linux.conf.au, January 2003
      PRINCIPLES: Race Conditions

We cannot call any function which might sleep (schedule()) while using ANY of these.

There are special primitives for protecting from other user contexts while in user context: they sleep if they have to wait:

down_interruptible(struct semaphore *) & up(struct semaphore *)

down_read(struct rw_semaphore *) & up_read(struct rw_semaphore *)
down_write(struct rw_semaphore *) & up_write(struct rw_semaphore*)