[linux.git] / lib / dec_and_lock.c

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

/*
 * This is an 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".
 */
int _atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock)
{
	/* Subtract 1 from counter unless that drops it to 0 (ie. it was 1) */
	if (atomic_add_unless(atomic, -1, 1))
		return 0;

	/* Otherwise do it the slow way */
	spin_lock(lock);
	if (atomic_dec_and_test(atomic))
		return 1;
	spin_unlock(lock);
	return 0;
}

EXPORT_SYMBOL(_atomic_dec_and_lock);
Commit	Line	Data
1da177e4 LT	1	#include <linux/module.h>
	2	#include <linux/spinlock.h>
	3	#include <asm/atomic.h>
	4
4db2ce01 DM	5	/*
	6	* This is an implementation of the notion of "decrement a
	7	* reference count, and return locked if it decremented to zero".
	8	*
1da177e4 LT	9	* NOTE NOTE NOTE! This is _not_ equivalent to
	10	*
	11	* if (atomic_dec_and_test(&atomic)) {
	12	* spin_lock(&lock);
	13	* return 1;
	14	* }
	15	* return 0;
	16	*
	17	* because the spin-lock and the decrement must be
	18	* "atomic".
1da177e4	19	*/
1da177e4 LT	20	int _atomic_dec_and_lock(atomic_t atomic, spinlock_t lock)
1da177e4 LT	21	{
a57004e1 NP	22	/* Subtract 1 from counter unless that drops it to 0 (ie. it was 1) */
	23	if (atomic_add_unless(atomic, -1, 1))
	24	return 0;
417dcdf9	25
a57004e1	26	/* Otherwise do it the slow way */
1da177e4 LT	27	spin_lock(lock);
	28	if (atomic_dec_and_test(atomic))
	29	return 1;
	30	spin_unlock(lock);
	31	return 0;
	32	}
	33
	34	EXPORT_SYMBOL(_atomic_dec_and_lock);