Wait Queue

An application waits on a device fd with poll() or epoll_wait(). The hardware has already raised an interrupt, but the application never wakes. Or the application wakes repeatedly, but read() returns no data.

This kind of bug usually does not live in one function. It comes from interrupts, buffers, wait queues, and poll semantics not agreeing with each other.

A hardware interrupt does not deliver data directly to an application. It only tells the CPU that a device event happened. The driver must turn that event into kernel-visible state and wake the processes waiting for it.