kernel/locking/mcs_spinlock.h

   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
  18 struct 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)                                 \
  30 do {                                                                    \
  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  */
  62 static inline
  63 void 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  */
  99 static inline
 100 void 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 */