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Concurrency (of Threads)
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How can two threads concurrently (and correctly) increment a number?
Not possible at all
Just do it
Yes, using the "++" operator
Use Locks
Use Compare & Swap
Use Peterson's algorithm
Can Peterson's algorithm be used to implement a lock?
Yes
Yes, but only if the shared variables are marked "volatile"
No: Even though the variables were marked volatile, the compiler reordered the instructions
No: Even though the variables were marked volatile, the CPU reordered instructions
Mutual Exclusion vs. Locks
Locks can be used to achieve mutual exclusion
Mutual exclusion is a way to implement a lock
Hardware primitives (e.g. test-and-set, swap) are usually used to implement locks
In Java, every object contains a lock
What is the problem of the code in Figure 6.7?
There is a bug, it does not work
It works but it is unfair, as some thread may never get the lock
It works but it is inefficient
What is the advantage of a Semaphore over a Lock?
More powerful (can do what locks cannot)
A thread waiting for a semaphore can immediately be woken
Semaphores can directly provide access to more than just a single resource
Dining Philosophers ...
... applies to Pizza eating
... can be used to explain deadlocks
... can be used to demonstrate parallelism
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