Suggested Exercises 5

Question #1

• Find a creative/funny example of synchronization that can demonstrate the difficulty of developing a monitor-based solution (in pseudocode) similar to the “Readers-Writers.”

• Make sure that you vigorously discuss the correctness and pitfalls of your solution.

Gas Station Problem

• There is a common price for gas– Consumers never modify the price – they just

pump gas– Owners read modify the price

• We want one owner at a time, but many consumers at the same time

Developing the Solution

• Global states:

activeConsumers = 0;

activeOwners = 0;

waitingConsumers = 0;

waitingOwners = 0;

Condition okToPump = NULL;

Condition okToChangePrice = NULL;

Lock lock = FREE;

Sample Code Solution – up to you to prove correctness

Consumer() {

lock.Acquire();

while (activeOwners > 0

|| waitingOwners > 0) {

++waitingConsumers;

okToPump.Wait(&lock);

--waitingConsumers;

}

++activeConsumers;

lock.Release();

// access price and pumps

lock.Acquire();

--activeConsumers;

if (activeConsumers == 0 &&

waitingOwners > 0) {

okToChangePrice.Signal(&lock);

}

lock.Release();

}

Writer() {

lock.Acquire();

while (activeOwners > 0

|| activeConsumers > 0) {

++waitingOwners;

okToChangePrice.Wait(&lock);

--waitingOwners;

}

++activeOwners;

lock.Release();

// access database

lock.Acquire();

--activeOwners;

if (waitingOwners > 0) {

okToChangePrice.Signal(&lock);

} else if (waitingConsumers > 0) {

okToPump.Broadcast(&lock);

}

lock.Release();

}

Question #2

• Find a creative/funny example of deadlock, and apply various deadlock prevention approaches to the live example.

• Describe the advantages and disadvantages of each approach.

A Tale of Two Fisherman

• Two fisherman are out fishing, but between them they only have one fishing pole and one bait.– If one takes the pole, and the other takes the

bait…• They deadlock!

Solutions

1. Infinite resources– Buy more poles and bait

2. No sharing– They each have their own poles and bait, fish

alone

3. Allocate all resources at the beginning– They each either get both things or none

Solutions

3. Allocate all resources at the beginning– They each either get both things or none

semaphore pole= 1semaphore bait= 1lock = 1;fisher(int j) {

while (TRUE) {P(s);

P(pole);P(bait);// fishV(bait);V(pole);

V(s);}

}

Solutions

4. Make everyone use the same ordering– Fisherman 1 goes first, then fisherman 2 (round

robin)

Question #3

• Please summarize the steps to configuring, compiling, and installing a kernel and its modules.

• Include the step of rebooting into the new kernel.

Kernel Steps

• Configuring a kernel– Download kernel source– Build new configuration from old configuration• Copy old configuration into kernel source• Issue ‘make oldconfig’

– Customize your kernel• Issue ‘make menuconfig’ and select/de-select options

as you see fit

Kernel Steps

• Compiling a kernel– cd into kernel source, type ‘make’

Kernel Steps• Installing a kernel– cd into kernel source– Issue ‘make modules_install’ to copy the modules to

installation location for new kernel– Issue ‘make install’ to install the kernel executable

to /boot– Make an initrd image

• Issue ‘makeinitramfs –o [output file] [kernel version]– Update the boot loader

• Issue ‘update-grub’, make sure grub finds both kernel image and initrd image

• Reboot!