chp 6 and 7: process synchronization

what are race conidtions

when the output depends on the timing/interleaving of operations

• processes can run concurrently

• processes can be interrupted at any time

• shared data can end up inconsistent

what is the critical section

any section of code that uses shared data; when one process is in its critical section, no other may be in its critical section

what are examples of code that can be found in the critical section

• Changing common variables

• Updating tables

• Writing files

what is the general structure of a process

entry section

critical section

exit section

remainder section

what are the bad solutions of the critical section problem

• disabling interrupts: impractical and can cause starvation, and too much control to users/developers

• shared flags: one process can block unnecessarily even when critical section is free

what are the 3 requirements for a correct critical section problem solution

1, mutual exclusion: one process in critical section at a time

2. progress: if critical section is free, can’t postpone forever

3. bounded waiting: a bound must exist on how long a process can be forced to wait

what is peterson’s solution

use turn (whose turn it is) and flag[2] (who wants to enter)

what are the 3 hardware solutions to the critical section problem

memory barrier instructions:

hardware instructions

atomic variables

what are memory barrier instructions

instructions to force memory updates to all processors ensuring that all threads use the correct data

what are hardware instructions

instructions guaranteed to function atomically (uninterruptible)

what are atomic variables

variables with operations that use only atomic hardware instructionw

what are mutex locks

a user-friendly approach built by kernel developers for application developers to provide mutual exclusion; a boolean variable indicating if a lock is available or not

what do acquire() and release() do for mutex locks

acquire(): acquires the lock and enters the critical section

release() releases the lock and exits the critical section

what is busy-waiting

any process that wishes to enter its critical section must loop continuously white it waits

what are the two types of semphores

binary semaphore: integer value can range only between 0 and 1 (mutex)

counting semaphore: Integer value can range over an unrestricted domain

what do wait(S) and signal(S) do

wait(S) obtains a resource if available

signal(S) releases a resource

If the value of S reaches 0, then no resources are available and the call wait(S) will block until one becomes available