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

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“Paradise Lost”

Sep. 20, 2013

Dave Eckhardt Dave Eckhardt Todd Mowry Todd Mowry L11a_Lost

“What could possibly go wrong?”

Outline

When to use When to use if()if() vs.vs. while()while()

What's Wrong With This Picture?

workitem * find_work(void) { workitem *w; mutex_lock(&m); if (going_out_of_business) w = (workitem *) 0; else w = (workitem *) dequeue(q); mutex_unlock(&m); return (w); }

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);

What Went Wrong?

What went wrong? What went wrong?

What Went Wrong?

What went wrong? What went wrong?  (^) Nothing!

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);

Have We Seen This Before?

What went wrong? 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”? When have we seen this “happiness revocation”?

Summary

if() vs. while() if() vs. while()  (^) If somebody can revoke your happiness, you'd better check

Related Work

TOCTTOU TOCTTOU

 (^) This is “just a sub-case of 'TOCTTOU bugs'”, but...  (^) Many people think TOCCTOU bugs are always security bugs  (^) Fundamentally, we expect the revoked condition to become unrevoked again (soon!)  (^) Unlike the general case, this can be fxed in less than a line of code!