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Release 4.15 kernel/locking/mcs_spinlock.h

Directory: kernel/locking
/* 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.


#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_load_acquire() provides a memory barrier that ensures
 * subsequent operations happen after the lock is acquired.

#define arch_mcs_spin_lock_contended(l)					\
do {                                                                    \
        while (!(smp_load_acquire(l)))                                  \
                cpu_relax();                                            \
} while (0)

#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)

 * 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); }


Tim Chen6989.61%120.00%
Davidlohr Bueso A33.90%120.00%
Will Deacon22.60%120.00%
Peter Zijlstra22.60%120.00%
Jason Low11.30%120.00%

/* * 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); }


Tim Chen7689.41%116.67%
Jason Low33.53%116.67%
Davidlohr Bueso A33.53%233.33%
Will Deacon22.35%116.67%
Christian Bornträger11.18%116.67%

#endif /* __LINUX_MCS_SPINLOCK_H */

Overall Contributors

Tim Chen17376.89%218.18%
Will Deacon3113.78%19.09%
Davidlohr Bueso A73.11%327.27%
Jason Low52.22%19.09%
Waiman Long41.78%19.09%
Christian Bornträger20.89%19.09%
Peter Zijlstra20.89%19.09%
Greg Kroah-Hartman10.44%19.09%
Directory: kernel/locking
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