linux/kernel/locking/mcs_spinlock.h

/* SPDX-License-Identifier: GPL-2.0 */
/*
 * 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 bounces 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 { … };

#ifndef arch_mcs_spin_lock_contended
/*
 * Using smp_cond_load_acquire() provides the acquire semantics
 * required so that subsequent operations happen after the
 * lock is acquired. Additionally, some architectures such as
 * ARM64 would like to do spin-waiting instead of purely
 * spinning, and smp_cond_load_acquire() provides that behavior.
 */
#define arch_mcs_spin_lock_contended(l) …
#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) …
#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)
{ … }

/*
 * 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)
{ … }

#endif /* __LINUX_MCS_SPINLOCK_H */