Until early versions of 2.6 kernel releases, semaphores were the primary form of sleep locks. A typical semaphore implementation comprises a counter, wait queue, and set of operations that can increment/decrement the counter atomically.
When a semaphore is used to protect a shared resource, its counter is initialized to a number greater than zero, which is considered to be unlocked state. A task seeking access to a shared resource begins by invoking the decrement operation on the semaphore. This call checks the semaphore counter; if it is found to be greater than zero, the counter is decremented and the function returns success. However, if the counter is found to be zero, the decrement operation puts the caller task to sleep until ...
