78 lines
2.1 KiB
C
78 lines
2.1 KiB
C
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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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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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};
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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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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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prev = xchg(lock, node);
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if (likely(prev == NULL)) {
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/* Lock acquired */
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node->locked = 1;
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return;
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}
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ACCESS_ONCE(prev->next) = node;
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/*
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* Wait until the lock holder passes the lock down.
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* Using smp_load_acquire() provides a memory barrier that
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* ensures subsequent operations happen after the lock is acquired.
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*/
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while (!(smp_load_acquire(&node->locked)))
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arch_mutex_cpu_relax();
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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 = ACCESS_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 (cmpxchg(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 = ACCESS_ONCE(node->next)))
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arch_mutex_cpu_relax();
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}
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/*
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* Pass lock to next waiter.
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* smp_store_release() provides a memory barrier to ensure
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* all operations in the critical section has been completed
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* before unlocking.
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*/
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smp_store_release(&next->locked, 1);
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}
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#endif /* __LINUX_MCS_SPINLOCK_H */
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