[537] Concurrency Bugs Chapter 32 Tyler Harter 10/22/14 Review - PowerPoint PPT Presentation

[537] Concurrency Bugs Chapter 32 Tyler Harter 10/22/14

Review Semaphores

CV’s vs. Semaphores CV rules of thumb: - Keep state in addition to CV’s - Always do wait/signal with lock held - Whenever you acquire a lock, recheck state � How do semaphores eliminate these needs?

Condition Variable (CV) Thread Queue: Thread Queue: Signal Queue: Semaphore

Condition Variable (CV) Thread Queue: A wait() Thread Queue: Signal Queue: A Semaphore

Condition Variable (CV) Thread Queue: A Thread Queue: Signal Queue: A Semaphore

Condition Variable (CV) Thread Queue: A signal() Thread Queue: Signal Queue: A Semaphore

Condition Variable (CV) Thread Queue: signal() Thread Queue: Signal Queue: Semaphore

Condition Variable (CV) Thread Queue: Thread Queue: Signal Queue: Semaphore

Condition Variable (CV) Thread Queue: signal() Thread Queue: Signal Queue: signal Semaphore

Condition Variable (CV) Thread Queue: Thread Queue: Signal Queue: signal Semaphore

Condition Variable (CV) Thread Queue: B wait() Thread Queue: Signal Queue: signal B Semaphore

Condition Variable (CV) Thread Queue: B wait() Thread Queue: Signal Queue: Semaphore

Condition Variable (CV) Thread Queue: B Thread Queue: Signal Queue: Semaphore

Condition Variable (CV) Thread Queue: may wait forever B (if not careful) Thread Queue: Signal Queue: Semaphore

Condition Variable (CV) Thread Queue: may wait forever B (if not careful) Thread Queue: Signal Queue: just use counter Semaphore

Join w/ CV int done = 0; mutex_t m = MUTEX_INIT; cond_t c = COND_INIT; void *child(void *arg) { printf(“child\n”); Mutex_lock(&m); done = 1; cond_signal(&c); Mutex_unlock(&m); } � int main(int argc, char *argv[]) { pthread_t c; printf(“parent: begin\n”); Pthread_create(c, NULL, child, NULL); Mutex_lock(&m); while(done == 0) Cond_wait(&c, &m); Mutex_unlock(&m); printf(“parent: end\n”); }

Join w/ CV int done = 0; mutex_t m = MUTEX_INIT; extra state and mutex cond_t c = COND_INIT; void *child(void *arg) { printf(“child\n”); Mutex_lock(&m); locks around state/signal done = 1; cond_signal(&c); Mutex_unlock(&m); } � int main(int argc, char *argv[]) { pthread_t c; printf(“parent: begin\n”); Pthread_create(c, NULL, child, NULL); Mutex_lock(&m); while(done == 0) while loop for checking state Cond_wait(&c, &m); Mutex_unlock(&m); printf(“parent: end\n”); }

Join w/ CV int done = 0; mutex_t m = MUTEX_INIT; cond_t c = COND_INIT; void *child(void *arg) { printf(“child\n”); Mutex_lock(&m); done = 1; cond_signal(&c); Mutex_unlock(&m); } � int main(int argc, char *argv[]) { pthread_t c; printf(“parent: begin\n”); Pthread_create(c, NULL, child, NULL); Mutex_lock(&m); while(done == 0) Cond_wait(&c, &m); Mutex_unlock(&m); printf(“parent: end\n”); }

sem_t s; Join w/ Semaphore void *child(void *arg) { printf(“child\n”); sem_post(&s); } � int main(int argc, char *argv[]) { sem_init(&s, 0); pthread_t c; printf(“parent: begin\n”); Pthread_create(c, NULL, child, NULL); sem_wait(&s); printf(“parent: end\n”); }

Semaphore Uses For the following init’s, what might the use be? � (a) sem_init(&s, 0 ); � (b) sem_init(&s, 1 ); � (c) sem_init(&s, N );

Producer/Consumer How many semaphores do we need?

Producer/Consumer How many semaphores do we need? � � Sem_init(&empty, max); // max are empty Sem_init(&full, 0); // 0 are full Sem_init(&mutex, 1); // mutex

Producer/Consumer void *producer(void *arg) { void *consumer(void *arg) { for (int i = 0; i < loops; i++) { while (1) { Sem_wait(&empty); Sem_wait(&full); Sem_wait(&mutex); Sem_wait(&mutex); do_fill(i); tmp = do_get(); Sem_post(&mutex); Sem_post(&mutex); Sem_post(&full); Sem_post(&empty); } printf("%d\n", tmp); } } }

Producer/Consumer void *producer(void *arg) { void *consumer(void *arg) { for (int i = 0; i < loops; i++) { while (1) { Sem_wait(&empty); Sem_wait(&full); Sem_wait(&mutex); Sem_wait(&mutex); do_fill(i); tmp = do_get(); Sem_post(&mutex); Sem_post(&mutex); Sem_post(&full); Sem_post(&empty); } printf("%d\n", tmp); } } } Mutual Exclusion

Producer/Consumer void *producer(void *arg) { void *consumer(void *arg) { for (int i = 0; i < loops; i++) { while (1) { Sem_wait(&empty); Sem_wait(&full); Sem_wait(&mutex); Sem_wait(&mutex); do_fill(i); tmp = do_get(); Sem_post(&mutex); Sem_post(&mutex); Sem_post(&full); Sem_post(&empty); } printf("%d\n", tmp); } } } Signaling

Concurrency Bugs

Concurrency in Medicine: Therac-25 “The accidents occurred when the high-power electron beam was activated instead of the intended low power beam, and without the beam spreader plate rotated into place. Previous models had hardware interlocks in place to prevent this, but Therac-25 had removed them, depending instead on software interlocks for safety. The software interlock could fail due to a race condition .” Source: http://en.wikipedia.org/wiki/Therac-25

Concurrency in Medicine: Therac-25 “The accidents occurred when the high-power electron beam was activated instead of the intended low power beam, and without the beam spreader plate rotated into place. Previous models had hardware interlocks in place to prevent this, but Therac-25 had removed them, depending instead on software interlocks for safety. The software interlock could fail due to a race condition .” � “…in three cases, the injured patients later died .” Source: http://en.wikipedia.org/wiki/Therac-25

Concurrency in Medicine: Therac-25 “The accidents occurred when the high-power electron beam was activated instead of the intended low power beam, and without the beam spreader plate rotated into place. Previous models had hardware interlocks in place to prevent this, but Therac-25 had removed them, depending instead on software interlocks for safety. The software interlock could fail due to a race condition .” � “…in three cases, the injured patients later died .” � Getting concurrency right can sometimes save lives! Source: http://en.wikipedia.org/wiki/Therac-25

Concurrency Bugs are Common and Various Atomicity Order Deadlock Other Lu etal. Study: � 60 � For four major projects, 45 search for concurrency bugs among >500K bug Bugs 30 reports. Analyze small sample to identify 15 common types of concurrency bugs. 0 MySQL Apache Mozilla OpenOffice Source: http://pages.cs.wisc.edu/~shanlu/paper/asplos122-lu.pdf

Atomicity: MySQL Thread 1: Thread 2: � � if (thd->proc_info) { thd->proc_info = NULL; … fputs(thd->proc_info, …); … } What’s wrong?

Atomicity: MySQL Thread 1: Thread 2: � � pthread_mutex_lock(&lock); pthread_mutex_lock(&lock); if (thd->proc_info) { thd->proc_info = NULL; … pthread_mutex_unlock(&lock); fputs(thd->proc_info, …); … } pthread_mutex_unlock(&lock);

Ordering: Mozilla Thread 1: Thread 2: � � void init() { void mMain(…) { … … mThread mState = mThread->State; = PR_CreateThread(mMain, …); … … } }

Ordering: Mozilla Thread 1: Thread 2: � � void init() { void mMain(…) { … … mThread Mutex_lock(&mtLock); = PR_CreateThread(mMain, …); while(mtInit == 0) Cond_wait(&mtCond, &mtLock); � pthread_mutex_lock(&mtLock); Mutex_unlock(&mtLock); mtInit = 1; � pthread_cond_signal(&mtCond); mState = mThread->State; pthread_mutex_unlock(&mtLock); … … } }

[537] Concurrency Bugs Chapter 32 Tyler Harter 10/22/14 Review - PowerPoint PPT Presentation

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