include/linux/mcs_spinlock.h

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