This interface is an implementation of sleeping reader-writer exclusion, which serves as an alternative for spinning ones. Reader-writer semaphores are represented by struct rw_semaphore, declared in the kernel header <linux/rwsem.h>:

struct rw_semaphore {
        atomic_long_t count;
        struct list_head wait_list;
        raw_spinlock_t wait_lock;
#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
       struct optimistic_spin_queue osq; /* spinner MCS lock */
       /*
       * Write owner. Used as a speculative check to see
       * if the owner is running on the cpu.
       */
      struct task_struct *owner;
#endif
#ifdef CONFIG_DEBUG_LOCK_ALLOC
     struct lockdep_map dep_map;
#endif
};

This structure is identical to that of a mutex, and is designed to support optimistic spinning with osq; it also includes debug support through the kernel's lockdep. Count serves as an exclusion counter, which is set to 1, allowing a maximum of one writer to own the lock at a point in time. This works since mutual exclusion is only enforced between contending writers, and any number of readers can concurrently share the read lock. wait_lock is a spinlock which protects the semaphore wait_list.

An rw_semaphore can be instantiated and initialized statically through DECLARE_RWSEM(name), and alternatively, it can be dynamically initialized through init_rwsem(sem).

As with the case of rw-spinlocks, this interface too offers distinct routines for lock acquisition in reader and writer paths. Following is a list of interface operations:

/* reader interfaces */
   void down_read(struct rw_semaphore *sem);
   void up_read(struct rw_semaphore *sem);
/* trylock for reading -- returns 1 if successful, 0 if contention */
   int down_read_trylock(struct rw_semaphore *sem);
   void up_read(struct rw_semaphore *sem);

/* writer Interfaces */
   void down_write(struct rw_semaphore *sem);
   int __must_check down_write_killable(struct rw_semaphore *sem);

/* trylock for writing -- returns 1 if successful, 0 if contention */
   int down_write_trylock(struct rw_semaphore *sem); 
   void up_write(struct rw_semaphore *sem);
/* downgrade write lock to read lock */
   void downgrade_write(struct rw_semaphore *sem); 

/* check if rw-sem is currently locked */  
   int rwsem_is_locked(struct rw_semaphore *sem);

These operations are implemented in the source file <kernel/locking/rwsem.c>; the code is quite self explanatory and we will not discuss it any further.