1. 1. Concurrency and threads 2. Synchronization: basic 3. Synchronization: advanced

2. Client / Server Session Iterative Echo Server ClientServer socket bind listen rio_readlineb rio_writenrio_readlineb rio_writen Connection request rio_readlineb close EOF Await connection request from next client open_listenfd open_clientfd acceptconnect

3. Iterative Servers Iterative servers process one request at a time client 1serverclient 2 connect accept connect write read call read close accept write read close Wait for Client 1 call read write ret read write ret read

4. Creating Concurrent Flows  Allow server to handle multiple clients simultaneously 1. Processes  Kernel automatically interleaves multiple logical flows  Each flow has its own private address space 2. Threads  Kernel automatically interleaves multiple logical flows  Each flow shares the same address space 3. I/O multiplexing with select()  Programmer manually interleaves multiple logical flows  All flows share the same address space  Relies on lower-level system abstractions

5. Concurrent Servers: Multiple Processes Spawn separate process for each client client 1serverclient 2 call connect call accept call read ret connect ret accept call connect call fgets fork child 1 User goes out to lunch Client 1 blocks waiting for user to type in data call accept ret connect ret accept call fgets writefork call read child 2 write call read end read close...

6. Review: Iterative Echo Server int main(int argc, char **argv) { int listenfd, connfd; int port = atoi(argv[1]); struct sockaddr_in clientaddr; int clientlen = sizeof(clientaddr); listenfd = Open_listenfd(port); while (1) { connfd = Accept(listenfd, (SA *)&clientaddr, &clientlen); echo(connfd); Close(connfd); } exit(0); }  Accept a connection request  Handle echo requests until client terminates

7. int main(int argc, char **argv) { int listenfd, connfd; int port = atoi(argv[1]); struct sockaddr_in clientaddr; int clientlen=sizeof(clientaddr); Signal(SIGCHLD, sigchld_handler); listenfd = Open_listenfd(port); while (1) { connfd = Accept(listenfd, (SA *) &clientaddr, &clientlen); if (Fork() == 0) { Close(listenfd); /* Child closes its listening socket */ echo(connfd); /* Child services client */ Close(connfd); /* Child closes connection with client */ exit(0); /* Child exits */ } Close(connfd); /* Parent closes connected socket (important!) */ } Process-Based Concurrent Server Fork separate process for each client Does not allow any communication between different client handlers

8. Process-Based Concurrent Server (cont) void sigchld_handler(int sig) { while (waitpid(-1, 0, WNOHANG) > 0) ; return; }  Reap all zombie children

9. Process Execution Model  Each client handled by independent process  No shared state between them  Both parent & child have copies of listenfd and connfd  Parent must close connfd  Child must close listenfd Client 1 Server Process Client 2 Server Process Listening Server Process Connection Requests Client 1 dataClient 2 data

10. Concurrent Server: accept Illustrated listenfd(3) Client 1. Server blocks in accept, waiting for connection request on listening descriptor listenfd clientfd Server listenfd(3) Client clientfd Server 2. Client makes connection request by calling and blocking in connect Connection request listenfd(3) Client clientfd Server 3. Server returns connfd from accept. Forks child to handle client. Client returns from connect. Connection is now established between clientfd and connfd Server Child connfd(4)

11. Implementation Must-dos With Process-Based Designs Listening server process must reap zombie children  to avoid fatal memory leak Listening server process must close its copy of connfd  Kernel keeps reference for each socket/open file  After fork, refcnt(connfd) = 2  Connection will not be closed until refcnt(connfd) == 0

12. Pros and Cons of Process-Based Designs + Handle multiple connections concurrently + Clean sharing model  descriptors (no)  file tables (yes)  global variables (no) + Simple and straightforward – Additional overhead for process control – Nontrivial to share data between processes  Requires IPC (interprocess communication) mechanisms  FIFO’s (named pipes), System V shared memory and semaphores

13. Approach #2: Multiple Threads Very similar to approach #1 (multiple processes)  but, with threads instead of processes

14. Traditional View of a Process Process = process context + code, data, and stack shared libraries run-time heap 0 read/write data Program context: Data registers Condition codes Stack pointer (SP) Program counter (PC) Kernel context: VM structures Descriptor table brk pointer Code, data, and stack read-only code/data stack SP PC brk Process context

15. Alternate View of a Process Process = thread + code, data, and kernel context shared libraries run-time heap 0 read/write data Thread context: Data registers Condition codes Stack pointer (SP) Program counter (PC) Code and Data read-only code/data stack SP PC brk Thread (main thread) Kernel context: VM structures Descriptor table brk pointer

16. A Process With Multiple Threads Multiple threads can be associated with a process  Each thread has its own logical control flow  Each thread shares the same code, data, and kernel context  Share common virtual address space (inc. stacks)  Each thread has its own thread id (TID) shared libraries run-time heap 0 read/write data Thread 1 context: Data registers Condition codes SP1 PC1 Shared code and data read-only code/data stack 1 Thread 1 (main thread) Kernel context: VM structures Descriptor table brk pointer Thread 2 context: Data registers Condition codes SP2 PC2 stack 2 Thread 2 (peer thread)

17. Logical View of Threads Threads associated with process form a pool of peers  Unlike processes which form a tree hierarchy P0 P1 sh foo bar T1 Process hierarchy Threads associated with process foo T2 T4 T5 T3 shared code, data and kernel context

18. Thread Execution Single Core Processor  Simulate concurrency by time slicing Multi-Core Processor  Can have true concurrency Time Thread AThread BThread C Thread AThread BThread C Run 3 threads on 2 cores

19. Threads vs. Processes How threads and processes are similar  Each has its own logical control flow  Each can run concurrently with others (possibly on different cores)  Each is context switched How threads and processes are different  Threads share code and some data  Processes (typically) do not  Threads are somewhat less expensive than processes  Process control (creating and reaping) is twice as expensive as thread control  Linux numbers: –~20K cycles to create and reap a process –~10K cycles (or less) to create and reap a thread

20. Posix Threads (Pthreads) Interface Pthreads: Standard interface for ~60 functions that manipulate threads from C programs  Creating and reaping threads  pthread_create()  pthread_join()  Determining your thread ID  pthread_self()  Terminating threads  pthread_cancel()  pthread_exit()  exit() [terminates all threads], RET [terminates current thread]  Synchronizing access to shared variables  pthread_mutex_init  pthread_mutex_[un]lock  pthread_cond_init  pthread_cond_[timed]wait

21. /* thread routine */ void *thread(void *vargp) { printf("Hello, world!\n"); return NULL; } The Pthreads "hello, world" Program /* * hello.c - Pthreads "hello, world" program */ #include "csapp.h" void *thread(void *vargp); int main() { pthread_t tid; Pthread_create(&tid, NULL, thread, NULL); Pthread_join(tid, NULL); exit(0); } Thread attributes (usually NULL) Thread arguments (void *p) return value (void **p) hello.c

22. Execution of Threaded “hello, world” main thread peer thread return NULL; main thread waits for peer thread to terminate exit() terminates main thread and any peer threads call Pthread_create() call Pthread_join() Pthread_join() returns printf() (peer thread terminates) Pthread_create() returns

23. Thread-Based Concurrent Echo Server int main(int argc, char **argv) { int port = atoi(argv[1]); struct sockaddr_in clientaddr; int clientlen=sizeof(clientaddr); pthread_t tid; int listenfd = Open_listenfd(port); while (1) { int *connfdp = Malloc(sizeof(int)); *connfdp = Accept(listenfd, (SA *) &clientaddr, &clientlen); Pthread_create(&tid, NULL, echo_thread, connfdp); }  Spawn new thread for each client  Pass it copy of connection file descriptor  Note use of Malloc()!  Without corresponding Free()

24. Thread-Based Concurrent Server (cont) /* thread routine */ void *echo_thread(void *vargp) { int connfd = *((int *)vargp); Pthread_detach(pthread_self()); Free(vargp); echo(connfd); Close(connfd); return NULL; }  Run thread in “detached” mode  Runs independently of other threads  Reaped when it terminates  Free storage allocated to hold clientfd  “Producer-Consumer” model

25. Threaded Execution Model  Multiple threads within single process  Some state between them  File descriptors Client 1 Server Client 2 Server Listening Server Connection Requests Client 1 dataClient 2 data

26. Potential Form of Unintended Sharing main thread peer 1 while (1) { int connfd = Accept(listenfd, (SA *) &clientaddr, &clientlen); Pthread_create(&tid, NULL, echo_thread, (void *) &connfd); } connfd Main thread stack vargp Peer 1 stack vargp Peer 2 stack peer 2 connfd = connfd 1 connfd = *vargp connfd = connfd 2 connfd = *vargp Race! Why would both copies of vargp point to same location?

27. Could this race occur? int i; for (i = 0; i < 100; i++) { Pthread_create(&tid, NULL, thread, &i); } Race Test  If no race, then each thread would get different value of i  Set of saved values would consist of one copy each of 0 through 99. Main void *thread(void *vargp) { int i = *((int *)vargp); Pthread_detach(pthread_self()); save_value(i); return NULL; } Thread

28. Experimental Results The race can really happen! No Race Multicore server Single core laptop

29. Issues With Thread-Based Servers Must run “detached” to avoid memory leak.  At any point in time, a thread is either joinable or detached.  Joinable thread can be reaped and killed by other threads.  must be reaped (with pthread_join ) to free memory resources.  Detached thread cannot be reaped or killed by other threads.  resources are automatically reaped on termination.  Default state is joinable.  use pthread_detach(pthread_self()) to make detached. Must be careful to avoid unintended sharing.  For example, passing pointer to main thread’s stack Pthread_create(&tid, NULL, thread, (void *)&connfd); All functions called by a thread must be thread-safe  Stay tuned

30. Pros and Cons of Thread-Based Designs + Easy to share data structures between threads  e.g., logging information, file cache. + Threads are more efficient than processes. – Unintentional sharing can introduce subtle and hard-to- reproduce errors!  The ease with which data can be shared is both the greatest strength and the greatest weakness of threads.  Hard to know which data shared & which private  Hard to detect by testing  Probability of bad race outcome very low  But nonzero!

31. 2. Synchronization: basic

32. Synchronization: basic Sharing Mutual exclusion Semaphores

33. Shared Variables in Threaded C Programs Question: Which variables in a threaded C program are shared?  The answer is not as simple as “global variables are shared” and “stack variables are private” Requires answers to the following questions:  What is the memory model for threads?  How are instances of variables mapped to memory?  How many threads might reference each of these instances? Def: A variable x is shared if and only if multiple threads reference some instance of x.

34. Threads Memory Model Conceptual model:  Multiple threads run within the context of a single process  Each thread has its own separate thread context  Thread ID, stack, stack pointer, PC, condition codes, and GP registers  All threads share the remaining process context  Code, data, heap, and shared library segments of the process virtual address space  Open files and installed handlers Operationally, this model is not strictly enforced:  Register values are truly separate and protected, but…  Any thread can read and write the stack of any other thread The mismatch between the conceptual and operation model is a source of confusion and errors

35. Example Program to Illustrate Sharing char **ptr; /* global */ int main() { int i; pthread_t tid; char *msgs[2] = { "Hello from foo", "Hello from bar" }; ptr = msgs; for (i = 0; i < 2; i++) Pthread_create(&tid, NULL, thread, (void *)i); Pthread_exit(NULL); } /* thread routine */ void *thread(void *vargp) { int myid = (int) vargp; static int cnt = 0; printf("[%d]: %s (svar=%d)\n", myid, ptr[myid], ++cnt); } Peer threads reference main thread’s stack indirectly through global ptr variable

36. Mapping Variable Instances to Memory Global variables  Def: Variable declared outside of a function  Virtual memory contains exactly one instance of any global variable Local variables  Def: Variable declared inside function without static attribute  Each thread stack contains one instance of each local variable Local static variables  Def: Variable declared inside function with the static attribute  Virtual memory contains exactly one instance of any local static variable.

37. Mapping Variable Instances to Memory char **ptr; /* global */ int main() { int i; pthread_t tid; char *msgs[2] = { "Hello from foo", "Hello from bar" }; ptr = msgs; for (i = 0; i < 2; i++) Pthread_create(&tid, NULL, thread, (void *)i); Pthread_exit(NULL); } /* thread routine */ void *thread(void *vargp) { int myid = (int)vargp; static int cnt = 0; printf("[%d]: %s (svar=%d)\n", myid, ptr[myid], ++cnt); } Global var: 1 instance ( ptr [data]) Local static var: 1 instance ( cnt [data]) Local vars: 1 instance ( i.m, msgs.m ) Local var: 2 instances ( myid.p0 [peer thread 0’s stack], myid.p1 [peer thread 1’s stack] )

38. Shared Variable Analysis Which variables are shared? Answer: A variable x is shared iff multiple threads reference at least one instance of x. Thus: ptr, cnt, and msgs are shared i and myid are not shared Variable Referenced byReferenced by Referenced by instance main thread?peer thread 0?peer thread 1? ptr cnt i.m msgs.m myid.p0 myid.p1 yes noyes no yes noyesno yes

39. Synchronization: basic Sharing Mutual exclusion Semaphores

40. badcnt.c : Improper Synchronization volatile int cnt = 0; /* global */ int main(int argc, char **argv) { int niters = atoi(argv[1]); pthread_t tid1, tid2; Pthread_create(&tid1, NULL, thread, &niters); Pthread_create(&tid2, NULL, thread, &niters); Pthread_join(tid1, NULL); Pthread_join(tid2, NULL); /* Check result */ if (cnt != (2 * niters)) printf("BOOM! cnt=%d\n”, cnt); else printf("OK cnt=%d\n", cnt); exit(0); } /* Thread routine */ void *thread(void *vargp) { int i, niters = *((int *)vargp); for (i = 0; i < niters; i++) cnt++; return NULL; } linux>./badcnt 1000000 OK cnt=2000000 linux>./badcnt 1000000 BOOM! cnt=1302251 linux> cnt should equal 2,000,000. What went wrong?

41. Assembly Code for Counter Loop movl (%rdi),%ecx movl $0,%edx cmpl %ecx,%edx jge.L13.L11: movl cnt(%rip),%eax incl %eax movl %eax,cnt(%rip) incl %edx cmpl %ecx,%edx jl.L11.L13: Corresponding assembly code for (i=0; i < niters; i++) cnt++; C code for counter loop in thread i Head (H i ) Tail (T i ) Load cn t (L i ) Update cn t (U i ) Store cnt (S i )

42. Concurrent Execution Key idea: In general, any sequentially consistent interleaving is possible, but some give an unexpected result!  I i denotes that thread i executes instruction I  %eax i is the content of %eax in thread i’s context H1H1 L1L1 U1U1 S1S1 H2H2 L2L2 U2U2 S2S2 T2T2 T1T1 1 1 1 1 2 2 2 2 2 1 - 0 1 1 - - - - - 1 0 0 0 1 1 1 1 2 2 2 i (thread) instr i cnt%eax 1 OK - - - - - 1 2 2 2 - %eax 2 Thread 1 critical section Thread 2 critical section

43. Concurrent Execution (cont) Incorrect ordering: two threads increment the counter, but the result is 1 instead of 2 H1H1 L1L1 U1U1 H2H2 L2L2 S1S1 T1T1 U2U2 S2S2 T2T2 1 1 1 2 2 1 1 2 2 2 - 0 1 - - 1 1 - - - 0 0 0 0 0 1 1 1 1 1 i (thread) instr i cnt%eax 1 - - - - 0 - - 1 1 1 %eax 2 Oops!

44. Concurrent Execution (cont) How about this ordering? We can analyze the behavior using a progress graph H1H1 L1L1 H2H2 L2L2 U2U2 S2S2 U1U1 S1S1 T1T1 T2T2 1 1 2 2 2 2 1 1 1 2 i (thread) instr i cnt%eax 1 %eax 2 0 0 0 1 1 1 1 1 1 1 Oops!

45. Progress Graphs A progress graph depicts the discrete execution state space of concurrent threads. Each axis corresponds to the sequential order of instructions in a thread. Each point corresponds to a possible execution state (Inst 1, Inst 2 ). E.g., (L 1, S 2 ) denotes state where thread 1 has completed L 1 and thread 2 has completed S 2. H1H1 L1L1 U1U1 S1S1 T1T1 H2H2 L2L2 U2U2 S2S2 T2T2 Thread 1 Thread 2 (L 1, S 2 )

46. Trajectories in Progress Graphs A trajectory is a sequence of legal state transitions that describes one possible concurrent execution of the threads. Example: H1, L1, U1, H2, L2, S1, T1, U2, S2, T2 H1H1 L1L1 U1U1 S1S1 T1T1 H2H2 L2L2 U2U2 S2S2 T2T2 Thread 1 Thread 2

47. Critical Sections and Unsafe Regions L, U, and S form a critical section with respect to the shared variable cnt Instructions in critical sections (wrt to some shared variable) should not be interleaved Sets of states where such interleaving occurs form unsafe regions H1H1 L1L1 U1U1 S1S1 T1T1 H2H2 L2L2 U2U2 S2S2 T2T2 Thread 1 Thread 2 critical section wrt cnt Unsafe region

48. Critical Sections and Unsafe Regions H1H1 L1L1 U1U1 S1S1 T1T1 H2H2 L2L2 U2U2 S2S2 T2T2 Thread 1 Thread 2 critical section wrt cnt Unsafe region Def: A trajectory is safe iff it does not enter any unsafe region Claim: A trajectory is correct (wrt cnt ) iff it is safe unsafe safe

49. Enforcing Mutual Exclusion Question: How can we guarantee a safe trajectory? Answer: We must synchronize the execution of the threads so that they never have an unsafe trajectory.  i.e., need to guarantee mutually exclusive access to critical regions Classic solution:  Semaphores (Edsger Dijkstra) Other approaches (out of our scope)  Mutex and condition variables (Pthreads)  Monitors (Java)

50. Synchronization: basic Sharing Mutual exclusion Semaphores

51. Semaphore: non-negative global integer synchronization variable Manipulated by P and V operations:  P(s): [ while (s == 0) wait(); s--; ]  Dutch for "Proberen" (test)  V(s): [ s++; ]  Dutch for "Verhogen" (increment) OS kernel guarantees that operations between brackets [ ] are executed indivisibly  Only one P or V operation at a time can modify s.  When while loop in P terminates, only that P can decrement s Semaphore invariant: (s >= 0)

52. C Semaphore Operations Pthreads functions: #include int sem_init(sem_t *sem, 0, unsigned int val);} /* s = val */ int sem_wait(sem_t *s); /* P(s) */ int sem_post(sem_t *s); /* V(s) */ CS:APP wrapper functions: #include "csapp.h” void P(sem_t *s); /* Wrapper function for sem_wait */ void V(sem_t *s); /* Wrapper function for sem_post */

53. badcnt.c : Improper Synchronization volatile int cnt = 0; /* global */ int main(int argc, char **argv) { int niters = atoi(argv[1]); pthread_t tid1, tid2; Pthread_create(&tid1, NULL, thread, &niters); Pthread_create(&tid2, NULL, thread, &niters); Pthread_join(tid1, NULL); Pthread_join(tid2, NULL); /* Check result */ if (cnt != (2 * niters)) printf("BOOM! cnt=%d\n”, cnt); else printf("OK cnt=%d\n", cnt); exit(0); } /* Thread routine */ void *thread(void *vargp) { int i, niters = *((int *)vargp); for (i = 0; i < niters; i++) cnt++; return NULL; } How can we fix this using semaphores?

54. Using Semaphores for Mutual Exclusion Basic idea:  Associate a unique semaphore mutex, initially 1, with each shared variable (or related set of shared variables).  Surround corresponding critical sections with P(mutex) and V(mutex) operations. Terminology:  Binary semaphore: semaphore whose value is always 0 or 1  Mutex: binary semaphore used for mutual exclusion  P operation: “locking” the mutex  V operation: “unlocking” or “releasing” the mutex  “Holding” a mutex: locked and not yet unlocked.  Counting semaphore: used as a counter for set of available resources.

55. goodcnt.c: Proper Synchronization Define and initialize a mutex for the shared variable cnt: volatile int cnt = 0; /* Counter */ sem_t mutex; /* Semaphore that protects cnt */ Sem_init(&mutex, 0, 1); /* mutex = 1 */ Surround critical section with P and V: for (i = 0; i < niters; i++) { P(&mutex); cnt++; V(&mutex); } linux>./goodcnt 1000000 OK cnt=2000000 linux>./goodcnt 1000000 OK cnt=2000000 linux> Warning: It’s much slower than badcnt.c.

56. Unsafe region Why Mutexes Work Provide mutually exclusive access to shared variable by surrounding critical section with P and V operations on semaphore s (initially set to 1) Semaphore invariant creates a forbidden region that encloses unsafe region that cannot be entered by any trajectory. H1H1 P(s)V(s)T1T1 Thread 1 Thread 2 L1L1 U1U1 S1S1 H2H2 P(s) V(s) T2T2 L2L2 U2U2 S2S2 11000011 11000011 00 00 00 00 00 00 00 00 11000011 11000011 Initially s = 1 Forbidden region

57. Summary Programmers need a clear model of how variables are shared by threads. Variables shared by multiple threads must be protected to ensure mutually exclusive access. Semaphores are a fundamental mechanism for enforcing mutual exclusion.

58. 3. Synchronization: Advanced

59. Synchronization: advanced Producer-consumer problem Thread safety Races Deadlocks

60. Using Semaphores to Schedule Access to Shared Resources Basic idea: Thread uses a semaphore operation to notify another thread that some condition has become true  Use counting semaphores to keep track of resource state.  Use binary semaphores to notify other threads. Two classic examples:  The Producer-Consumer Problem  The Readers-Writers Problem

61. Producer-Consumer Problem Common synchronization pattern:  Producer waits for empty slot, inserts item in buffer, and notifies consumer  Consumer waits for item, removes it from buffer, and notifies producer Examples  Multimedia processing:  Producer creates MPEG video frames, consumer renders them  Event-driven graphical user interfaces  Producer detects mouse clicks, mouse movements, and keyboard hits and inserts corresponding events in buffer  Consumer retrieves events from buffer and paints the display producer thread shared buffer consumer thread

62. Producer-Consumer on 1-element Buffer #include “csapp.h” #define NITERS 5 void *producer(void *arg); void *consumer(void *arg); struct { int buf; /* shared var */ sem_t full; /* sems */ sem_t empty; } shared; int main() { pthread_t tid_producer; pthread_t tid_consumer; /* Initialize the semaphores */ Sem_init(&shared.empty, 0, 1); Sem_init(&shared.full, 0, 0); /* Create threads and wait */ Pthread_create(&tid_producer, NULL, producer, NULL); Pthread_create(&tid_consumer, NULL, consumer, NULL); Pthread_join(tid_producer, NULL); Pthread_join(tid_consumer, NULL); exit(0); }

63. Producer-Consumer on 1-element Buffer void *producer(void *arg) { int i, item; for (i=0; i<NITERS; i++) { /* Produce item */ item = i; printf("produced %d\n", item); /* Write item to buf */ P(&shared.empty); shared.buf = item; V(&shared.full); } return NULL; } void *consumer(void *arg) { int i, item; for (i=0; i<NITERS; i++) { /* Read item from buf */ P(&shared.full); item = shared.buf; V(&shared.empty); /* Consume item */ printf("consumed %d\n“, item); } return NULL; } Initially: empty==1, full==0 Producer ThreadConsumer Thread

64. Producer-Consumer on an n-element Buffer Requires a mutex and two counting semaphores:  mutex : enforces mutually exclusive access to the the buffer  slots : counts the available slots in the buffer  items : counts the available items in the buffer Implemented using a shared buffer package called sbuf.

65. sbuf Package - Declarations #include "csapp.h” typedef struct { int *buf; /* Buffer array */ int n; /* Maximum number of slots */ int front; /* buf[(front+1)%n] is first item */ int rear; /* buf[rear%n] is last item */ sem_t mutex; /* Protects accesses to buf */ sem_t slots; /* Counts available slots */ sem_t items; /* Counts available items */ } sbuf_t; void sbuf_init(sbuf_t *sp, int n); void sbuf_deinit(sbuf_t *sp); void sbuf_insert(sbuf_t *sp, int item); int sbuf_remove(sbuf_t *sp); sbuf.h

66. sbuf Package - Implementation /* Create an empty, bounded, shared FIFO buffer with n slots */ void sbuf_init(sbuf_t *sp, int n) { sp->buf = Calloc(n, sizeof(int)); sp->n = n; /* Buffer holds max of n items */ sp->front = sp->rear = 0; /* Empty buffer iff front == rear */ Sem_init(&sp->mutex, 0, 1); /* Binary semaphore for locking */ Sem_init(&sp->slots, 0, n); /* Initially, buf has n empty slots */ Sem_init(&sp->items, 0, 0); /* Initially, buf has zero items */ } /* Clean up buffer sp */ void sbuf_deinit(sbuf_t *sp) { Free(sp->buf); } sbuf.c Initializing and deinitializing a shared buffer:

67. sbuf Package - Implementation /* Insert item onto the rear of shared buffer sp */ void sbuf_insert(sbuf_t *sp, int item) { P(&sp->slots); /* Wait for available slot */ P(&sp->mutex); /* Lock the buffer */ sp->buf[(++sp->rear)%(sp->n)] = item; /* Insert the item */ V(&sp->mutex); /* Unlock the buffer */ V(&sp->items); /* Announce available item */ } sbuf.c Inserting an item into a shared buffer:

68. sbuf Package - Implementation /* Remove and return the first item from buffer sp */ int sbuf_remove(sbuf_t *sp) { int item; P(&sp->items); /* Wait for available item */ P(&sp->mutex); /* Lock the buffer */ item = sp->buf[(++sp->front)%(sp->n)]; /* Remove the item */ V(&sp->mutex); /* Unlock the buffer */ V(&sp->slots); /* Announce available slot */ return item; } sbuf.c Removing an item from a shared buffer:

69. Case Study: Prethreaded Concurrent Server Master thread Buffer... Accept connections Insert descriptors Remove descriptors Worker thread Worker thread Client... Service client Pool of worker threads

70. Prethreaded Concurrent Server sbuf_t sbuf; /* Shared buffer of connected descriptors */ int main(int argc, char **argv) { int i, listenfd, connfd, port; socklen_t clientlen=sizeof(struct sockaddr_in); struct sockaddr_in clientaddr; pthread_t tid; port = atoi(argv[1]); sbuf_init(&sbuf, SBUFSIZE); listenfd = Open_listenfd(port); for (i = 0; i < NTHREADS; i++) /* Create worker threads */ Pthread_create(&tid, NULL, thread, NULL); while (1) { connfd = Accept(listenfd, (SA *) &clientaddr, &clientlen); sbuf_insert(&sbuf, connfd); /* Insert connfd in buffer */ } echoservert_pre.c

71. Prethreaded Concurrent Server void *thread(void *vargp) { Pthread_detach(pthread_self()); while (1) { int connfd = sbuf_remove(&sbuf); /* Remove connfd from buffer */ echo_cnt(connfd); /* Service client */ Close(connfd); } echoservert_pre.c Worker thread routine:

72. Prethreaded Concurrent Server static int byte_cnt; /* Byte counter */ static sem_t mutex; /* and the mutex that protects it */ static void init_echo_cnt(void) { Sem_init(&mutex, 0, 1); byte_cnt = 0; } echo_cnt.c echo_cnt initialization routine:

73. Prethreaded Concurrent Server void echo_cnt(int connfd) { int n; char buf[MAXLINE]; rio_t rio; static pthread_once_t once = PTHREAD_ONCE_INIT; Pthread_once(&once, init_echo_cnt); Rio_readinitb(&rio, connfd); while((n = Rio_readlineb(&rio, buf, MAXLINE)) != 0) { P(&mutex); byte_cnt += n; printf("thread %d received %d (%d total) bytes on fd %d\n”, (int) pthread_self(), n, byte_cnt, connfd); V(&mutex); Rio_writen(connfd, buf, n); } Worker thread service routine: echo_cnt.c

74. Synchronization: advanced Producer-consumer problem Readers-writers problem Thread safety Races Deadlocks

75. Crucial concept: Thread Safety Functions called from a thread must be thread-safe Def: A function is thread-safe iff it will always produce correct results when called repeatedly from multiple concurrent threads. Classes of thread-unsafe functions:  Class 1: Functions that do not protect shared variables.  Class 2: Functions that keep state across multiple invocations.  Class 3: Functions that return a pointer to a static variable.  Class 4: Functions that call thread-unsafe functions.

76. Thread-Unsafe Functions (Class 1) Failing to protect shared variables  Fix: Use P and V semaphore operations  Example: goodcnt.c  Issue: Synchronization operations will slow down code

77. Thread-Unsafe Functions (Class 2) Relying on persistent state across multiple function invocations  Example: Random number generator that relies on static state static unsigned int next = 1; /* rand: return pseudo-random integer on 0..32767 */ int rand(void) { next = next*1103515245 + 12345; return (unsigned int)(next/65536) % 32768; } /* srand: set seed for rand() */ void srand(unsigned int seed) { next = seed; }

78. Thread-Safe Random Number Generator Pass state as part of argument  and, thereby, eliminate static state Consequence: programmer using rand_r must maintain seed /* rand_r - return pseudo-random integer on 0..32767 */ int rand_r(int *nextp) { *nextp = *nextp*1103515245 + 12345; return (unsigned int)(*nextp/65536) % 32768; }

79. Thread-Unsafe Functions (Class 3) Returning a pointer to a static variable Fix 1. Rewrite function so caller passes address of variable to store result  Requires changes in caller and callee Fix 2. Lock-and-copy  Requires simple changes in caller (and none in callee)  However, caller must free memory. /* lock-and-copy version */ char *ctime_ts(const time_t *timep, char *privatep) { char *sharedp; P(&mutex); sharedp = ctime(timep); strcpy(privatep, sharedp); V(&mutex); return privatep; } Warning: Some functions like gethostbyname require a deep copy. Use reentrant gethostbyname_r version instead.

80. Thread-Unsafe Functions (Class 4) Calling thread-unsafe functions  Calling one thread-unsafe function makes the entire function that calls it thread-unsafe  Fix: Modify the function so it calls only thread-safe functions

81. Reentrant Functions Def: A function is reentrant iff it accesses no shared variables when called by multiple threads.  Important subset of thread-safe functions.  Require no synchronization operations.  Only way to make a Class 2 function thread-safe is to make it reentrant (e.g., rand_r ) Reentrant functions All functions Thread-unsafe functions Thread-safe functions

82. Thread-Safe Library Functions All functions in the Standard C Library (at the back of your K&R text) are thread-safe  Examples: malloc, free, printf, scanf Most Unix system calls are thread-safe, with a few exceptions: Thread-unsafe functionClassReentrant version asctime 3asctime_r ctime 3ctime_r gethostbyaddr 3gethostbyaddr_r gethostbyname 3gethostbyname_r inet_ntoa 3(none) localtime 3localtime_r rand 2rand_r

83. Synchronization: advanced Producer-consumer problem Readers-writers problem Thread safety Races Deadlocks

84. One Worry: Races A race occurs when correctness of the program depends on one thread reaching point x before another thread reaches point y /* a threaded program with a race */ int main() { pthread_t tid[N]; int i; for (i = 0; i < N; i++) Pthread_create(&tid[i], NULL, thread, &i); for (i = 0; i < N; i++) Pthread_join(tid[i], NULL); exit(0); } /* thread routine */ void *thread(void *vargp) { int myid = *((int *)vargp); printf("Hello from thread %d\n", myid); return NULL; } race.c

85. Race Elimination Make sure don’t have unintended sharing of state /* a threaded program without the race */ int main() { pthread_t tid[N]; int i; for (i = 0; i < N; i++) { int *valp = malloc(sizeof(int)); *valp = i; Pthread_create(&tid[i], NULL, thread, valp); } for (i = 0; i < N; i++) Pthread_join(tid[i], NULL); exit(0); } /* thread routine */ void *thread(void *vargp) { int myid = *((int *)vargp); free(vargp); printf("Hello from thread %d\n", myid); return NULL; } norace.c

86. Synchronization: advanced Producer-consumer problem Readers-writers problem Thread safety Races Deadlocks

87. Another Worry: Deadlock Def: A process is deadlocked iff it is waiting for a condition that will never be true. Typical Scenario  Processes 1 and 2 needs two resources (A and B) to proceed  Process 1 acquires A, waits for B  Process 2 acquires B, waits for A  Both will wait forever!

88. Deadlocking With Semaphores int main() { pthread_t tid[2]; Sem_init(&mutex[0], 0, 1); /* mutex[0] = 1 */ Sem_init(&mutex[1], 0, 1); /* mutex[1] = 1 */ Pthread_create(&tid[0], NULL, count, (void*) 0); Pthread_create(&tid[1], NULL, count, (void*) 1); Pthread_join(tid[0], NULL); Pthread_join(tid[1], NULL); printf("cnt=%d\n", cnt); exit(0); } void *count(void *vargp) { int i; int id = (int) vargp; for (i = 0; i < NITERS; i++) { P(&mutex[id]); P(&mutex[1-id]); cnt++; V(&mutex[id]); V(&mutex[1-id]); } return NULL; } Tid[0]: P(s 0 ); P(s 1 ); cnt++; V(s 0 ); V(s 1 ); Tid[1]: P(s 1 ); P(s 0 ); cnt++; V(s 1 ); V(s 0 );

89. Deadlock Visualized in Progress Graph Locking introduces the potential for deadlock: waiting for a condition that will never be true Any trajectory that enters the deadlock region will eventually reach the deadlock state, waiting for either s 0 or s 1 to become nonzero Other trajectories luck out and skirt the deadlock region Unfortunate fact: deadlock is often nondeterministic Thread 1 Thread 2 P(s 0 )V(s 0 )P(s 1 )V(s 1 ) P(s 1 ) P(s 0 ) V(s 0 ) Forbidden region for s 0 Forbidden region for s 1 Deadlock state Deadlock region s 0 = s 1 =1

90. Avoiding Deadlock int main() { pthread_t tid[2]; Sem_init(&mutex[0], 0, 1); /* mutex[0] = 1 */ Sem_init(&mutex[1], 0, 1); /* mutex[1] = 1 */ Pthread_create(&tid[0], NULL, count, (void*) 0); Pthread_create(&tid[1], NULL, count, (void*) 1); Pthread_join(tid[0], NULL); Pthread_join(tid[1], NULL); printf("cnt=%d\n", cnt); exit(0); } void *count(void *vargp) { int i; int id = (int) vargp; for (i = 0; i < NITERS; i++) { P(&mutex[0]); P(&mutex[1]); cnt++; V(&mutex[id]); V(&mutex[1-id]); } return NULL; } Tid[0]: P(s0); P(s1); cnt++; V(s0); V(s1); Tid[1]: P(s0); P(s1); cnt++; V(s1); V(s0); Acquire shared resources in same order

91. Avoided Deadlock in Progress Graph Thread 1 Thread 2 P(s 0 )V(s 0 )P(s 1 )V(s 1 ) P(s 1 ) P(s 0 ) V(s 0 ) Forbidden region for s 0 Forbidden region for s 1 s 0 = s 1 =1 No way for trajectory to get stuck Processes acquire locks in same order Order in which locks released immaterial

92. Threads Summary Threads provide another mechanism for writing concurrent programs Threads are growing in popularity  Somewhat cheaper than processes  Easy to share data between threads However, the ease of sharing has a cost:  Easy to introduce subtle synchronization errors  Tread carefully with threads! For more info:  D. Butenhof, “Programming with Posix Threads”, Addison-Wesley, 1997