linux/lib/dec_and_lock.c

#include <linux/module.h>
#include <linux/spinlock.h>
#include <asm/atomic.h>
#include <asm/system.h>

#ifdef __HAVE_ARCH_CMPXCHG
/*
 * This is an implementation of the notion of "decrement a
 * reference count, and return locked if it decremented to zero".
 *
 * This implementation can be used on any architecture that
 * has a cmpxchg, and where atomic->value is an int holding
 * the value of the atomic (i.e. the high bits aren't used
 * for a lock or anything like that).
 */
int _atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock)
{
	int counter;
	int newcount;

	for (;;) {
		counter = atomic_read(atomic);
		newcount = counter - 1;
		if (!newcount)
			break;		/* do it the slow way */

		newcount = cmpxchg(&atomic->counter, counter, newcount);
		if (newcount == counter)
			return 0;
	}

	spin_lock(lock);
	if (atomic_dec_and_test(atomic))
		return 1;
	spin_unlock(lock);
	return 0;
}
#else
/*
 * This is an architecture-neutral, but slow,
 * implementation of the notion of "decrement
 * a reference count, and return locked if it
 * decremented to zero".
 *
 * NOTE NOTE NOTE! This is _not_ equivalent to
 *
 *	if (atomic_dec_and_test(&atomic)) {
 *		spin_lock(&lock);
 *		return 1;
 *	}
 *	return 0;
 *
 * because the spin-lock and the decrement must be
 * "atomic".
 *
 * This slow version gets the spinlock unconditionally,
 * and releases it if it isn't needed. Architectures
 * are encouraged to come up with better approaches,
 * this is trivially done efficiently using a load-locked
 * store-conditional approach, for example.
 */
int _atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock)
{
	spin_lock(lock);
	if (atomic_dec_and_test(atomic))
		return 1;
	spin_unlock(lock);
	return 0;
}
#endif

EXPORT_SYMBOL(_atomic_dec_and_lock);