Programming with Threads Dec 5, 2002Shared Variables in Threaded C ProgramsThreads Memory ModelExample of Threads Accessing Another Thread’s StackMapping Variables to Mem. InstancesShared Variable Analysisbadcnt.c: An Improperly Synchronized Threaded ProgramAssembly Code for Counter LoopConcurrent ExecutionConcurrent Execution (cont)Slide 11Progress GraphsTrajectories in Progress GraphsCritical Sections and Unsafe RegionsSafe and Unsafe TrajectoriesSemaphoresSafe Sharing with SemaphoresSafe Sharing With SemaphoresPOSIX SemaphoresSharing With POSIX SemaphoresSignaling With SemaphoresProducer-Consumer on a Buffer That Holds One ItemProducer-Consumer (cont)Thread SafetyThread-Unsafe FunctionsThread-Unsafe Functions (cont)Slide 27Slide 28Reentrant FunctionsThread-Safe Library FunctionsRacesDeadlockThreads SummaryProgramming with ThreadsDec 5, 2002Programming with ThreadsDec 5, 2002TopicsTopicsShared variablesThe need for synchronizationSynchronizing with semaphoresThread safety and reentrancyRaces and deadlocksclass29.ppt15-213“The course that gives CMU its Zip!”– 2 –15-213, F’02Shared Variables in Threaded C ProgramsShared Variables in Threaded C ProgramsQuestion: Which variables in a threaded C program are Question: Which variables in a threaded C program are shared variables?shared variables?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: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, F’02Threads Memory ModelThreads Memory ModelConceptual model:Conceptual model:Each thread runs in the context of a process.Each thread has its own separate thread context.Thread ID, stack, stack pointer, program counter, condition codes, and general purpose 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 handlersOperationally, this model is not strictly enforced:Operationally, 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 Mismatch between the conceptual and operation model is a source of confusion and errors.of confusion and errors.– 4 –15-213, F’02Example of Threads Accessing Another Thread’s StackExample 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, 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 iff multiple threads reference at least one instance of x. Thus:Answer: 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, F’02badcnt.c: An Improperly Synchronized Threaded Programbadcnt.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); 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 consistent Key idea: In general, any sequentially consistent interleaving is possible, but some are incorrect!interleaving is possible, but some are incorrect!Ii denotes 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, F’02Concurrent Execution (cont)Concurrent Execution (cont)Incorrect ordering: two threads increment the counter, Incorrect ordering: two threads increment the counter, but the result
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