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