include/asm-generic/qrwlock.h

   1 /*
   2  * Queue read/write lock
   3  *
   4  * This program is free software; you can redistribute it and/or modify
   5  * it under the terms of the GNU General Public License as published by
   6  * the Free Software Foundation; either version 2 of the License, or
   7  * (at your option) any later version.
   8  *
   9  * This program is distributed in the hope that it will be useful,
  10  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  11  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  12  * GNU General Public License for more details.
  13  *
  14  * (C) Copyright 2013-2014 Hewlett-Packard Development Company, L.P.
  15  *
  16  * Authors: Waiman Long <waiman.long@hp.com>
  17  */
  18 #ifndef __ASM_GENERIC_QRWLOCK_H
  19 #define __ASM_GENERIC_QRWLOCK_H
  20
  21 #include <linux/atomic.h>
  22 #include <asm/barrier.h>
  23 #include <asm/processor.h>
  24
  25 #include <asm-generic/qrwlock_types.h>
  26
  27 /*
  28  * Writer states & reader shift and bias
  29  */
  30 #define _QW_WAITING     1               /* A writer is waiting     */
  31 #define _QW_LOCKED      0xff            /* A writer holds the lock */
  32 #define _QW_WMASK       0xff            /* Writer mask             */
  33 #define _QR_SHIFT       8               /* Reader count shift      */
  34 #define _QR_BIAS        (1U << _QR_SHIFT)
  35
  36 /*
  37  * External function declarations
  38  */
  39 extern void queue_read_lock_slowpath(struct qrwlock *lock);
  40 extern void queue_write_lock_slowpath(struct qrwlock *lock);
  41
  42 /**
  43  * queue_read_can_lock- would read_trylock() succeed?
  44  * @lock: Pointer to queue rwlock structure
  45  */
  46 static inline int queue_read_can_lock(struct qrwlock *lock)
  47 {
  48         return !(atomic_read(&lock->cnts) & _QW_WMASK);
  49 }
  50
  51 /**
  52  * queue_write_can_lock- would write_trylock() succeed?
  53  * @lock: Pointer to queue rwlock structure
  54  */
  55 static inline int queue_write_can_lock(struct qrwlock *lock)
  56 {
  57         return !atomic_read(&lock->cnts);
  58 }
  59
  60 /**
  61  * queue_read_trylock - try to acquire read lock of a queue rwlock
  62  * @lock : Pointer to queue rwlock structure
  63  * Return: 1 if lock acquired, 0 if failed
  64  */
  65 static inline int queue_read_trylock(struct qrwlock *lock)
  66 {
  67         u32 cnts;
  68
  69         cnts = atomic_read(&lock->cnts);
  70         if (likely(!(cnts & _QW_WMASK))) {
  71                 cnts = (u32)atomic_add_return(_QR_BIAS, &lock->cnts);
  72                 if (likely(!(cnts & _QW_WMASK)))
  73                         return 1;
  74                 atomic_sub(_QR_BIAS, &lock->cnts);
  75         }
  76         return 0;
  77 }
  78
  79 /**
  80  * queue_write_trylock - try to acquire write lock of a queue rwlock
  81  * @lock : Pointer to queue rwlock structure
  82  * Return: 1 if lock acquired, 0 if failed
  83  */
  84 static inline int queue_write_trylock(struct qrwlock *lock)
  85 {
  86         u32 cnts;
  87
  88         cnts = atomic_read(&lock->cnts);
  89         if (unlikely(cnts))
  90                 return 0;
  91
  92         return likely(atomic_cmpxchg(&lock->cnts,
  93                                      cnts, cnts | _QW_LOCKED) == cnts);
  94 }
  95 /**
  96  * queue_read_lock - acquire read lock of a queue rwlock
  97  * @lock: Pointer to queue rwlock structure
  98  */
  99 static inline void queue_read_lock(struct qrwlock *lock)
 100 {
 101         u32 cnts;
 102
 103         cnts = atomic_add_return(_QR_BIAS, &lock->cnts);
 104         if (likely(!(cnts & _QW_WMASK)))
 105                 return;
 106
 107         /* The slowpath will decrement the reader count, if necessary. */
 108         queue_read_lock_slowpath(lock);
 109 }
 110
 111 /**
 112  * queue_write_lock - acquire write lock of a queue rwlock
 113  * @lock : Pointer to queue rwlock structure
 114  */
 115 static inline void queue_write_lock(struct qrwlock *lock)
 116 {
 117         /* Optimize for the unfair lock case where the fair flag is 0. */
 118         if (atomic_cmpxchg(&lock->cnts, 0, _QW_LOCKED) == 0)
 119                 return;
 120
 121         queue_write_lock_slowpath(lock);
 122 }
 123
 124 /**
 125  * queue_read_unlock - release read lock of a queue rwlock
 126  * @lock : Pointer to queue rwlock structure
 127  */
 128 static inline void queue_read_unlock(struct qrwlock *lock)
 129 {
 130         /*
 131          * Atomically decrement the reader count
 132          */
 133         smp_mb__before_atomic();
 134         atomic_sub(_QR_BIAS, &lock->cnts);
 135 }
 136
 137 #ifndef queue_write_unlock
 138 /**
 139  * queue_write_unlock - release write lock of a queue rwlock
 140  * @lock : Pointer to queue rwlock structure
 141  */
 142 static inline void queue_write_unlock(struct qrwlock *lock)
 143 {
 144         /*
 145          * If the writer field is atomic, it can be cleared directly.
 146          * Otherwise, an atomic subtraction will be used to clear it.
 147          */
 148         smp_mb__before_atomic();
 149         atomic_sub(_QW_LOCKED, &lock->cnts);
 150 }
 151 #endif
 152
 153 /*
 154  * Remapping rwlock architecture specific functions to the corresponding
 155  * queue rwlock functions.
 156  */
 157 #define arch_read_can_lock(l)   queue_read_can_lock(l)
 158 #define arch_write_can_lock(l)  queue_write_can_lock(l)
 159 #define arch_read_lock(l)       queue_read_lock(l)
 160 #define arch_write_lock(l)      queue_write_lock(l)
 161 #define arch_read_trylock(l)    queue_read_trylock(l)
 162 #define arch_write_trylock(l)   queue_write_trylock(l)
 163 #define arch_read_unlock(l)     queue_read_unlock(l)
 164 #define arch_write_unlock(l)    queue_write_unlock(l)
 165
 166 #endif /* __ASM_GENERIC_QRWLOCK_H */