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 bounces 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_cond_load_acquire() provides the acquire semantics
  27 * required so that subsequent operations happen after the
  28 * lock is acquired. Additionally, some architectures such as
  29 * ARM64 would like to do spin-waiting instead of purely
  30 * spinning, and smp_cond_load_acquire() provides that behavior.
  31 */
  32#define arch_mcs_spin_lock_contended(l)                                 \
  33do {                                                                    \
  34        smp_cond_load_acquire(l, VAL);                                  \
  35} while (0)
  36#endif
  37
  38#ifndef arch_mcs_spin_unlock_contended
  39/*
  40 * smp_store_release() provides a memory barrier to ensure all
  41 * operations in the critical section has been completed before
  42 * unlocking.
  43 */
  44#define arch_mcs_spin_unlock_contended(l)                               \
  45        smp_store_release((l), 1)
  46#endif
  47
  48/*
  49 * Note: the smp_load_acquire/smp_store_release pair is not
  50 * sufficient to form a full memory barrier across
  51 * cpus for many architectures (except x86) for mcs_unlock and mcs_lock.
  52 * For applications that need a full barrier across multiple cpus
  53 * with mcs_unlock and mcs_lock pair, smp_mb__after_unlock_lock() should be
  54 * used after mcs_lock.
  55 */
  56
  57/*
  58 * In order to acquire the lock, the caller should declare a local node and
  59 * pass a reference of the node to this function in addition to the lock.
  60 * If the lock has already been acquired, then this will proceed to spin
  61 * on this node->locked until the previous lock holder sets the node->locked
  62 * in mcs_spin_unlock().
  63 */
  64static inline
  65void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
  66{
  67        struct mcs_spinlock *prev;
  68
  69        /* Init node */
  70        node->locked = 0;
  71        node->next   = NULL;
  72
  73        /*
  74         * We rely on the full barrier with global transitivity implied by the
  75         * below xchg() to order the initialization stores above against any
  76         * observation of @node. And to provide the ACQUIRE ordering associated
  77         * with a LOCK primitive.
  78         */
  79        prev = xchg(lock, node);
  80        if (likely(prev == NULL)) {
  81                /*
  82                 * Lock acquired, don't need to set node->locked to 1. Threads
  83                 * only spin on its own node->locked value for lock acquisition.
  84                 * However, since this thread can immediately acquire the lock
  85                 * and does not proceed to spin on its own node->locked, this
  86                 * value won't be used. If a debug mode is needed to
  87                 * audit lock status, then set node->locked value here.
  88                 */
  89                return;
  90        }
  91        WRITE_ONCE(prev->next, node);
  92
  93        /* Wait until the lock holder passes the lock down. */
  94        arch_mcs_spin_lock_contended(&node->locked);
  95}
  96
  97/*
  98 * Releases the lock. The caller should pass in the corresponding node that
  99 * was used to acquire the lock.
 100 */
 101static inline
 102void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
 103{
 104        struct mcs_spinlock *next = READ_ONCE(node->next);
 105
 106        if (likely(!next)) {
 107                /*
 108                 * Release the lock by setting it to NULL
 109                 */
 110                if (likely(cmpxchg_release(lock, node, NULL) == node))
 111                        return;
 112                /* Wait until the next pointer is set */
 113                while (!(next = READ_ONCE(node->next)))
 114                        cpu_relax();
 115        }
 116
 117        /* Pass lock to next waiter. */
 118        arch_mcs_spin_unlock_contended(&next->locked);
 119}
 120
 121#endif /* __LINUX_MCS_SPINLOCK_H */
 122