[PATCH] atomic: dec_and_lock use atomic primitives

#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)) {

 *

 * 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)

{

#ifdef CONFIG_SMP

	/* Subtract 1 from counter unless that drops it to 0 (ie. it was 1) */

	if (atomic_add_unless(atomic, -1, 1))

		return 0;

#endif

	/* Otherwise do it the slow way */

	spin_lock(lock);

	if (atomic_dec_and_test(atomic))

		return 1;

	spin_unlock(lock);

	return 0;

}

#endif

EXPORT_SYMBOL(_atomic_dec_and_lock);