Programming with ThreadsDec 5, 2002Programming with ThreadsDec 5, 2002TopicsTopicsn Shared variablesn The need for synchronizationn Synchronizing with semaphoresn Thread safety and reentrancyn Races and deadlocksclass29.ppt15-213“The course that gives CMU its Zip!”– 2 –15-213, F’02Shared Variables in Threaded CProgramsShared Variables in Threaded CProgramsQuestion: Which variables in a threaded C program areQuestion: Which variables in a threaded C program areshared variables?shared variables?n The answer is not as simple as “global variables are shared”and “stack variables are private”.Requires answers to the following questions:Requires answers to the following questions:n What is the memory model for threads?n How are variables mapped to memory instances?n How many threads reference each of these instances?– 3 –15-213, F’02Threads Memory ModelThreads Memory ModelConceptual model:Conceptual model:n Each thread runs in the context of a process.n Each thread has its own separate thread context.l Thread ID, stack, stack pointer, program counter, condition codes, andgeneral purpose registers.n All threads share the remaining process context.l Code, data, heap, and shared library segments of the process virtualaddress space.l Open files and installed handlersOperationally, this model is not strictly enforced:Operationally, this model is not strictly enforced:n While register values are truly separate and protected....n Any thread can read and write the stack of any other thread.Mismatch between the conceptual and operation model is a sourceMismatch between the conceptual and operation model is a sourceof confusion and errors.of confusion and errors.– 4 –15-213, F’02Example of Threads AccessingAnother Thread’s StackExample of Threads AccessingAnother 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, F’02Mapping Variables to Mem. InstancesMapping 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, F’02Shared Variable AnalysisShared Variable AnalysisWhich variables are shared?Which 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 Answer: A variable x is shared iff iff multiple threadsmultiple threadsreference at least one instance of x. Thus:reference at least one instance of x. Thus:n ptr, svar, and msgs are shared.n i and myid are NOT shared.– 7 –15-213, F’02badcnt.c: An ImproperlySynchronized Threaded Programbadcnt.c: An ImproperlySynchronized 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); else printf("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, F’02Assembly Code for Counter LoopAssembly 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, F’02Concurrent ExecutionConcurrent ExecutionKey idea: In general, any sequentially consistentKey idea: In general, any sequentially consistentinterleaving is possible, but some are incorrect!interleaving is possible, but some are incorrect!n Ii denotes that thread i executes instruction In %eaxi is the contents of %eax in thread i’s contextH1L1U1S1H2L2U2S2T2T11111222221-011-----10001111222i (thread)instricnt%eax1OK-----1222-%eax2– 10 –15-213, F’02Concurrent Execution (cont)Concurrent Execution (cont)Incorrect ordering: two threads increment the counter,Incorrect ordering: two threads increment the counter,but the result is 1 instead of 2.but the result is 1 instead of 2.H1L1U1H2L2S1T1U2S2T21112211222-01--11---0000011111i (thread)instricnt%eax1----0--111%eax2Oops!– 11 –15-213, F’02Concurrent Execution (cont)Concurrent Execution (cont)How about this ordering?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, F’02Progress GraphsProgress GraphsA progress graph depictsthe discrete execution state space of concurrent threads.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 L1 and thread2 has completed S2.H1L1U1S1T1H2L2U2S2T2Thread 1Thread 2(L1, S2)– 13 –15-213, F’02Trajectories in Progress GraphsTrajectories in Progress GraphsA trajectory is a sequence of
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