/* 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 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_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)					\do {									\	smp_cond_load_acquire(l, VAL);					\} 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 inlinevoid mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node){	struct mcs_spinlock *prev; 	/* Init node */	node->locked = 0;	node->next   = NULL; 	/*	 * We rely on the full barrier with global transitivity implied by the	 * below xchg() to order the initialization stores above against any	 * observation of @node. And to provide the ACQUIRE ordering associated	 * with a LOCK primitive.	 */	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 inlinevoid 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_release(lock, node, NULL) == node))			return;		/* Wait until the next pointer is set */		while (!(next = READ_ONCE(node->next)))			cpu_relax();	} 	/* Pass lock to next waiter. */	arch_mcs_spin_unlock_contended(&next->locked);} #endif /* __LINUX_MCS_SPINLOCK_H */