It took years for the Internet to reach its first 100 computers. Today, 100 new ones join each second. And running deep within the silicon souls of most of these machines is the work of a technical wizard of remarkable power, a man described as a genius and a bully, a spiritual leader and a benevolent dictator. ... Linus Torvalds — who in person could be mistaken for just another paunchy, middle-aged suburban dad who happens to have a curiously large collection of stuffed penguin dolls — looms over the future of computing much as Bill Gates and the late Steve Jobs loom over its past and present. For Linux, the operating system that Torvalds created and named after himself, has come to dominate the exploding online world, making it more popular overall than rivals from Microsoft and Apple. ... But while Linux is fast, flexible and free, a growing chorus of critics warn that it has security weaknesses that could be fixed but haven’t been. Worse, as Internet security has surged as a subject of international concern, Torvalds has engaged in an occasionally profane standoff with experts on the subject. ... Linux has thrived in part because of Torvalds’s relentless focus on performance and reliability, both of which could suffer if more security features were added. Linux works on almost any chip in the world and is famously stable as it manages the demands of many programs at once, allowing computers to hum along for years at a time without rebooting. ... Yet even among Linux’s many fans there is growing unease about vulnerabilities in the operating system’s most basic, foundational elements — housed in something called “the kernel,” which Torvalds has personally managed since its creation in 1991.