1. Deadlock and Starvation
Deadlock – two or more processes are waiting indefinitely for an event that can be caused by only one of the waiting processes
Example:
Process1
Process2
wait(b);
wait(a);

2. Deadlock and Starvation
Starvation – indefinite blocking. A process may never be removed from the semaphore queue in which it is suspended.
Priority Inversion - Scheduling problem when lower-priority process holds a lock needed by higher-priority process.

3. Classical Problems of Synchronization
Bounded-Buffer Problem
Readers and Writers Problem
Dining-Philosophers Problem

4. Bounded-Buffer Problem
N buffers, each can hold one item
Semaphore mutex initialized to the value 1
可以到buffer裡面存取嗎
Semaphore full initialized to the value 0
還有東西嗎？
Semaphore empty initialized to the value N.
還有空位嗎？

5. Bounded Buffer Problem Producer
do { // produce an item in nextp wait (empty); wait (mutex); // add the item to the buffer signal (mutex); signal (full); } while (TRUE);

6. Bounded Buffer Problem consumer
do { // produce an item in nextp wait (full); wait (mutex); // remove the item from the buffer signal (mutex); signal (empty); } while (TRUE);

7. Bounded Buffer Problem consumer(????)
do { // produce an item in nextp wait (mutex); wait (full); // remove the item from the buffer signal (empty); signal (mutex); } while (TRUE);

8. Bounded Buffer Problem consumer(????)
do { // produce an item in nextp
wait (mutex);
wait (full);
　// remove the item from the buffer
signal (empty);
signal (mutex);
} while (TRUE);
錯誤，想想看，如果buffer已經空了，producer可以進入嗎？
buffer空了，但producer進不去！？

9. Readers-Writers Problem
A data set is shared among a number of concurrent processes
Readers – only read the data set; they do not perform any updates
Writers – can both read and write
Problem
Allow multiple readers to read at the same time
Only one single writer can access the shared data at the same time

10. 解決這個問題很重要， 因為大部份的thread/process只會進行讀取

11. rw-spinlock key=10000; reader writer
While((key--)<0) { key++ } //CS key=key+1;
while((key=key-10000)<0) { key=key+10000; } //CS

12. Readers-Writers Problem variables
Shared Data
Data set
Semaphore mutex initialized to 1
Semaphore wrt initialized to 1
Integer readcount initialized to 0

13. Readers-Writers Problem Writer
do { wait (wrt) ; //writing is performed signal (wrt) ; } while (TRUE);

14. Readers-Writers Problem Reader
do { wait (mutex) ; readcount ++ ; if (readcount == 1) wait (wrt) ; signal (mutex); // reading is performed readcount - - ; if (readcount == 0) signal (wrt) ; signal (mutex) ; } while (TRUE);

15. Dining-Philosophers Problem

16. Dining-Philosophers Problem
Shared data
Bowl of rice (data set)
Semaphore chopstick [5] initialized to 1

17. Dining-Philosophers Problem The structure of Philosopher i: (???)
do { wait ( chopstick[i] ); wait ( chopStick[ (i + 1) % 5] ); // eat signal ( chopstick[i] ); signal (chopstick[ (i + 1) % 5] ); // think } while (TRUE);

18. Dining-Philosophers Problem
每個人都拿到「一支」筷子，大家都無法用餐

19. Monitor 物件導向版的signal

20. Problems with Semaphores
Incorrect use of semaphore operations:
signal (mutex) …. wait (mutex)
wait (mutex) … wait (mutex)
Omitting of wait (mutex) or signal (mutex) (or both)

21. Monitors
A high-level abstraction that provides a convenient and effective mechanism for process synchronization
Only one process may be active within the monitor at a time

22. Monitors
monitor monitor-name { // shared variable declarations procedure P1 (…) { …. } … procedure Pn (…) {……} Initialization code ( ….) { … } }

23. Schematic view of a Monitor

24. Condition Variables condition x, y;
Two operations on a condition variable:
x.wait () – a process that invokes the operation is suspended.
x.signal () – resumes one of processes (if any) that invoked x.wait ()

25. Monitor with Condition Variables

26. Solution to Dining Philosophers
monitor DingPhilosophers { enum { THINKING, HUNGRY, EATING} state [5] ; condition self [5]; void pickup (int i) { state[i] = HUNGRY; test(i); if (state[i] != EATING) self [i].wait(); } void putdown (int i) { state[i] = THINKING; // test left and right neighbors test((i + 4) % 5); test((i + 1) % 5);

27. Solution to Dining Philosophers (cont.)
void test (int i) { if ( (state[(i + 4) % 5] != EATING) && (state[i] == HUNGRY) && (state[(i + 1) % 5] != EATING) ) { state[i] = EATING ; self[i].signal() ; } initialization_code() { for (int i = 0; i < 5; i++) state[i] = THINKING; } /*end of monitor DingPhilosophers*/

28. Solution to Dining Philosophers (cont)
Each philosopher I invokes the operations pickup() and putdown() in the following sequence:
DiningPhilosophters DP1;
DP1.pickup (i);
//EAT
DP1.putdown (i);

29. Monitor Implementation Using Semaphores（補充）
Variables
semaphore mutex; // (initially = 1)
semaphore next; // (initially = 0)
int next_count = 0;
Each procedure F will be replaced by
wait(mutex); …
body of F;
if (next_count > 0)
signal(next)
else
signal(mutex);
Mutual exclusion within a monitor is ensured.

30. Monitor Implementation （補充）
For each condition variable x, we have:
semaphore x_sem; // (initially = 0)
int x_count = 0;
The operation x.wait() can be implemented as:
x_count++;
if (next_count > 0)
signal(next);
else
signal(mutex);
wait(x_sem);
x_count--;

31. Monitor Implementation （補充）
The operation x.signal() can be implemented as:
if (x_count > 0) {
next_count++;
signal(x_sem);
wait(next);
next_count--; }

32. A Monitor to Allocate Single Resource （補充）
monitor ResourceAllocator {
　boolean busy;
　condition x;
　void acquire(int time) {
　　if (busy)
　　x.wait(time);
　　busy = TRUE; 　}
　void release() {
　　busy = FALSE;
　　x.signal();
　initialization code() { }

33. Synchronization Examples
Solaris
Windows XP
Linux
Pthreads
Java

34. Solaris Synchronization
Implements a variety of locks to support multithreading and real-time processing
adaptive mutex lock
Spinlock ==> mutex
readers-writers lock
# reader >> # writer
turnstiles
Mutex + priority inheritance protocol

35. Windows XP Synchronization
Uses interrupt masks to protect access to global resources on uniprocessor systems
Uses spinlocks on multiprocessor systems
Also provides dispatcher objects which may act as either mutexes and semaphores
Dispatcher objects may also provide events
An event acts much like a condition variable

36. Linux Synchronization (lightweight locks)
Single processor
multiprocessor
Disable kernel preemption (scheduler/interrupt)
Acquire spin lock
Enable kernel preemption
Release spin lock

37. Linux Synchronization (kernel preemption)
Linux 2.6: fully preemptive kernel
~ Linux 2.4: non-preemptive kernel
The design of the fully preemptive kernel (Linux)
disables/enable interrupts to implement short critical sections (for per-processor variables)
uses a counter, preempt_count, to indicate the number of locks being held by a task (task_struct)
if (preempt_count==0) {the kernel is preemptable}
if (preempt_count!=0) {the kernel is not preemptable}

38. Linux Synchronization (spinlock and semaphore)
Spinlock -- along with enabling and disabling kernel preemption – are used in the kernel only when the lock is held for a short duration.
When a lock must held for a longer period, semaphores are appropriate for use.

39. Pthreads Synchronization
Pthreads API is OS-independent
It provides:
mutex locks
condition variables
Non-portable extensions include:
read-write locks
spin locks

40. Java
Java provide a monitor-like concurrency mechanism for thread synchronization.
Keyword: synchronized
Each object in Java has associated with it a single lock.
Java also provides wait() and notify() methods, which are similar in function to the wait() and signal() statements for a monitor.