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/* 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 {
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 inline
void 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 inline
void 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 */
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