[deliverable/linux.git] / kernel / locking / 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

#include <asm/mcs_spinlock.h>

struct mcs_spinlock {
	struct mcs_spinlock *next;
	int locked; /* 1 if lock acquired */
	int count;  /* nesting count, see qspinlock.c */
};

#ifndef arch_mcs_spin_lock_contended
/*
 * Using smp_load_acquire() provides a memory barrier that ensures
 * subsequent operations happen after the lock is acquired.
 */
#define arch_mcs_spin_lock_contended(l)					\
do {									\
	while (!(smp_load_acquire(l)))					\
		cpu_relax_lowlatency();					\
} while (0)
#endif

#ifndef arch_mcs_spin_unlock_contended
/*
 * smp_store_release() provides a memory barrier to ensure all
 * operations in the critical section has been completed before
 * unlocking.
 */
#define arch_mcs_spin_unlock_contended(l)				\
	smp_store_release((l), 1)
#endif

/*
 * 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.
 */

/*
 * In order to acquire the lock, the caller should declare a local node and
 * pass a reference of the node to this function in addition to the lock.
 * If the lock has already been acquired, then this will proceed to spin
 * on this node->locked until the previous lock holder sets the node->locked
 * in mcs_spin_unlock().
 */
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, don't need to set node->locked to 1. Threads
		 * only spin on its own node->locked value for lock acquisition.
		 * However, since this thread can immediately acquire the lock
		 * and does not proceed to spin on its own node->locked, this
		 * value won't be used. If a debug mode is needed to
		 * audit lock status, then set node->locked value here.
		 */
		return;
	}
	WRITE_ONCE(prev->next, node);

	/* Wait until the lock holder passes the lock down. */
	arch_mcs_spin_lock_contended(&node->locked);
}

/*
 * Releases the lock. The caller should pass in the corresponding node that
 * was used to acquire the lock.
 */
static inline
void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
{
	struct mcs_spinlock *next = READ_ONCE(node->next);

	if (likely(!next)) {
		/*
		 * Release the lock by setting it to NULL
		 */
		if (likely(cmpxchg(lock, node, NULL) == node))
			return;
		/* Wait until the next pointer is set */
		while (!(next = READ_ONCE(node->next)))
			cpu_relax_lowlatency();
	}

	/* Pass lock to next waiter. */
	arch_mcs_spin_unlock_contended(&next->locked);
}

#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
ddf1d169 TC	15	#include <asm/mcs_spinlock.h>
ddf1d169 TC	16
e7224674 TC	17	struct mcs_spinlock {
	18	struct mcs_spinlock *next;
	19	int locked; /* 1 if lock acquired */
a33fda35	20	int count; /* nesting count, see qspinlock.c */
e7224674 TC	21	};
e7224674 TC	22
e207552e WD	23	#ifndef arch_mcs_spin_lock_contended
	24	/*
	25	* Using smp_load_acquire() provides a memory barrier that ensures
	26	* subsequent operations happen after the lock is acquired.
	27	*/
	28	#define arch_mcs_spin_lock_contended(l) \
	29	do { \
	30	while (!(smp_load_acquire(l))) \
3a6bfbc9	31	cpu_relax_lowlatency(); \
e207552e WD	32	} while (0)
	33	#endif
	34
	35	#ifndef arch_mcs_spin_unlock_contended
	36	/*
	37	* smp_store_release() provides a memory barrier to ensure all
	38	* operations in the critical section has been completed before
	39	* unlocking.
	40	*/
	41	#define arch_mcs_spin_unlock_contended(l) \
	42	smp_store_release((l), 1)
	43	#endif
	44
e7224674 TC	45	/*
	46	* Note: the smp_load_acquire/smp_store_release pair is not
	47	* sufficient to form a full memory barrier across
	48	* cpus for many architectures (except x86) for mcs_unlock and mcs_lock.
	49	* For applications that need a full barrier across multiple cpus
	50	* with mcs_unlock and mcs_lock pair, smp_mb__after_unlock_lock() should be
	51	* used after mcs_lock.
	52	*/
5faeb8ad JL	53
	54	/*
	55	* In order to acquire the lock, the caller should declare a local node and
	56	* pass a reference of the node to this function in addition to the lock.
	57	* If the lock has already been acquired, then this will proceed to spin
	58	* on this node->locked until the previous lock holder sets the node->locked
	59	* in mcs_spin_unlock().
5faeb8ad	60	*/
e7224674 TC	61	static inline
	62	void mcs_spin_lock(struct mcs_spinlock *lock, struct mcs_spinlock node)
	63	{
	64	struct mcs_spinlock *prev;
	65
	66	/* Init node */
	67	node->locked = 0;
	68	node->next = NULL;
	69
	70	prev = xchg(lock, node);
	71	if (likely(prev == NULL)) {
5faeb8ad JL	72	/*
	73	* Lock acquired, don't need to set node->locked to 1. Threads
	74	* only spin on its own node->locked value for lock acquisition.
	75	* However, since this thread can immediately acquire the lock
	76	* and does not proceed to spin on its own node->locked, this
	77	* value won't be used. If a debug mode is needed to
	78	* audit lock status, then set node->locked value here.
	79	*/
e7224674 TC	80	return;
e7224674 TC	81	}
4d3199e4	82	WRITE_ONCE(prev->next, node);
e207552e WD	83
	84	/* Wait until the lock holder passes the lock down. */
	85	arch_mcs_spin_lock_contended(&node->locked);
e7224674 TC	86	}
e7224674 TC	87
5faeb8ad JL	88	/*
	89	* Releases the lock. The caller should pass in the corresponding node that
	90	* was used to acquire the lock.
	91	*/
e7224674 TC	92	static inline
	93	void mcs_spin_unlock(struct mcs_spinlock *lock, struct mcs_spinlock node)
	94	{
4d3199e4	95	struct mcs_spinlock *next = READ_ONCE(node->next);
e7224674 TC	96
	97	if (likely(!next)) {
	98	/*
	99	* Release the lock by setting it to NULL
	100	*/
5faeb8ad	101	if (likely(cmpxchg(lock, node, NULL) == node))
e7224674 TC	102	return;
e7224674 TC	103	/* Wait until the next pointer is set */
4d3199e4	104	while (!(next = READ_ONCE(node->next)))
3a6bfbc9	105	cpu_relax_lowlatency();
e7224674	106	}
e207552e WD	107
	108	/* Pass lock to next waiter. */
	109	arch_mcs_spin_unlock_contended(&next->locked);
e7224674 TC	110	}
	111
	112	#endif /* __LINUX_MCS_SPINLOCK_H */