function.
Step 6: Implement the Producer-Consumer Solution
In the lectures, we will cover the celebrated Producer-Consumer problem, and introduce a solution
that uses a Mutex and two Semaphores. Implement this in KPL using the classes Mutex and
Semaphore. Your solution needs to deal with multiple producers and multiple consumers, all sharing
a single bounded buffer.
At the time when you try to complete this lab, our lectures may have just reached the point of
discussing the use of Mutex locks and Semaphores to implement a solution to the producerconsumer
problem correctly. In this case, if you wish to complete this part of the lab early, you may
need to read ahead a little bit in the “Three Easy Pieces” texbook, from Chapter 26 up to and including
Chapter 31.4 (“The Producer-Consumer (Bounded Buffer) Problem”), before the lecture coverage
reaches that point. It is fine if you cannot understand the solution completely, as we will present a
detailed coverage of this problem in upcoming lectures.
The Main package contains a part of the solution code that will serve as a framework for your
complete solution. The bounded buffer is called buffer and contains up to BUFFER_SIZE (e.g., 5)
characters. There are 5 producer threads and 3 consumer threads, in addition to the main thread that
creates the other ones. You only need to supply the missing portion of the code to support thread
Each producer will loop, adding 5 characters to the buffer. The first producer will add five ‘A’
characters, the second producer will add five ‘B’s, etc. However, since the execution of these threads
will be interleaved, the characters will be added in a somewhat random order. The provided file
DesiredOutput2.pdf provides you with a sample of the correct output.
What to Submit
Complete all the above steps.
Please submit Synch.h, Synch.c, Main.c.
Grading for this Lab
Your submitted solution will also be marked (out of the remaining 5 marks) using test cases, such as
the provided function TestMutex. The maximum possible mark for this lab assignment is 10.