/* SPDX-License-Identifier: GPL-2.0 */
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/*
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* MCS lock defines
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*
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* This file contains the main data structure and API definitions of MCS lock.
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*
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* The MCS lock (proposed by Mellor-Crummey and Scott) is a simple spin-lock
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* with the desirable properties of being fair, and with each cpu trying
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* to acquire the lock spinning on a local variable.
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* It avoids expensive cache bouncings that common test-and-set spin-lock
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* implementations incur.
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*/
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#ifndef __LINUX_MCS_SPINLOCK_H
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#define __LINUX_MCS_SPINLOCK_H
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#include <asm/mcs_spinlock.h>
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struct mcs_spinlock {
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struct mcs_spinlock *next;
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int locked; /* 1 if lock acquired */
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int count; /* nesting count, see qspinlock.c */
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};
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#ifndef arch_mcs_spin_lock_contended
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/*
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* Using smp_cond_load_acquire() provides the acquire semantics
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* required so that subsequent operations happen after the
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* lock is acquired. Additionally, some architectures such as
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* ARM64 would like to do spin-waiting instead of purely
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* spinning, and smp_cond_load_acquire() provides that behavior.
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*/
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#define arch_mcs_spin_lock_contended(l) \
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do { \
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smp_cond_load_acquire(l, VAL); \
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} while (0)
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#endif
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#ifndef arch_mcs_spin_unlock_contended
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/*
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* smp_store_release() provides a memory barrier to ensure all
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* operations in the critical section has been completed before
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* unlocking.
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*/
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#define arch_mcs_spin_unlock_contended(l) \
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smp_store_release((l), 1)
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#endif
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/*
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* Note: the smp_load_acquire/smp_store_release pair is not
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* sufficient to form a full memory barrier across
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* cpus for many architectures (except x86) for mcs_unlock and mcs_lock.
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* For applications that need a full barrier across multiple cpus
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* with mcs_unlock and mcs_lock pair, smp_mb__after_unlock_lock() should be
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* used after mcs_lock.
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*/
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/*
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* In order to acquire the lock, the caller should declare a local node and
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* pass a reference of the node to this function in addition to the lock.
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* If the lock has already been acquired, then this will proceed to spin
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* on this node->locked until the previous lock holder sets the node->locked
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* in mcs_spin_unlock().
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*/
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static inline
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void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
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{
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struct mcs_spinlock *prev;
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/* Init node */
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node->locked = 0;
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node->next = NULL;
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/*
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* We rely on the full barrier with global transitivity implied by the
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* below xchg() to order the initialization stores above against any
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* observation of @node. And to provide the ACQUIRE ordering associated
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* with a LOCK primitive.
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*/
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prev = xchg(lock, node);
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if (likely(prev == NULL)) {
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/*
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* Lock acquired, don't need to set node->locked to 1. Threads
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* only spin on its own node->locked value for lock acquisition.
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* However, since this thread can immediately acquire the lock
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* and does not proceed to spin on its own node->locked, this
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* value won't be used. If a debug mode is needed to
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* audit lock status, then set node->locked value here.
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*/
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return;
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}
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WRITE_ONCE(prev->next, node);
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/* Wait until the lock holder passes the lock down. */
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arch_mcs_spin_lock_contended(&node->locked);
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}
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/*
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* Releases the lock. The caller should pass in the corresponding node that
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* was used to acquire the lock.
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*/
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static inline
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void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
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{
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struct mcs_spinlock *next = READ_ONCE(node->next);
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if (likely(!next)) {
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/*
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* Release the lock by setting it to NULL
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*/
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if (likely(cmpxchg_release(lock, node, NULL) == node))
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return;
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/* Wait until the next pointer is set */
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while (!(next = READ_ONCE(node->next)))
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cpu_relax();
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}
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/* Pass lock to next waiter. */
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arch_mcs_spin_unlock_contended(&next->locked);
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}
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#endif /* __LINUX_MCS_SPINLOCK_H */
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