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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