Paradise Lost - Operating System Design and Implementation - Lecture Slides, Slides of Operating Systems

Download Paradise Lost - Operating System Design and Implementation - Lecture Slides and more Slides Operating Systems in PDF only on Docsity! 15-410, F'131 “Paradise Lost” Sep. 20, 2013 Dave Eckhardt Todd Mowry L11a_Lost 15-410 “What could possibly go wrong?” Outline When to use if () vs. while() 15-410, F'13 15-410, F'135 Better? mutex_lock(&m); if (going_out_of_business) w = (workitem *) 0; else { if (!(w = (workitem *) dequeue(q))) { cond_wait(&new_work, &m); w = (workitem *) dequeue(queue); } } mutex_unlock(&m); return (w); 15-410, F'136 What We Hope For find_work() queue_work() mutex_lock(&m); if (!..dequeue(..)) cond_wait(&new, &m); mutex_lock(&m); enqueue(...) cond_signal(&new); mutex_unlock(&m); w = dequeue(..); mutex_unlock(&m); 15-410, F'137 What Went Wrong? What went wrong? 15-410, F'1310 Not Exactly What We Hope For find_work() queue_work() find_work() lock(&m); if (!..deq(.).) cwait(&new, &m); lock(&m); enqueue(...) csignal(&new); unlock(&m); lock(&m); if (!..deq(.).) unlock(&m); w = deq(.)... return(w); return (0); 15-410, F'1311 Have We Seen This Before? What went wrong?  Protected world state wasn't ready for us  We blocked  Somebody prepared the world for us to run  We ran  We assumed nobody else had run  We assumed the world state was still ready for us When have we seen this “happiness revocation”? 15-410, F'1312 To “if()” Or Not To “if()”? mutex_lock(&m); if (going_out_of_business) w = (workitem *) 0; else { while (!(w = (workitem *) dequeue(q))) cond_wait(&new_work, &m); } mutex_unlock(&m); return (w); /* XXX still wrong! - rewrite after class */