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kernel/linux-imx6_3.14.28/include/asm-generic/mutex-xchg.h 3.72 KB
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  /*
   * include/asm-generic/mutex-xchg.h
   *
   * Generic implementation of the mutex fastpath, based on xchg().
   *
   * NOTE: An xchg based implementation might be less optimal than an atomic
   *       decrement/increment based implementation. If your architecture
   *       has a reasonable atomic dec/inc then you should probably use
   *	 asm-generic/mutex-dec.h instead, or you could open-code an
   *	 optimized version in asm/mutex.h.
   */
  #ifndef _ASM_GENERIC_MUTEX_XCHG_H
  #define _ASM_GENERIC_MUTEX_XCHG_H
  
  /**
   *  __mutex_fastpath_lock - try to take the lock by moving the count
   *                          from 1 to a 0 value
   *  @count: pointer of type atomic_t
   *  @fail_fn: function to call if the original value was not 1
   *
   * Change the count from 1 to a value lower than 1, and call <fail_fn> if it
   * wasn't 1 originally. This function MUST leave the value lower than 1
   * even when the "1" assertion wasn't true.
   */
  static inline void
  __mutex_fastpath_lock(atomic_t *count, void (*fail_fn)(atomic_t *))
  {
  	if (unlikely(atomic_xchg(count, 0) != 1))
  		/*
  		 * We failed to acquire the lock, so mark it contended
  		 * to ensure that any waiting tasks are woken up by the
  		 * unlock slow path.
  		 */
  		if (likely(atomic_xchg(count, -1) != 1))
  			fail_fn(count);
  }
  
  /**
   *  __mutex_fastpath_lock_retval - try to take the lock by moving the count
   *                                 from 1 to a 0 value
   *  @count: pointer of type atomic_t
   *
   * Change the count from 1 to a value lower than 1. This function returns 0
   * if the fastpath succeeds, or -1 otherwise.
   */
  static inline int
  __mutex_fastpath_lock_retval(atomic_t *count)
  {
  	if (unlikely(atomic_xchg(count, 0) != 1))
  		if (likely(atomic_xchg(count, -1) != 1))
  			return -1;
  	return 0;
  }
  
  /**
   *  __mutex_fastpath_unlock - try to promote the mutex from 0 to 1
   *  @count: pointer of type atomic_t
   *  @fail_fn: function to call if the original value was not 0
   *
   * try to promote the mutex from 0 to 1. if it wasn't 0, call <function>
   * In the failure case, this function is allowed to either set the value to
   * 1, or to set it to a value lower than one.
   * If the implementation sets it to a value of lower than one, the
   * __mutex_slowpath_needs_to_unlock() macro needs to return 1, it needs
   * to return 0 otherwise.
   */
  static inline void
  __mutex_fastpath_unlock(atomic_t *count, void (*fail_fn)(atomic_t *))
  {
  	if (unlikely(atomic_xchg(count, 1) != 0))
  		fail_fn(count);
  }
  
  #define __mutex_slowpath_needs_to_unlock()		0
  
  /**
   * __mutex_fastpath_trylock - try to acquire the mutex, without waiting
   *
   *  @count: pointer of type atomic_t
   *  @fail_fn: spinlock based trylock implementation
   *
   * Change the count from 1 to a value lower than 1, and return 0 (failure)
   * if it wasn't 1 originally, or return 1 (success) otherwise. This function
   * MUST leave the value lower than 1 even when the "1" assertion wasn't true.
   * Additionally, if the value was < 0 originally, this function must not leave
   * it to 0 on failure.
   *
   * If the architecture has no effective trylock variant, it should call the
   * <fail_fn> spinlock-based trylock variant unconditionally.
   */
  static inline int
  __mutex_fastpath_trylock(atomic_t *count, int (*fail_fn)(atomic_t *))
  {
  	int prev = atomic_xchg(count, 0);
  
  	if (unlikely(prev < 0)) {
  		/*
  		 * The lock was marked contended so we must restore that
  		 * state. If while doing so we get back a prev value of 1
  		 * then we just own it.
  		 *
  		 * [ In the rare case of the mutex going to 1, to 0, to -1
  		 *   and then back to 0 in this few-instructions window,
  		 *   this has the potential to trigger the slowpath for the
  		 *   owner's unlock path needlessly, but that's not a problem
  		 *   in practice. ]
  		 */
  		prev = atomic_xchg(count, prev);
  		if (prev < 0)
  			prev = 0;
  	}
  
  	return prev;
  }
  
  #endif