failing like never before


Something is Not Quite Right

Take a look at this C function, and see if you can can catch whats wrong with it. (That is, what about this function could produce an error?)

void wait_for_ready(void)
    while ((inb(0x1F7) & 0x80) || !(inb(0x1F7) & 0x40))

This question showed up on my midterm and stumped the hell out of me at the time. Now that I know the answer, I feel like a complete idiot for not spotting the problem initially, seeing as its so amazingly obvious. When I took the exam, I spent way too much time on this problem, completely overthinking it.

In this function, we're using a machine level instruction, inb, to read the value of some hardware port on an x86 machines. So far so good, or so I thought when I was taking my exam. But the problem is, is that the data we're reading can be a shared resource, that is, other applications could be writing and reading to it at the same time, so a race condition ensues. Even on a machine with a single processor, this is still a problem, since wait_for_ready() could read the data memory into register, then a context switch could occur, some application could write to that location, and then wait_for_read() regains control but operates with an old data value.

So simple. And now I feel like an idiot.