mcs_spinlock.h

  1/* SPDX-License-Identifier: GPL-2.0 */
  2/*
  3 * MCS lock defines
  4 *
  5 * This file contains the main data structure and API definitions of MCS lock.
  6 *
  7 * The MCS lock (proposed by Mellor-Crummey and Scott) is a simple spin-lock
  8 * with the desirable properties of being fair, and with each cpu trying
  9 * to acquire the lock spinning on a local variable.
 10 * It avoids expensive cache bounces that common test-and-set spin-lock
 11 * implementations incur.
 12 */
 13#ifndef __LINUX_MCS_SPINLOCK_H
 14#define __LINUX_MCS_SPINLOCK_H
 15
 16#include <asm/mcs_spinlock.h>
 17
 18#ifndef arch_mcs_spin_lock_contended
 19/*
 20 * Using smp_cond_load_acquire() provides the acquire semantics
 21 * required so that subsequent operations happen after the
 22 * lock is acquired. Additionally, some architectures such as
 23 * ARM64 would like to do spin-waiting instead of purely
 24 * spinning, and smp_cond_load_acquire() provides that behavior.
 25 */
 26#define arch_mcs_spin_lock_contended(l)					\
 27	smp_cond_load_acquire(l, VAL)
 28#endif
 29
 30#ifndef arch_mcs_spin_unlock_contended
 31/*
 32 * smp_store_release() provides a memory barrier to ensure all
 33 * operations in the critical section has been completed before
 34 * unlocking.
 35 */
 36#define arch_mcs_spin_unlock_contended(l)				\
 37	smp_store_release((l), 1)
 38#endif
 39
 40/*
 41 * Note: the smp_load_acquire/smp_store_release pair is not
 42 * sufficient to form a full memory barrier across
 43 * cpus for many architectures (except x86) for mcs_unlock and mcs_lock.
 44 * For applications that need a full barrier across multiple cpus
 45 * with mcs_unlock and mcs_lock pair, smp_mb__after_unlock_lock() should be
 46 * used after mcs_lock.
 47 */
 48
 49/*
 50 * In order to acquire the lock, the caller should declare a local node and
 51 * pass a reference of the node to this function in addition to the lock.
 52 * If the lock has already been acquired, then this will proceed to spin
 53 * on this node->locked until the previous lock holder sets the node->locked
 54 * in mcs_spin_unlock().
 55 */
 56static inline
 57void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
 58{
 59	struct mcs_spinlock *prev;
 60
 61	/* Init node */
 62	node->locked = 0;
 63	node->next   = NULL;
 64
 65	/*
 66	 * We rely on the full barrier with global transitivity implied by the
 67	 * below xchg() to order the initialization stores above against any
 68	 * observation of @node. And to provide the ACQUIRE ordering associated
 69	 * with a LOCK primitive.
 70	 */
 71	prev = xchg(lock, node);
 72	if (likely(prev == NULL)) {
 73		/*
 74		 * Lock acquired, don't need to set node->locked to 1. Threads
 75		 * only spin on its own node->locked value for lock acquisition.
 76		 * However, since this thread can immediately acquire the lock
 77		 * and does not proceed to spin on its own node->locked, this
 78		 * value won't be used. If a debug mode is needed to
 79		 * audit lock status, then set node->locked value here.
 80		 */
 81		return;
 82	}
 83	WRITE_ONCE(prev->next, node);
 84
 85	/* Wait until the lock holder passes the lock down. */
 86	arch_mcs_spin_lock_contended(&node->locked);
 87}
 88
 89/*
 90 * Releases the lock. The caller should pass in the corresponding node that
 91 * was used to acquire the lock.
 92 */
 93static inline
 94void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
 95{
 96	struct mcs_spinlock *next = READ_ONCE(node->next);
 97
 98	if (likely(!next)) {
 99		/*
100		 * Release the lock by setting it to NULL
101		 */
102		if (likely(cmpxchg_release(lock, node, NULL) == node))
103			return;
104		/* Wait until the next pointer is set */
105		while (!(next = READ_ONCE(node->next)))
106			cpu_relax();
107	}
108
109	/* Pass lock to next waiter. */
110	arch_mcs_spin_unlock_contended(&next->locked);
111}
112
113#endif /* __LINUX_MCS_SPINLOCK_H */