DOC PREVIEW
CMU CS 15213 - Lecture

This preview shows page 1-2-3 out of 9 pages.

Save
View full document
View full document
Premium Document
Do you want full access? Go Premium and unlock all 9 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 9 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 9 pages.
Access to all documents
Download any document
Ad free experience
Premium Document
Do you want full access? Go Premium and unlock all 9 pages.
Access to all documents
Download any document
Ad free experience

Unformatted text preview:

Programming with ThreadsApril 29, 2004Topics Shared variables The need for synchronization Synchronizing with semaphores Thread safety and reentrancy Races and deadlocksclass29.ppt15-213“The course that gives CMU its Zip!”–2–15-213, S’04Shared Variables in Threaded C ProgramsQuestion: Which variables in a threaded C program are shared variables? 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 variables mapped to memory instances? How many threads reference each of these instances?–3–15-213, S’04Threads Memory ModelConceptual model: Each thread runs in the context of a process. Each thread has its own separate thread context.z Thread ID, stack, stack pointer, program counter, condition codes, and general purpose registers. All threads share the remaining process context.z Code, data, heap, and shared library segments of the process virtual address space.z Open files and installed handlersOperationally, this model is not strictly enforced: While register values are truly separate and protected.... Any thread can read and write the stack of any other thread. Mismatch between the conceptual and operation model is a source of confusion and errors.–4–15-213, S’04Example of Threads Accessing Another Thread’s Stackchar **ptr; /* global */int main(){int i;pthread_t tid;char *msgs[N] = {"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 svar = 0;printf("[%d]: %s (svar=%d)\n", myid, ptr[myid], ++svar);}Peer threads access main thread’s stackindirectly through global ptr variable–5–15-213, S’04Mapping Variables to Mem. Instanceschar **ptr; /* global */int main(){int i;pthread_t tid;char *msgs[N] = {"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 svar = 0;printf("[%d]: %s (svar=%d)\n", myid, ptr[myid], ++svar);}Global var: 1 instance (ptr [data])Local static var: 1 instance (svar [data])Local automatic vars: 1 instance (i.m, msgs.m )Local automatic var: 2 instances (myid.p0[peer thread 0’s stack],myid.p1[peer thread 1’s stack])–6–15-213, S’04Shared Variable AnalysisWhich variables are shared?Variable Referenced by Referenced by Referenced byinstance main thread? peer thread 0? peer thread 1?ptr yes yes yessvar no yes yesi.m yes no nomsgs.m yes yes yesmyid.p0 no yes nomyid.p1 no no yesAnswer: A variable x is shared iff multiple threads reference at least one instance of x. Thus: ptr, svar, and msgs are shared. i and myid are NOT shared.–7–15-213, S’04badcnt.c: An Improperly Synchronized Threaded Programunsigned int cnt = 0; /* shared */int main() {pthread_t tid1, tid2;Pthread_create(&tid1, NULL, count, NULL);Pthread_create(&tid2, NULL, count, NULL);Pthread_join(tid1, NULL);Pthread_join(tid2, NULL);if (cnt != (unsigned)NITERS*2)printf("BOOM! cnt=%d\n", cnt);elseprintf("OK cnt=%d\n", cnt);}/* thread routine */void *count(void *arg) {int i;for (i=0; i<NITERS; i++)cnt++;return NULL;}linux> ./badcntBOOM! cnt=198841183linux> ./badcntBOOM! cnt=198261801linux> ./badcntBOOM! cnt=198269672cnt should beequal to 200,000,000. What went wrong?!–8–15-213, S’04Assembly Code for Counter Loop.L9:movl -4(%ebp),%eaxcmpl $99999999,%eaxjle .L12jmp .L10.L12:movl cnt,%eax # Loadleal 1(%eax),%edx # Updatemovl %edx,cnt # Store.L11:movl -4(%ebp),%eaxleal 1(%eax),%edxmovl %edx,-4(%ebp)jmp .L9.L10:Corresponding asm code(gcc -O0 -fforce-mem) for (i=0; i<NITERS; i++)cnt++;C code for counter loopHead (Hi)Tail (Ti)Load cnt (Li)Update cnt (Ui)Store cnt (Si)–9–15-213, S’04Concurrent ExecutionKey idea: In general, any sequentially consistent interleaving is possible, but some are incorrect! Iidenotes that thread i executes instruction I %eaxi is the contents of %eax in thread i’s contextH1L1U1S1H2L2U2S2T2T11111222221-011-----10001111222i (thread)instricnt%eax1OK-----1222-%eax2–10–15-213, S’04Concurrent Execution (cont)Incorrect ordering: two threads increment the counter, but the result is 1 instead of 2.H1L1U1H2L2S1T1U2S2T21112211222-01--11---0000011111i (thread)instricnt%eax1----0--111%eax2Oops!–11–15-213, S’04Concurrent Execution (cont)How about this ordering?H1L1H2L2U2S2U1S1T1T21122221112i (thread)instricnt%eax1%eax2We can clarify our understanding of concurrentexecution with the help of the progress graph–12–15-213, S’04Progress GraphsA progress graph depictsthe discrete execution state space of concurrentthreads.Each axis corresponds tothe sequential order ofinstructions in a thread.Each point corresponds toa possible execution state(Inst1, Inst2).E.g., (L1, S2) denotes statewhere thread 1 hascompleted L1and thread2 has completed S2.H1L1U1S1T1H2L2U2S2T2Thread 1Thread 2(L1, S2)–13–15-213, S’04Trajectories in Progress GraphsA trajectory is a sequence of legal state transitions that describes one possible concurrent execution ofthe threads.Example:H1, L1, U1, H2, L2, S1, T1, U2, S2, T2H1L1U1S1T1H2L2U2S2T2Thread 1Thread 2–14–15-213, S’04Critical Sections and Unsafe RegionsL, U, and S form a critical section withrespect to the sharedvariable cnt.Instructions in criticalsections (wrt to someshared variable) should not be interleaved.Sets of states where suchinterleaving occursform unsafe regions.H1L1U1S1T1H2L2U2S2T2Thread 1Thread 2Unsafe regioncritical section wrt cntcritical section wrt cnt–15–15-213, S’04Safe and Unsafe TrajectoriesDef: A trajectory is safeiff it doesn’t touch any part of an unsafe region.Claim: A trajectory is correct (wrt cnt) iff it issafe.H1L1U1S1T1H2L2U2S2T2Thread 1Thread 2Unsafe regionUnsafetrajectorySafe trajectorycritical section wrt cntcritical section wrt cnt–16–15-213, S’04SemaphoresQuestion: How can we guarantee a safe trajectory? We must synchronize the threads so that they never enter an unsafe state.Classic solution: Dijkstra's P and V operations on semaphores. semaphore: non-negative integer synchronization variable.z P(s): [ while (s == 0) wait(); s--; ]» Dutch for "Proberen" (test)z V(s): [ s++; ]» Dutch for "Verhogen" (increment) OS


View Full Document

CMU CS 15213 - Lecture

Documents in this Course
lecture

lecture

14 pages

lecture

lecture

46 pages

Caches

Caches

9 pages

lecture

lecture

39 pages

Lecture

Lecture

36 pages

Lecture

Lecture

45 pages

Lecture

Lecture

56 pages

lecture

lecture

11 pages

lecture

lecture

9 pages

Lecture

Lecture

36 pages

Lecture

Lecture

37 pages

Exam

Exam

16 pages

Lecture

Lecture

10 pages

Lecture

Lecture

43 pages

Lecture

Lecture

8 pages

Lecture

Lecture

8 pages

Lecture

Lecture

36 pages

Lecture

Lecture

43 pages

Lecture

Lecture

12 pages

Lecture

Lecture

37 pages

Lecture

Lecture

6 pages

Lecture

Lecture

40 pages

coding

coding

2 pages

Exam

Exam

17 pages

Exam

Exam

14 pages

Lecture

Lecture

29 pages

Lecture

Lecture

34 pages

Exam

Exam

11 pages

Lecture

Lecture

9 pages

Lecture

Lecture

37 pages

Lecture

Lecture

36 pages

lecture

lecture

46 pages

Lecture

Lecture

33 pages

Lecture

Lecture

57 pages

Lecture

Lecture

32 pages

Lecture

Lecture

46 pages

Lecture

Lecture

40 pages

Lecture

Lecture

11 pages

Lecture

Lecture

6 pages

Lecture

Lecture

43 pages

Lecture

Lecture

12 pages

Lecture

Lecture

18 pages

Exam

Exam

10 pages

Lecture

Lecture

45 pages

Lecture

Lecture

37 pages

Exam

Exam

24 pages

class09

class09

21 pages

class22

class22

37 pages

class20

class20

30 pages

class27

class27

33 pages

class25

class25

21 pages

class04

class04

31 pages

Lecture

Lecture

59 pages

class01a

class01a

14 pages

class12

class12

45 pages

class29

class29

33 pages

Lecture

Lecture

39 pages

Lecture

Lecture

6 pages

class03

class03

34 pages

lecture

lecture

42 pages

Lecture

Lecture

40 pages

Lecture

Lecture

47 pages

Exam

Exam

19 pages

R06-B

R06-B

25 pages

class17

class17

37 pages

class25

class25

31 pages

Lecture

Lecture

15 pages

final-f06

final-f06

17 pages

lecture

lecture

9 pages

Exam

Exam

15 pages

Lecture

Lecture

22 pages

class11

class11

45 pages

lecture

lecture

50 pages

Linking

Linking

37 pages

Lecture

Lecture

64 pages

Integers

Integers

40 pages

Exam

Exam

11 pages

Lecture

Lecture

37 pages

Lecture

Lecture

44 pages

Lecture

Lecture

37 pages

Lecture

Lecture

9 pages

Lecture

Lecture

37 pages

Lecture

Lecture

45 pages

Final

Final

25 pages

lecture

lecture

9 pages

Lecture

Lecture

30 pages

Lecture

Lecture

16 pages

Final

Final

17 pages

Lecture

Lecture

8 pages

Exam

Exam

11 pages

Lecture

Lecture

47 pages

Lecture

Lecture

9 pages

lecture

lecture

39 pages

Exam

Exam

11 pages

lecture

lecture

41 pages

lecture

lecture

37 pages

Lecture

Lecture

59 pages

Lecture

Lecture

45 pages

Exam 1

Exam 1

18 pages

Lecture

Lecture

41 pages

Lecture

Lecture

32 pages

Lecture

Lecture

30 pages

Lecture

Lecture

9 pages

Lecture

Lecture

9 pages

Lecture

Lecture

15 pages

Lecture

Lecture

11 pages

Lecture

Lecture

9 pages

Lecture

Lecture

34 pages

Lecture

Lecture

40 pages

Lecture

Lecture

4 pages

Lecture

Lecture

46 pages

Lecture

Lecture

8 pages

Lecture

Lecture

65 pages

Lecture

Lecture

38 pages

Lecture

Lecture

35 pages

Lecture

Lecture

8 pages

Lecture

Lecture

34 pages

Lecture

Lecture

8 pages

Exam

Exam

13 pages

Lecture

Lecture

43 pages

Lecture

Lecture

9 pages

Lecture

Lecture

12 pages

Lecture

Lecture

9 pages

Lecture

Lecture

34 pages

Lecture

Lecture

43 pages

Lecture

Lecture

7 pages

Lecture

Lecture

45 pages

Lecture

Lecture

24 pages

Lecture

Lecture

47 pages

Lecture

Lecture

12 pages

Lecture

Lecture

20 pages

Lecture

Lecture

9 pages

Exam

Exam

11 pages

Lecture

Lecture

52 pages

Lecture

Lecture

20 pages

Exam

Exam

11 pages

Lecture

Lecture

35 pages

Lecture

Lecture

47 pages

Lecture

Lecture

18 pages

Lecture

Lecture

30 pages

Lecture

Lecture

59 pages

Lecture

Lecture

37 pages

Lecture

Lecture

22 pages

Lecture

Lecture

35 pages

Exam

Exam

23 pages

Lecture

Lecture

9 pages

Lecture

Lecture

22 pages

class12

class12

32 pages

Lecture

Lecture

8 pages

Lecture

Lecture

39 pages

Lecture

Lecture

44 pages

Lecture

Lecture

38 pages

Lecture

Lecture

69 pages

Lecture

Lecture

41 pages

Lecture

Lecture

12 pages

Lecture

Lecture

52 pages

Lecture

Lecture

59 pages

Lecture

Lecture

39 pages

Lecture

Lecture

83 pages

Lecture

Lecture

59 pages

class01b

class01b

17 pages

Exam

Exam

21 pages

class07

class07

47 pages

Lecture

Lecture

11 pages

Odyssey

Odyssey

18 pages

multicore

multicore

66 pages

Lecture

Lecture

6 pages

lecture

lecture

41 pages

lecture

lecture

55 pages

lecture

lecture

52 pages

lecture

lecture

33 pages

lecture

lecture

46 pages

lecture

lecture

55 pages

lecture

lecture

17 pages

lecture

lecture

49 pages

Exam

Exam

17 pages

lecture

lecture

56 pages

Exam 2

Exam 2

16 pages

Exam 2

Exam 2

16 pages

Notes

Notes

37 pages

Lecture

Lecture

40 pages

Lecture

Lecture

36 pages

Lecture

Lecture

43 pages

Lecture

Lecture

25 pages

Exam

Exam

13 pages

Lecture

Lecture

32 pages

Lecture

Lecture

12 pages

Lecture

Lecture

58 pages

Lecture

Lecture

29 pages

Lecture

Lecture

59 pages

Lecture

Lecture

41 pages

Lecture

Lecture

50 pages

Exam

Exam

17 pages

Lecture

Lecture

29 pages

Lecture

Lecture

44 pages

Lecture

Lecture

41 pages

Lecture

Lecture

52 pages

Lecture

Lecture

40 pages

Lecture

Lecture

33 pages

lecture

lecture

10 pages

Lecture

Lecture

27 pages

Lecture

Lecture

29 pages

Lecture

Lecture

39 pages

Lecture

Lecture

9 pages

Lecture

Lecture

29 pages

Lecture

Lecture

8 pages

Lecture

Lecture

43 pages

Lecture

Lecture

43 pages

Lecture

Lecture

75 pages

Lecture

Lecture

55 pages

Exam

Exam

12 pages

Lecture

Lecture

43 pages

Lecture

Lecture

35 pages

lecture

lecture

36 pages

Exam

Exam

33 pages

lecture

lecture

56 pages

lecture

lecture

64 pages

lecture

lecture

8 pages

Exam

Exam

14 pages

Lecture

Lecture

43 pages

Lecture

Lecture

36 pages

lecture

lecture

56 pages

lecture

lecture

75 pages

lecture

lecture

36 pages

Lecture

Lecture

50 pages

Lecture

Lecture

45 pages

Lecture

Lecture

13 pages

Exam

Exam

23 pages

Lecture

Lecture

10 pages

Lecture

Lecture

48 pages

Lecture

Lecture

83 pages

lecture

lecture

57 pages

Lecture

Lecture

33 pages

Lecture

Lecture

39 pages

Lecture

Lecture

33 pages

lecture

lecture

54 pages

Lecture

Lecture

30 pages

Exam

Exam

13 pages

Lecture

Lecture

36 pages

Lecture

Lecture

40 pages

Exam

Exam

17 pages

Lecture

Lecture

9 pages

Exam

Exam

15 pages

Lecture

Lecture

44 pages

Lecture

Lecture

34 pages

Lecture

Lecture

24 pages

Lecture

Lecture

29 pages

class12

class12

43 pages

lecture

lecture

43 pages

class22

class22

22 pages

R06-B

R06-B

25 pages

class01b

class01b

19 pages

lecture

lecture

29 pages

lab1

lab1

8 pages

Caches

Caches

36 pages

lecture

lecture

55 pages

Lecture,

Lecture,

37 pages

Integers

Integers

40 pages

Linking

Linking

38 pages

lecture

lecture

45 pages

Lecture

Lecture

61 pages

Linking

Linking

33 pages

lecture

lecture

40 pages

lecture

lecture

40 pages

Lecture

Lecture

32 pages

lecture

lecture

48 pages

lecture

lecture

44 pages

Exam

Exam

11 pages

Lecture

Lecture

31 pages

Lecture

Lecture

46 pages

Lecture

Lecture

40 pages

Lecture

Lecture

40 pages

Exam

Exam

12 pages

Lecture

Lecture

42 pages

Lecture

Lecture

36 pages

Lecture

Lecture

45 pages

Lecture

Lecture

41 pages

Lecture

Lecture

13 pages

Lecture

Lecture

35 pages

Lecture

Lecture

20 pages

Final

Final

19 pages

Lecture

Lecture

33 pages

Lecture

Lecture

50 pages

Lecture

Lecture

33 pages

Lecture

Lecture

27 pages

Lecture

Lecture

6 pages

Exam

Exam

15 pages

Lecture

Lecture

24 pages

Lecture

Lecture

23 pages

Lecture

Lecture

43 pages

Lecture

Lecture

32 pages

Lecture

Lecture

52 pages

Lecture

Lecture

37 pages

Lecture

Lecture

36 pages

Lecture

Lecture

34 pages

Lecture

Lecture

40 pages

Lecture

Lecture

15 pages

lecture

lecture

21 pages

Lecture

Lecture

58 pages

Lecture

Lecture

49 pages

Lecture

Lecture

36 pages

Lecture

Lecture

11 pages

Lecture

Lecture

12 pages

Lecture

Lecture

58 pages

Lecture

Lecture

33 pages

Exam

Exam

15 pages

Lecture

Lecture

35 pages

Lecture

Lecture

10 pages

Lecture

Lecture

25 pages

Lecture

Lecture

31 pages

Lecture

Lecture

24 pages

Lecture

Lecture

34 pages

Lecture

Lecture

50 pages

lecture

lecture

35 pages

Lecture

Lecture

11 pages

Lecture

Lecture

39 pages

Lecture

Lecture

45 pages

Lecture

Lecture

41 pages

exam1-f05

exam1-f05

11 pages

Lecture

Lecture

4 pages

Lecture

Lecture

17 pages

Exam

Exam

17 pages

malloc()

malloc()

12 pages

Lecture

Lecture

57 pages

Lecture

Lecture

30 pages

Lecture

Lecture

30 pages

Lecture

Lecture

47 pages

Lecture

Lecture

33 pages

Exam

Exam

12 pages

Lecture

Lecture

43 pages

Lectures

Lectures

33 pages

Lecture

Lecture

36 pages

lecture

lecture

33 pages

Exam

Exam

14 pages

Lecture

Lecture

43 pages

Lecture

Lecture

25 pages

Load more
Download Lecture
Our administrator received your request to download this document. We will send you the file to your email shortly.
Loading Unlocking...
Login

Join to view Lecture and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view Lecture 2 2 and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?