Synchronization December 6, 2006Shared 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 11Beware of Optimizing Compilers!Controlling Optimizing Compilers!Progress GraphsTrajectories in Progress GraphsCritical Sections and Unsafe RegionsSafe and Unsafe TrajectoriesSemaphoresSafe Sharing with SemaphoresSafe Sharing With SemaphoresWrappers on POSIX SemaphoresSharing With POSIX SemaphoresSignaling With SemaphoresProducer-Consumer on a Buffer That Holds One ItemProducer-Consumer (cont)Thread SafetyThread-Unsafe FunctionsThread-Unsafe Functions (cont)Slide 29Slide 30Reentrant FunctionsThread-Safe Library FunctionsRacesDeadlockDeadlocking With POSIX SemaphoresSlide 36Avoiding DeadlockRemoved DeadlockThreads SummarySynchronizationDecember 6, 2006TopicsShared variablesThe need for synchronizationSynchronizing with semaphoresThread safety and reentrancyRaces and deadlocksclass26.ppt15-213“The course that gives CMU its Zip!”– 2 –15-213, F’06Shared 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, F’06Threads Memory ModelConceptual model:Multiple threads run within the context of a single process.Each thread has its own separate thread contextThread 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 spaceOpen 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, F’06Example 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’06Mapping 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’06Shared 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, F’06badcnt.c: An Improperly Synchronized Threaded Program/* shared */volatile unsigned int cnt = 0;#define NITERS 100000000 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’06Assembly 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 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’06Concurrent ExecutionKey idea: In general, any sequentially consistent 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’06Concurrent 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, F’06Concurrent 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, F’06Beware of Optimizing Compilers!Global variable cnt shared between threadsMultiple threads could be trying to update within their iterationsCompiler moved access to cnt out of loopOnly shared accesses to cnt occur before loop (read) or after (write)What are possible program outcomes?#define NITERS 100000000/* shared counter variable */unsigned
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