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     0x100           /* A writer is waiting     */
  31 #define _QW_LOCKED      0x0ff           /* A writer holds the lock */
  32 #define _QW_WMASK       0x1ff           /* Writer mask             */
  33 #define _QR_SHIFT       9               /* Reader count shift      */
  34 #define _QR_BIAS        (1U << _QR_SHIFT)
  35
  36 /*
  37  * External function declarations
  38  */
  39 extern void queued_read_lock_slowpath(struct qrwlock *lock);
  40 extern void queued_write_lock_slowpath(struct qrwlock *lock);
  41
  42 /**
  43  * queued_read_trylock - try to acquire read lock of a queue rwlock
  44  * @lock : Pointer to queue rwlock structure
  45  * Return: 1 if lock acquired, 0 if failed
  46  */
  47 static inline int queued_read_trylock(struct qrwlock *lock)
  48 {
  49         u32 cnts;
  50
  51         cnts = atomic_read(&lock->cnts);
  52         if (likely(!(cnts & _QW_WMASK))) {
  53                 cnts = (u32)atomic_add_return_acquire(_QR_BIAS, &lock->cnts);
  54                 if (likely(!(cnts & _QW_WMASK)))
  55                         return 1;
  56                 atomic_sub(_QR_BIAS, &lock->cnts);
  57         }
  58         return 0;
  59 }
  60
  61 /**
  62  * queued_write_trylock - try to acquire write lock of a queue rwlock
  63  * @lock : Pointer to queue rwlock structure
  64  * Return: 1 if lock acquired, 0 if failed
  65  */
  66 static inline int queued_write_trylock(struct qrwlock *lock)
  67 {
  68         u32 cnts;
  69
  70         cnts = atomic_read(&lock->cnts);
  71         if (unlikely(cnts))
  72                 return 0;
  73
  74         return likely(atomic_cmpxchg_acquire(&lock->cnts,
  75                                              cnts, cnts | _QW_LOCKED) == cnts);
  76 }
  77 /**
  78  * queued_read_lock - acquire read lock of a queue rwlock
  79  * @lock: Pointer to queue rwlock structure
  80  */
  81 static inline void queued_read_lock(struct qrwlock *lock)
  82 {
  83         u32 cnts;
  84
  85         cnts = atomic_add_return_acquire(_QR_BIAS, &lock->cnts);
  86         if (likely(!(cnts & _QW_WMASK)))
  87                 return;
  88
  89         /* The slowpath will decrement the reader count, if necessary. */
  90         queued_read_lock_slowpath(lock);
  91 }
  92
  93 /**
  94  * queued_write_lock - acquire write lock of a queue rwlock
  95  * @lock : Pointer to queue rwlock structure
  96  */
  97 static inline void queued_write_lock(struct qrwlock *lock)
  98 {
  99         /* Optimize for the unfair lock case where the fair flag is 0. */
 100         if (atomic_cmpxchg_acquire(&lock->cnts, 0, _QW_LOCKED) == 0)
 101                 return;
 102
 103         queued_write_lock_slowpath(lock);
 104 }
 105
 106 /**
 107  * queued_read_unlock - release read lock of a queue rwlock
 108  * @lock : Pointer to queue rwlock structure
 109  */
 110 static inline void queued_read_unlock(struct qrwlock *lock)
 111 {
 112         /*
 113          * Atomically decrement the reader count
 114          */
 115         (void)atomic_sub_return_release(_QR_BIAS, &lock->cnts);
 116 }
 117
 118 /**
 119  * queued_write_unlock - release write lock of a queue rwlock
 120  * @lock : Pointer to queue rwlock structure
 121  */
 122 static inline void queued_write_unlock(struct qrwlock *lock)
 123 {
 124         smp_store_release(&lock->wlocked, 0);
 125 }
 126
 127 /*
 128  * Remapping rwlock architecture specific functions to the corresponding
 129  * queue rwlock functions.
 130  */
 131 #define arch_read_lock(l)       queued_read_lock(l)
 132 #define arch_write_lock(l)      queued_write_lock(l)
 133 #define arch_read_trylock(l)    queued_read_trylock(l)
 134 #define arch_write_trylock(l)   queued_write_trylock(l)
 135 #define arch_read_unlock(l)     queued_read_unlock(l)
 136 #define arch_write_unlock(l)    queued_write_unlock(l)
 137
 138 #endif /* __ASM_GENERIC_QRWLOCK_H */