Download Module 7: Process Synchronization

Chapter 7: Process Synchronization
 Background
 The Critical-Section Problem
 Synchronization Hardware
 Semaphores
 Classical Problems of Synchronization
 Critical Regions
 Monitors
 Message Passing
 Synchronization in Solaris 2 & Windows 2000
Operating System Concepts
7.1
Silberschatz, Galvin and Gagne 2002
Background
 Concurrent access to shared data may result in data
inconsistency.
 Maintaining data consistency requires mechanisms to
ensure the orderly execution of cooperating processes.
 Shared-memory solution to bounded-butter problem
(Chapter 4) allows at most n – 1 items in buffer at the
same time. A solution, where all N buffers are used is not
simple.
 Suppose that we modify the producer-consumer code by
adding a variable counter, initialized to 0 and incremented
each time a new item is added to the buffer
Operating System Concepts
7.2
Silberschatz, Galvin and Gagne 2002
Bounded-Buffer
 Shared data
#define BUFFER_SIZE 10
typedef struct {
...
} item;
item buffer[BUFFER_SIZE];
int in = 0;
int out = 0;
int counter = 0;
Operating System Concepts
7.3
Silberschatz, Galvin and Gagne 2002
Bounded-Buffer
 Producer process
item nextProduced;
while (1) {
while (counter == BUFFER_SIZE)
; /* do nothing */
buffer[in] = nextProduced;
in = (in + 1) % BUFFER_SIZE;
counter++;
}
Operating System Concepts
7.4
Silberschatz, Galvin and Gagne 2002
Bounded-Buffer
 Consumer process
item nextConsumed;
while (1) {
while (counter == 0)
; /* do nothing */
nextConsumed = buffer[out];
out = (out + 1) % BUFFER_SIZE;
counter--;
}
Operating System Concepts
7.5
Silberschatz, Galvin and Gagne 2002
Bounded Buffer
 The statements
counter++;
counter--;
must be performed atomically.
 Atomic operation means an operation that completes in
its entirety without interruption.
Operating System Concepts
7.6
Silberschatz, Galvin and Gagne 2002
Bounded Buffer
 The statement “count++” may be implemented in
machine language as:
register1 = counter
register1 = register1 + 1
counter = register1
 The statement “count—” may be implemented as:
register2 = counter
register2 = register2 – 1
counter = register2
Operating System Concepts
7.7
Silberschatz, Galvin and Gagne 2002
Bounded Buffer
 If both the producer and consumer attempt to update the
buffer concurrently, the assembly language statements
may get interleaved.
 Interleaving depends upon how the producer and
consumer processes are scheduled.
Operating System Concepts
7.8
Silberschatz, Galvin and Gagne 2002
Bounded Buffer
 Assume counter is initially 5. One interleaving of
statements is:
producer: register1 = counter (register1 = 5)
producer: register1 = register1 + 1 (register1 = 6)
consumer: register2 = counter (register2 = 5)
consumer: register2 = register2 – 1 (register2 = 4)
producer: counter = register1 (counter = 6)
consumer: counter = register2 (counter = 4)
 The value of count may be either 4 or 6, where the
correct result should be 5.
Operating System Concepts
7.9
Silberschatz, Galvin and Gagne 2002
Race Condition
 Race condition: The situation where several processes
access – and manipulate shared data concurrently. The
final value of the shared data depends upon which
process finishes last.
 To prevent race conditions, concurrent processes must be
synchronized.
Operating System Concepts
7.10
Silberschatz, Galvin and Gagne 2002
The Critical-Section Problem
 n processes all competing to use some shared data
 Each process has a code segment, called critical




section, in which the shared data is accessed.
Each process must request permission to enter its critical
section. The section of code implementing this request is
the entry section.
The critical section may be followed by an exit section.
The remaining code is the remainder section.
Problem – ensure that when one process is executing in
its critical section, no other process is allowed to execute
in its critical section.
Operating System Concepts
7.11
Silberschatz, Galvin and Gagne 2002
Solution to Critical-Section Problem
1. Mutual Exclusion. If process Pi is executing in its critical
section, then no other processes can be executing in their
critical sections.
2. Progress. If no process is executing in its critical section
and there exist some processes that wish to enter their
critical section, then the selection of the processes that
will enter the critical section next cannot be postponed
indefinitely.
3. Bounded Waiting. A bound must exist on the number of
times that other processes are allowed to enter their
critical sections after a process has made a request to
enter its critical section and before that request is granted
(No starvation).


Operating System Concepts
Assume that each process executes at a nonzero speed
No assumption concerning relative speeds of processes or
the number of processors (CPUs).
7.12
Silberschatz, Galvin and Gagne 2002
Initial Attempts to Solve Problem
 Only 2 processes, P0 and P1
 General structure of process Pi (other process Pj)
do {
entry section
critical section
exit section
reminder section
} while (1);
 Processes may share some common variables to
synchronize their actions.
Operating System Concepts
7.13
Silberschatz, Galvin and Gagne 2002
Algorithm 1 – Strict Alteration
 Shared variables:
 int turn;
initially turn = 0
 turn - i  Pi can enter its critical section
 Process Pi
do {
while (turn != i) ;
critical section
turn = j;
reminder section
} while (1);
 Satisfies mutual exclusion, but not progress
Operating System Concepts
7.14
Silberschatz, Galvin and Gagne 2002
Algorithm 1 – Strict Alteration
 This solution may violate progress requirement. Since
the processes must strictly alternate entering their critical
sections, a process wanting to enter its critical section
twice in a row will be blocked until the other process
decides to enter (and leave) its critical section as shown
in the the table below.
 The solution of strict alteration is shown in Ex5.c. Be sure
to note the way shared memory is allocated using shmget
and shmat.
Operating System Concepts
turn
P0
P1
0
CS
while
1
RS
CS
0
RS
RS
0
RS
while
7.15
Silberschatz, Galvin and Gagne 2002
Algorithm 2 – Shared locks
 Shared variables
 boolean flag[2];
initially flag [0] = flag [1] = false.
 flag [i] = true  Pi ready to enter its critical section
 Process Pi
do {
flag[i] := true;
while (flag[j]) ;
critical section
flag [i] = false;
remainder section
} while (1);
 Satisfies mutual exclusion, but not progress requirement.
If flag[0] and flag[1] are both set to be true, both
processes are looping forever.
Operating System Concepts
7.16
Silberschatz, Galvin and Gagne 2002
Algorithm 3 – Peterson’s Solution
 Combined shared variables of algorithms 1 and 2.
 Process Pi
do {
flag [i]:= true;
turn = j;
while (flag [j] and turn = j) ;
critical section
flag [i] = false;
remainder section
} while (1);
 Meets all three requirements; solves the critical-section problem
for two processes.
 Unfortunately, this solution involves busy waiting in the while
loop. Busy waiting can lead to problems we will discuss below.
Operating System Concepts
7.17
Silberschatz, Galvin and Gagne 2002
Bakery Algorithm – Extension of
Peterson’s solution to n processes
Critical section for n processes
 Before entering its critical section, process receives a
number. Holder of the smallest number enters the critical
section.
 If processes Pi and Pj receive the same number, if i < j,
then Pi is served first; else Pj is served first.
 The numbering scheme always generates numbers in
increasing order of enumeration; i.e., 1,2,3,3,3,3,4,5...
Operating System Concepts
7.18
Silberschatz, Galvin and Gagne 2002
Bakery Algorithm
 Notation < lexicographical order (ticket #, process id #)
 (a,b) < (c,d) if a < c or if a = c and b < d
 max (a0,…, an-1) is a number, k, such that k  ai for i - 0,
…, n – 1
 Shared data
boolean choosing[n];
int number[n];
Data structures are initialized to false and 0 respectively
Operating System Concepts
7.19
Silberschatz, Galvin and Gagne 2002
Bakery Algorithm
do {
choosing[i] = true;
number[i] = max(number[0], number[1], …, number [n – 1])+1;
choosing[i] = false;
for (j = 0; j < n; j++) {
while (choosing[j]) ;
while ((number[j] != 0) && ((number[j] < number[i]) ||
(number[j] == number[i] && j < i))) ;
}
critical section
number[i] = 0;
remainder section
} while (1);
Operating System Concepts
7.20
Silberschatz, Galvin and Gagne 2002
Synchronization Hardware
 The critical-section problem could be solved simply in a
uniprocessor environment if we could forbid interrupts to
occur while a shared variable is being modified. This
solution is not feasible in a multiprocessor environment.
 Test and modify the content of a word atomically.
boolean TestAndSet(boolean &target) {
boolean rv = target;
tqrget = true;
return rv;
}
Operating System Concepts
7.21
Silberschatz, Galvin and Gagne 2002
Mutual Exclusion with Test-and-Set
 Shared data:
boolean lock = false;
 Process Pi
do {
while (TestAndSet(lock)) ;
critical section
lock = false;
remainder section
}
Operating System Concepts
7.22
Silberschatz, Galvin and Gagne 2002
Synchronization Hardware
 Atomically swap two variables.
void Swap(boolean &a, boolean &b) {
boolean temp = a;
a = b;
b = temp;
}
Operating System Concepts
7.23
Silberschatz, Galvin and Gagne 2002
Mutual Exclusion with Swap
 Shared data (initialized to false):
boolean lock;
boolean waiting[n];
 Process Pi
do {
key = true;
while (key == true)
Swap(lock,key);
critical section
lock = false;
remainder section
}
 These two solutions in Figure 7.7 and Figure 7.9 don’t
satisfy the bounded-waiting requirement.
 The algorithm in Figure 7.10 satisfies all the criticalsection requirement.
Operating System Concepts
7.24
Silberschatz, Galvin and Gagne 2002
Mutual Exclusion Solutions
1. Disable interrupts.
2. Shared lock variable.
3. Strict alteration. See Ex6.c. See the manual entry for
shared memory allocation: man shmget and Ex6.c and
programs in shm directory.
4. Peterson's solution [1981]. Challenge: modify the code in
Ex6.c to implement Peterson's solution for 2 processes.
Note several variables are shared:
shared int turn = 0;
shared int flag[2] = {FALSE,FALSE};
5. Hardware solution: Test-and-Set Locks (TSL).
Operating System Concepts
7.25
Silberschatz, Galvin and Gagne 2002
Mutual Exclusion Solutions
 NOTE: The last two solutions, 4 and 5, require BUSY-
WAITING; that is, a process executing the entry code will
sit in a tight loop using up CPU cycles, testing some
condition over and over, until it becomes true.
 For example, in 5, in the enter_region code, a process
keeps checking over and over to see if the flag has been
set to 0.
 Busy-waiting may lead to the PRIORITY-INVERSION
PROBLEM if simple priority scheduling is used to
schedule the processes.
Operating System Concepts
7.26
Silberschatz, Galvin and Gagne 2002
Mutual Exclusion with Swap
 Example: Test-and-set Locks:
P0 (low) - in cs -x
|
context
switch
|
P1 (high) -----TestAndSet... x-... forever.
 Note, since priority scheduling is used, P1 will keep
getting scheduled and waste time doing busy-waiting. :-(
 Thus, we have a situation in which a low-priority process
is blocking a high-priority process, and this is called
PRIORITY-INVERSION.
Operating System Concepts
7.27
Silberschatz, Galvin and Gagne 2002
Semaphores [E. W. Dijkstra, 1965]
 The solutions presented in the previous section are not easy to
generalize to more complex problems. To overcome this
difficulty, we can use a synchronization tool called a
semaphore.
 Synchronization tool that does not require busy waiting.
 A semaphore S is an integer variable that can only be accessed
via two indivisible (atomic) operations
wait (S):
while S 0 do no-op;
S--;
signal (S):
S++;
Note: These operations were originally termed P (wait, proberen)
and V (signal, verhogen). In order to distinguish with the C/UNIX
function wait and signal, in our programming examples we use
DOWN and UP to indicate wait and signal respectively.
Operating System Concepts
7.28
Silberschatz, Galvin and Gagne 2002
Critical Section of n Processes
 We can use semaphores to deal with the n-process
critical-section problem as shown Fig7-11.c.
 Shared data:
semaphore mutex; //initially mutex = 1
 Process Pi:
do {
wait(mutex);
critical section
signal(mutex);
remainder section
} while (1);
Operating System Concepts
7.29
Silberschatz, Galvin and Gagne 2002
Semaphore Implementation
 The original semaphore definition suffers from the similar
busy waiting as the previous mutual-exclusion solutions.
 This type of semaphore is also called spinlock because
the process spins while waiting for the lock.
 The advantage of a spinlock is that on context switch is
required when a process must wait on a lock, and a
context switch may take considerable time. They are
useful in multiprocessor systems.
 To overcome the need for busy waiting, we can modify
the definition of the wait and signal semaphore
operations.
Operating System Concepts
7.30
Silberschatz, Galvin and Gagne 2002
Semaphore Implementation
 Instead busy waiting, the process can block itself and is
placed in a waiting queue. It is restarted by a wakeup
operation.
 Define a semaphore as a record
typedef struct {
int value;
struct process *L;
} semaphore;
 Each semaphore has an integer value and a list of
processes.
 Assume two simple operations:
 block suspends the process that invokes it.
 wakeup(P) resumes the execution of a blocked
process P.
Operating System Concepts
7.31
Silberschatz, Galvin and Gagne 2002
Implementation
 There are 2 operations on semaphores, wait/DOWN and
signal/UP. These operations must be executed
atomically (that is in mutual exclusion). Suppose that P is
the process making the system call.
 Semaphore operations now defined as
wait(S):
S.value--;
if (S.value < 0) {
add this process to S.L;
block(P);
}
(a) enqueue the pid of P in S.L,
(b) block process P (remove the pid from
the ready queue), and
(c) pass control to the scheduler.
Operating System Concepts
7.32
Silberschatz, Galvin and Gagne 2002
Implementation
 Semaphore operations now defined as
signal(S):
S.value++;
if (S.value <= 0) {
remove a process P from S.L;
wakeup(P);
}
(a) remove a pid from S.L (the pid of P),
(b) put the pid in the ready queue, and
(c) pass control to the scheduler.
Operating System Concepts
7.33
Silberschatz, Galvin and Gagne 2002
Implementation
 The list of waiting processes can be implemented by a
link field in each process control block (PCB).
 This list can use a FIFO to ensure bounded waiting.
 However, the list may use any queueing strategy.
 We must guarantee that no two processes can execute
wait and signal operations on the same semaphore at the
same time.
 In a uniprocessor environment, we can simply inhibit
interrupts during the time the wait and signal operations
are executing.
 In a multiprocessor environment, inhibiting interrupts does
not work. If the hardware does not provide any special
instructions, we can employ any of the correct busy-waiting
solutions to the critical sections.
Operating System Concepts
7.34
Silberschatz, Galvin and Gagne 2002
Semaphore as a General Synchronization Tool
 Execute B in Pj only after A executed in Pi
 Use semaphore flag initialized to 0
 Code:
Pi

A
signal(flag)
Operating System Concepts
Pj

wait(flag)
B
7.35
Silberschatz, Galvin and Gagne 2002
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.
 Let S and Q be two semaphores initialized to 1
P0
P1
wait(S);
wait(Q);
wait(Q);
wait(S);


signal(S);
signal(Q);
signal(Q)
signal(S);
 The events with which we are mainly concerned here are
resource acquisition and release.
 Starvation – indefinite blocking. A process may never
be removed from the semaphore queue in which it is
suspended.
Operating System Concepts
7.36
Silberschatz, Galvin and Gagne 2002
Two Types of Semaphores
 Counting semaphore – integer value can range over
an unrestricted domain.
 Binary semaphore – integer value can range only
between 0 and 1; can be simpler to implement.
 Can implement a counting semaphore S as a binary
semaphore.
Operating System Concepts
7.37
Silberschatz, Galvin and Gagne 2002
Implementing S as a Binary Semaphore
 Data structures:
binary-semaphore S1, S2;
int C:
 Initialization:
S1 = 1
S2 = 0
C = initial value of semaphore S
Operating System Concepts
7.38
Silberschatz, Galvin and Gagne 2002
Implementing S
 wait operation
wait(S1);
C--;
if (C < 0) {
signal(S1);
wait(S2);
}
signal(S1);
 signal operation
wait(S1);
C ++;
if (C <= 0)
signal(S2);
else
signal(S1);
Operating System Concepts
7.39
Silberschatz, Galvin and Gagne 2002
Using Semaphores
 Mutual Exclusion Problem:
semaphore mutex = 1; /* set mutex.count = 1 */
DOWN(mutex);
- critical section UP(mutex);
 To see how semaphores are used to eliminate race
conditions in Ex4.c, see Ex7.c and sem.h. The library
sem.h contains a version of UP(semid) and
DOWN(semid) that correspond with UP and DOWN given
above.
 Another example is shown in Fig7-11.c.
Operating System Concepts
7.40
Silberschatz, Galvin and Gagne 2002
Using Semaphores
 Process Synchronization: Order process execution:
Suppose we have 4 processes: A, B, C, and D. A must finish
executing before B and C start. B and C must finish executing
before D starts.
S1
S2
A ----> B ----> D
|
^
| S1
S3 |
+-----> C ------+
Then, the processes may be synchronized using semaphores:
semaphore S1, S2, S3 = 0,0,0;
Operating System Concepts
7.41
Silberschatz, Galvin and Gagne 2002
Using Semaphores
 Process Synchronization: Order process execution:
Process A:
---------- do work of A
UP(S1);
/* Let B or C start */
UP(S1);
/* Let B or C start */
Process B:
---------DOWN(S1);
- do work of B
UP(S2);
Operating System Concepts
/* Block until A is finished */
7.42
Silberschatz, Galvin and Gagne 2002
Using Semaphores
Process C:
---------DOWN(S1);
- do work of C
UP(S3);
Process D:
---------DOWN(S2);
DOWN(S3);
- do work of D
Operating System Concepts
7.43
Silberschatz, Galvin and Gagne 2002
Using Semaphores
 Process Synchronization: Order process execution:
Suppose we have 4 processes: A, B, C, and D.
A must finish executing before B and C start.
B and C must finish executing before D starts. (Another
solution)
S1
S2
A ----> B ----> D
|
^
| S3
S4 |
+-----> C ------+
Then, the processes may be synchronized using semaphores:
semaphore S1, S2, S3, S4 = 0,0,0,0;
Operating System Concepts
7.44
Silberschatz, Galvin and Gagne 2002
Using Semaphores
 Process Synchronization: Order process execution:
Process A:
---------- do work of A
UP(S1);
/* Let B start */
UP(S3);
/* Let C start */
Process B:
---------DOWN(S1);
- do work of B
UP(S2);
Operating System Concepts
/* Block until A is finished */
7.45
Silberschatz, Galvin and Gagne 2002
Using Semaphores
Process C:
---------DOWN(S3);
- do work of C
UP(S4);
Process D:
---------DOWN(S2);
DOWN(S4);
- do work of D
Operating System Concepts
7.46
Silberschatz, Galvin and Gagne 2002
Classical Problems of Synchronization
 These problems are used for testing every newly
proposed synchronization scheme.
 Bounded-Buffer Problem
 Readers and Writers Problem
 Dining-Philosophers Problem
Operating System Concepts
7.47
Silberschatz, Galvin and Gagne 2002
Bounded-Buffer Problem
 Bounded Buffer Problem = Producer-Consumer Problem :
Consider a circular buffer that can hold N items.
Producers add items to the buffer and Consumers
remove items from the buffer. The Producer-Consumer
Problem is to restrict access to the buffer so
correct executions result.
 Shared data
semaphore full, empty, mutex;
Initially:
not_empty = 0, not_full = n, mutex = 1
Operating System Concepts
7.48
Silberschatz, Galvin and Gagne 2002
Bounded-Buffer Problem Producer Process
do {
…
produce an item in nextp
…
wait(not_full);
wait(mutex);
…
add nextp to buffer
…
signal(mutex);
signal(not_empty);
} while (1);
Operating System Concepts
7.49
Silberschatz, Galvin and Gagne 2002
Bounded-Buffer Problem Consumer Process
do {
wait(not_empty)
wait(mutex);
…
remove an item from buffer to nextc
…
signal(mutex);
signal(not_full);
…
consume the item in nextc
…
} while (1);
Operating System Concepts
7.50
Silberschatz, Galvin and Gagne 2002
Readers-Writers Problem
 The readers and writers problem models access to a
shared database. Only one writer may write at a time. Any
number of readers may read at the same time, but not
when a writer is writing.
 Weak reader Priority: An arriving writer waits until
there are no more active readers.
 Strong reader priority: Conditions of the weak
reader priority solution apply, but also a waiting reader
has priority over a waiting writer.
 Writer priority: An arriving reader waits until there are
no more active or waiting writers.
 Shared data
semaphore mutex, wrt;
Initially
mutex = 1, wrt = 1, readcount = 0
Operating System Concepts
7.51
Silberschatz, Galvin and Gagne 2002
Readers-Writers Problem Writer Process
wait(wrt);
…
writing is performed
…
signal(wrt);
Operating System Concepts
7.52
Silberschatz, Galvin and Gagne 2002
Readers-Writers Problem Reader Process
wait(mutex);
readcount++;
if (readcount == 1)
wait(rt);
signal(mutex);
…
reading is performed
…
wait(mutex);
readcount--;
if (readcount == 0)
signal(wrt);
signal(mutex):
Operating System Concepts
7.53
Silberschatz, Galvin and Gagne 2002
Dining-Philosophers Problem
 Problem: Five philosophers are seated around a table.
There is one chopstick between each pair of philosophers.
Each philosopher needs to grab the two adjacent
chopsticks in order to eat. Philosophers alternate
between eating and thinking. They only eat for finite
periods of time.
Operating System Concepts
7.54
Silberschatz, Galvin and Gagne 2002
Dining-Philosophers Problem
 Shared data
semaphore chopstick[5];
Initially all values are 1
 Philosopher i:
do {
wait(chopstick[i])
wait(chopstick[(i+1) % 5])
…
eat
…
signal(chopstick[i]);
signal(chopstick[(i+1) % 5]);
…
think
…
} while (1);
Operating System Concepts
7.55
Silberschatz, Galvin and Gagne 2002
Dining-Philosophers Problem
 Problem: Suppose all philosophers execute the first
wait/DOWN operation, before any have a chance to
execute the second wait/DOWN operation; that is, they all
grab one chopstick. Then, deadlock will occur and no
philosophers will be able to proceed. This is called a
CIRCULAR WAIT. In Section 7.7, a solution free from
deadlocks is presented.
 Other Solutions:
 Only allow up to four philosophers to try grabbing
their chopsticks.
 Pick-up the forks only if both are available. Note: this
solution may lead to starvation.
 Asymmetric solution: Odd numbered philosophers
grab their left chopstick first, whereas even numbered
philosophers grab their right chopstick first.
Operating System Concepts
7.56
Silberschatz, Galvin and Gagne 2002
Critical Regions
 Although semaphores provide a convenient and effective
mechanism for process synchronization, their incorrect
use can still result in timing errors.
 A process interchanges the order of the wait and signal
operations.
signal(mutex);
critical section
wait(mutex);
 A process replaces signal(mutex) with wait(mutex).
wait(mutex);
critical section
wait(mutex);
 A process omits the wait(mutex) or the signal(mutex).
Operating System Concepts
7.57
Silberschatz, Galvin and Gagne 2002
Critical Regions
 To deal with these errors, a number of high-level
language constructs have been introduced: the critical
region and monitor.
 High-level synchronization construct
 A shared variable v of type T, is declared as:
v: shared T
 Variable v accessed only inside statement
region v when B do S
where B is a boolean expression.
 The statement S is executed only when B is true and no
other process is the region associated with v.
 While statement S is being executed, no other process
can access variable v.
Operating System Concepts
7.58
Silberschatz, Galvin and Gagne 2002
Critical Regions
 Regions referring to the same shared variable exclude
each other in time.
 When a process tries to execute the region statement, the
Boolean expression B is evaluated. If B is true, statement
S is executed. If it is false, the process is delayed until B
becomes true and no other process is in the region
associated with v.
region v when (true) S1;
region v when (true) S2;
Operating System Concepts
7.59
Silberschatz, Galvin and Gagne 2002
Example – Bounded Buffer
 Shared data:
struct buffer {
int pool[n];
int count, in, out;
}
Operating System Concepts
7.60
Silberschatz, Galvin and Gagne 2002
Bounded Buffer Producer Process
 Producer process inserts nextp into the shared buffer
region buffer when( count < n) {
pool[in] = nextp;
in:= (in+1) % n;
count++;
}
Operating System Concepts
7.61
Silberschatz, Galvin and Gagne 2002
Bounded Buffer Consumer Process
 Consumer process removes an item from the shared
buffer and puts it in nextc
region buffer when (count > 0) {
nextc = pool[out];
out = (out+1) % n;
count--;
}
Operating System Concepts
7.62
Silberschatz, Galvin and Gagne 2002
Implementation region x when B do S
 Associate with the shared variable x, the following
variables:
semaphore mutex, first-delay, second-delay;
int first-count, second-count;
 Mutually exclusive access to the critical section is
provided by mutex.
 If a process cannot enter the critical section because the
Boolean expression B is false, it initially waits on the
first-delay semaphore; moved to the second-delay
semaphore before it is allowed to reevaluate B.
Operating System Concepts
7.63
Silberschatz, Galvin and Gagne 2002
Implementation
 Keep track of the number of processes waiting on first-
delay and second-delay, with first-count and secondcount respectively.
 The algorithm assumes a FIFO ordering in the queuing of
processes for a semaphore.
 For an arbitrary queuing discipline, a more complicated
implementation is required.
Operating System Concepts
7.64
Silberschatz, Galvin and Gagne 2002
Monitors
 High-level synchronization construct that allows the safe sharing
of an abstract data type among concurrent processes.
monitor monitor-name
{
shared variable declarations
procedure body P1 (…) {
...
}
procedure body P2 (…) {
...
}
procedure body Pn (…) {
...
}
{
initialization code
}
}
Operating System Concepts
7.65
Silberschatz, Galvin and Gagne 2002
Monitors
 A monitor is a collection of procedures, variables, and
data structures that can only be accessed by one
process at a time.
 To allow a process to wait within the monitor, a
condition variable must be declared, as
condition x, y;
 Condition variable can only be used with the
operations wait and signal.
 The operation
x.wait();
means that the process invoking this operation is
suspended until another process invokes
x.signal();
 The x.signal operation resumes exactly one suspended
process. If no process is suspended, then the signal
operation has no effect.
Operating System Concepts
7.66
Silberschatz, Galvin and Gagne 2002
Schematic View of a Monitor
Operating System Concepts
7.67
Silberschatz, Galvin and Gagne 2002
Monitor With Condition Variables
Operating System Concepts
7.68
Silberschatz, Galvin and Gagne 2002
Dining Philosophers Example
monitor dp
{
enum {thinking, hungry, eating} state[5];
condition self[5];
void pickup(int i)
// following slides
void putdown(int i)
// following slides
void test(int i)
// following slides
void init() {
for (int i = 0; i < 5; i++)
state[i] = thinking;
}
}
Operating System Concepts
7.69
Silberschatz, Galvin and Gagne 2002
Dining Philosophers
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);
}
Operating System Concepts
7.70
Silberschatz, Galvin and Gagne 2002
Dining Philosophers
void test(int i) {
if ( (state[(I + 4) % 5] != eating) &&
(state[i] == hungry) &&
(state[(i + 1) % 5] != eating)) {
state[i] = eating;
self[i].signal();
}
}
Operating System Concepts
7.71
Silberschatz, Galvin and Gagne 2002
Monitor Implementation Using Semaphores
 Variables
semaphore mutex; // (initially = 1)
semaphore next; // (initially = 0)
int next-count = 0;
 Each external 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.
Operating System Concepts
7.72
Silberschatz, Galvin and Gagne 2002
Monitor Implementation
 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--;
Operating System Concepts
7.73
Silberschatz, Galvin and Gagne 2002
Monitor Implementation
 The operation x.signal can be implemented as:
if (x-count > 0) {
next-count++;
signal(x-sem);
wait(next);
next-count--;
}
Operating System Concepts
7.74
Silberschatz, Galvin and Gagne 2002
Monitor Implementation
 Conditional-wait construct: x.wait(c);
 c – integer expression evaluated when the wait operation is
executed.
 value of c (a priority number) stored with the name of the
process that is suspended.
 when x.signal is executed, process with smallest
associated priority number is resumed next.
 Check two conditions to establish correctness of system:
 User processes must always make their calls on the monitor
in a correct sequence.
 Must ensure that an uncooperative process does not ignore
the mutual-exclusion gateway provided by the monitor, and
try to access the shared resource directly, without using the
access protocols.
Operating System Concepts
7.75
Silberschatz, Galvin and Gagne 2002
Message Passing
 Possible Approaches:
 Assign each process a unique address such as addr.
Then, send messages directly to the process:
blocking receive.
send(addr, msg);
recv(addr, msg);
Example: signals in UNIX.
 Use mailboxes: blocking receive.
send(mailbox, msg);
recv(mailbox, msg);
Example: pipes in UNIX.
 Rendezvous: blocking send and receive.
Example: Ada tasks.
Operating System Concepts
7.76
Silberschatz, Galvin and Gagne 2002
Pipe Implementation
 Pipe description:
 pipe is a unidirectional data structure.
 One end is for reading and one end is for writing.
 Use pipe function to create a pipe.
int mbox[2];
pipe(mbox);
 In our implementation, mbox[0] is for reading and
mbox[1] is for writing.
First End
Second End
mbox[0] <-oooooooooooooooooooo<- mbox[1]
Where o stands for token.
 Each pipe is used like a semaphore.
If the initial value of a semaphore is 0, then no token
is required to store in the pipe initially.
Operating System Concepts
7.77
Silberschatz, Galvin and Gagne 2002
Pipe Implementation
 Pipe description:
 If the initial value of a semaphore is more than 0, for
example, the initial value is 3, then it can be initialized
in this way:
int msg = 0;
for (i = 1; i <= 3; i++)
write(mbox1[1],&msg,sizeof(msg));
 DOWN(S) is equivalent to
read(mbox[0],&msg,sizeof(msg));
 UP(S) is equivalent to
write(mbox[1],&msg,sizeof(msg));
Operating System Concepts
7.78
Silberschatz, Galvin and Gagne 2002
Message Passing
 Here we consider the pipe approach.
 Producer-consumer Problem with bound N.
Producer:
- produce item recv(mbox1, msg);
send(mbox2, item);
Consumer:
for (i=0; i<N; i++)
send(mbox1, NULL_MSG);
recv(mbox2, item);
send(mbox1, NULL_MSG);
- consume item –
 See Ex10.c to see how this is implemented.
 See pipe-ps.c how to pipe to synchronize processes.
Operating System Concepts
7.79
Silberschatz, Galvin and Gagne 2002
Solaris 2 Synchronization
 Implements a variety of locks to support multitasking,
multithreading (including real-time threads), and
multiprocessing.
 Uses adaptive mutexes for efficiency when protecting
data from short code segments.
 Uses condition variables and readers-writers locks when
longer sections of code need access to data.
 Uses turnstiles to order the list of threads waiting to
acquire either an adaptive mutex or reader-writer lock.
Operating System Concepts
7.80
Silberschatz, Galvin and Gagne 2002
Windows 2000 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 wither
mutexes and semaphores.
 Dispatcher objects may also provide events. An event
acts much like a condition variable.
Operating System Concepts
7.81
Silberschatz, Galvin and Gagne 2002