[linux.git] / lib / dec_and_lock.c

// SPDX-License-Identifier: GPL-2.0
#include <linux/export.h>
#include <linux/spinlock.h>
#include <linux/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
b2441318	1	// SPDX-License-Identifier: GPL-2.0
8bc3bcc9	2	#include <linux/export.h>
1da177e4	3	#include <linux/spinlock.h>
60063497	4	#include <linux/atomic.h>
1da177e4	5
4db2ce01 DM	6	/*
	7	* This is an implementation of the notion of "decrement a
	8	* reference count, and return locked if it decremented to zero".
	9	*
1da177e4 LT	10	* NOTE NOTE NOTE! This is _not_ equivalent to
	11	*
	12	* if (atomic_dec_and_test(&atomic)) {
	13	* spin_lock(&lock);
	14	* return 1;
	15	* }
	16	* return 0;
	17	*
	18	* because the spin-lock and the decrement must be
	19	* "atomic".
1da177e4	20	*/
1da177e4 LT	21	int _atomic_dec_and_lock(atomic_t atomic, spinlock_t lock)
1da177e4 LT	22	{
a57004e1 NP	23	/* Subtract 1 from counter unless that drops it to 0 (ie. it was 1) */
	24	if (atomic_add_unless(atomic, -1, 1))
	25	return 0;
417dcdf9	26
a57004e1	27	/* Otherwise do it the slow way */
1da177e4 LT	28	spin_lock(lock);
	29	if (atomic_dec_and_test(atomic))
	30	return 1;
	31	spin_unlock(lock);
	32	return 0;
	33	}
	34
	35	EXPORT_SYMBOL(_atomic_dec_and_lock);