Re: [PATCH v11 0/4] Introducing a queue read/write lockimplementation

[Peter Zijlstra]

On Thu, Jan 30, 2014 at 10:50:41AM -0500, Waiman Long wrote:
> One more thing, I often see line like
> 
> #define queue_write_unlock queue_write_unlock
> 
> So exactly what effect does this macro have?

Makes sure the below doesn't emit another version.

#ifndef queue_write_unlock
/**
 * queue_write_unlock - release write lock of a queue rwlock
 * @lock : Pointer to queue rwlock structure
 */
static inline void queue_write_unlock(struct qrwlock *lock)
{
	/*
	 * If the writer field is atomic, it can be cleared directly.
	 * Otherwise, an atomic subtraction will be used to clear it.
	 */
	smp_mb__before_atomic_dec();
	atomic_sub(_QW_LOCKED, &lock->cnts);
}
#endif
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