LXR linux/kernel/locking/mcs

   1/* SPDX-License-Identifier: GPL-2.0 */
   2/*
   3 * MCS lock defines
   4 *
   5 * This file contains the main data structure and API definitions of MCS lock.
   6 *
   7 * The MCS lock (proposed by Mellor-Crummey and Scott) is a simple spin-lock
   8 * with the desirable properties of being fair, and with each cpu trying
   9 * to acquire the lock spinning on a local variable.
  10 * It avoids expensive cache bouncings that common test-and-set spin-lock
  11 * implementations incur.
  12 */
  13#ifndef __LINUX_MCS_SPINLOCK_H
  14#define __LINUX_MCS_SPINLOCK_H
  15
  16#include <asm/mcs_spinlock.h>
  17
  18struct mcs_spinlock {
  19        struct mcs_spinlock *next;
  20        int locked; /* 1 if lock acquired */
  21        int count;  /* nesting count, see qspinlock.c */
  22};
  23
  24#ifndef arch_mcs_spin_lock_contended
  25/*
  26 * Using smp_load_acquire() provides a memory barrier that ensures
  27 * subsequent operations happen after the lock is acquired.
  28 */
  29#define arch_mcs_spin_lock_contended(l)                                 \
  30do {                                                                    \
  31        while (!(smp_load_acquire(l)))                                  \
  32                cpu_relax();                                            \
  33} while (0)
  34#endif
  35
  36#ifndef arch_mcs_spin_unlock_contended
  37/*
  38 * smp_store_release() provides a memory barrier to ensure all
  39 * operations in the critical section has been completed before
  40 * unlocking.
  41 */
  42#define arch_mcs_spin_unlock_contended(l)                               \
  43        smp_store_release((l), 1)
  44#endif
  45
  46/*
  47 * Note: the smp_load_acquire/smp_store_release pair is not
  48 * sufficient to form a full memory barrier across
  49 * cpus for many architectures (except x86) for mcs_unlock and mcs_lock.
  50 * For applications that need a full barrier across multiple cpus
  51 * with mcs_unlock and mcs_lock pair, smp_mb__after_unlock_lock() should be
  52 * used after mcs_lock.
  53 */
  54
  55/*
  56 * In order to acquire the lock, the caller should declare a local node and
  57 * pass a reference of the node to this function in addition to the lock.
  58 * If the lock has already been acquired, then this will proceed to spin
  59 * on this node->locked until the previous lock holder sets the node->locked
  60 * in mcs_spin_unlock().
  61 */
  62static inline
  63void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
  64{
  65        struct mcs_spinlock *prev;
  66
  67        /* Init node */
  68        node->locked = 0;
  69        node->next   = NULL;
  70
  71        /*
  72         * We rely on the full barrier with global transitivity implied by the
  73         * below xchg() to order the initialization stores above against any
  74         * observation of @node. And to provide the ACQUIRE ordering associated
  75         * with a LOCK primitive.
  76         */
  77        prev = xchg(lock, node);
  78        if (likely(prev == NULL)) {
  79                /*
  80                 * Lock acquired, don't need to set node->locked to 1. Threads
  81                 * only spin on its own node->locked value for lock acquisition.
  82                 * However, since this thread can immediately acquire the lock
  83                 * and does not proceed to spin on its own node->locked, this
  84                 * value won't be used. If a debug mode is needed to
  85                 * audit lock status, then set node->locked value here.
  86                 */
  87                return;
  88        }
  89        WRITE_ONCE(prev->next, node);
  90
  91        /* Wait until the lock holder passes the lock down. */
  92        arch_mcs_spin_lock_contended(&node->locked);
  93}
  94
  95/*
  96 * Releases the lock. The caller should pass in the corresponding node that
  97 * was used to acquire the lock.
  98 */
  99static inline
 100void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
 101{
 102        struct mcs_spinlock *next = READ_ONCE(node->next);
 103
 104        if (likely(!next)) {
 105                /*
 106                 * Release the lock by setting it to NULL
 107                 */
 108                if (likely(cmpxchg_release(lock, node, NULL) == node))
 109                        return;
 110                /* Wait until the next pointer is set */
 111                while (!(next = READ_ONCE(node->next)))
 112                        cpu_relax();
 113        }
 114
 115        /* Pass lock to next waiter. */
 116        arch_mcs_spin_unlock_contended(&next->locked);
 117}
 118
 119#endif /* __LINUX_MCS_SPINLOCK_H */
 120