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 --activeConsumers; if (activeConsumers == 0 && waitingOwners > 0) { okToChangePrice.Signal(&lock); Writer() { lock.Acquire(); while (activeOwners > 0 || activeConsumers > 0) { ++waitingOwners; okToChangePrice.Wait(&lock); --waitingOwners; } ++activeOwners; lock.Release(); // access database --activeOwners; if (waitingOwners > 0) { okToChangePrice.Signal(&lock); } else if (waitingConsumers > 0) { okToPump.Broadcast(&lock);

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 Infinite resources No sharing Buy more poles and bait No sharing They each have their own poles and bait, fish alone Allocate all resources at the beginning They each either get both things or none

Solutions Allocate all resources at the beginning They each either get both things or none semaphore pole= 1 semaphore bait= 1 lock = 1; fisher(int j) { while (TRUE) { P(s); P(pole); P(bait); // fish V(bait); V(pole); V(s); }

Solutions 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 Make the default configuration (smaller than the distro configuration) Issue ‘make defconfig’ 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 Reboot! 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!