Semaphore acquisition

There are three functions to acquire a semaphore:

void down(struct semaphore *sem);
Blocks (uninterruptible) until sem is available

int down_interruptible(struct semaphore *sem);
Blocks until sem is available
Can be interrupted by signals
Non-zero return indicates signal
Semaphore *not* acquired
This form should usually be used

int down_trylock(struct semaphore *sem);
Will acquire semaphore if available
But will not sleep
Return value of zero indicates success