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= 1
semaphore bait= 1
lock = 1;
fisher(int j) {
while (TRUE) {
P(s);
}
}
V(s);
P(pole);
P(bait);
// fish
V(bait);
V(pole);
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
• 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
– 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!