[mirror_ubuntu-artful-kernel.git] / include / linux / mcs_spinlock.h

/*
 * MCS lock defines
 *
 * This file contains the main data structure and API definitions of MCS lock.
 *
 * The MCS lock (proposed by Mellor-Crummey and Scott) is a simple spin-lock
 * with the desirable properties of being fair, and with each cpu trying
 * to acquire the lock spinning on a local variable.
 * It avoids expensive cache bouncings that common test-and-set spin-lock
 * implementations incur.
 */
#ifndef __LINUX_MCS_SPINLOCK_H
#define __LINUX_MCS_SPINLOCK_H

struct mcs_spinlock {
	struct mcs_spinlock *next;
	int locked; /* 1 if lock acquired */
};

/*
 * Note: the smp_load_acquire/smp_store_release pair is not
 * sufficient to form a full memory barrier across
 * cpus for many architectures (except x86) for mcs_unlock and mcs_lock.
 * For applications that need a full barrier across multiple cpus
 * with mcs_unlock and mcs_lock pair, smp_mb__after_unlock_lock() should be
 * used after mcs_lock.
 */
static inline
void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
{
	struct mcs_spinlock *prev;

	/* Init node */
	node->locked = 0;
	node->next   = NULL;

	prev = xchg(lock, node);
	if (likely(prev == NULL)) {
		/* Lock acquired */
		node->locked = 1;
		return;
	}
	ACCESS_ONCE(prev->next) = node;
	/*
	 * Wait until the lock holder passes the lock down.
	 * Using smp_load_acquire() provides a memory barrier that
	 * ensures subsequent operations happen after the lock is acquired.
	 */
	while (!(smp_load_acquire(&node->locked)))
		arch_mutex_cpu_relax();
}

static inline
void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
{
	struct mcs_spinlock *next = ACCESS_ONCE(node->next);

	if (likely(!next)) {
		/*
		 * Release the lock by setting it to NULL
		 */
		if (cmpxchg(lock, node, NULL) == node)
			return;
		/* Wait until the next pointer is set */
		while (!(next = ACCESS_ONCE(node->next)))
			arch_mutex_cpu_relax();
	}
	/*
	 * Pass lock to next waiter.
	 * smp_store_release() provides a memory barrier to ensure
	 * all operations in the critical section has been completed
	 * before unlocking.
	 */
	smp_store_release(&next->locked, 1);
}

#endif /* __LINUX_MCS_SPINLOCK_H */
Commit	Line	Data
e7224674 TC	1	/*
	2	* MCS lock defines
	3	*
	4	* This file contains the main data structure and API definitions of MCS lock.
	5	*
	6	* The MCS lock (proposed by Mellor-Crummey and Scott) is a simple spin-lock
	7	* with the desirable properties of being fair, and with each cpu trying
	8	* to acquire the lock spinning on a local variable.
	9	* It avoids expensive cache bouncings that common test-and-set spin-lock
	10	* implementations incur.
	11	*/
	12	#ifndef __LINUX_MCS_SPINLOCK_H
	13	#define __LINUX_MCS_SPINLOCK_H
	14
	15	struct mcs_spinlock {
	16	struct mcs_spinlock *next;
	17	int locked; /* 1 if lock acquired */
	18	};
	19
	20	/*
	21	* Note: the smp_load_acquire/smp_store_release pair is not
	22	* sufficient to form a full memory barrier across
	23	* cpus for many architectures (except x86) for mcs_unlock and mcs_lock.
	24	* For applications that need a full barrier across multiple cpus
	25	* with mcs_unlock and mcs_lock pair, smp_mb__after_unlock_lock() should be
	26	* used after mcs_lock.
	27	*/
	28	static inline
	29	void mcs_spin_lock(struct mcs_spinlock *lock, struct mcs_spinlock node)
	30	{
	31	struct mcs_spinlock *prev;
	32
	33	/* Init node */
	34	node->locked = 0;
	35	node->next = NULL;
	36
	37	prev = xchg(lock, node);
	38	if (likely(prev == NULL)) {
	39	/* Lock acquired */
	40	node->locked = 1;
	41	return;
	42	}
	43	ACCESS_ONCE(prev->next) = node;
	44	/*
	45	* Wait until the lock holder passes the lock down.
	46	* Using smp_load_acquire() provides a memory barrier that
	47	* ensures subsequent operations happen after the lock is acquired.
	48	*/
	49	while (!(smp_load_acquire(&node->locked)))
	50	arch_mutex_cpu_relax();
	51	}
	52
	53	static inline
	54	void mcs_spin_unlock(struct mcs_spinlock *lock, struct mcs_spinlock node)
	55	{
	56	struct mcs_spinlock *next = ACCESS_ONCE(node->next);
	57
	58	if (likely(!next)) {
	59	/*
	60	* Release the lock by setting it to NULL
	61	*/
	62	if (cmpxchg(lock, node, NULL) == node)
	63	return;
	64	/* Wait until the next pointer is set */
65	while (!(next = ACCESS_ONCE(node->next)))
66	arch_mutex_cpu_relax();
67	}
68	/*
69	* Pass lock to next waiter.
70	* smp_store_release() provides a memory barrier to ensure
71	* all operations in the critical section has been completed
72	* before unlocking.
73	*/
74	smp_store_release(&next->locked, 1);
75	}
76
77	#endif /* __LINUX_MCS_SPINLOCK_H */