LXR linux/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 */
  39extern void queued_read_lock_slowpath(struct qrwlock *lock, u32 cnts);
  40extern void queued_write_lock_slowpath(struct qrwlock *lock);
  41
  42/**
  43 * queued_read_can_lock- would read_trylock() succeed?
  44 * @lock: Pointer to queue rwlock structure
  45 */
  46static inline int queued_read_can_lock(struct qrwlock *lock)
  47{
  48        return !(atomic_read(&lock->cnts) & _QW_WMASK);
  49}
  50
  51/**
  52 * queued_write_can_lock- would write_trylock() succeed?
  53 * @lock: Pointer to queue rwlock structure
  54 */
  55static inline int queued_write_can_lock(struct qrwlock *lock)
  56{
  57        return !atomic_read(&lock->cnts);
  58}
  59
  60/**
  61 * queued_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 */
  65static inline int queued_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_acquire(_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 * queued_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 */
  84static inline int queued_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_acquire(&lock->cnts,
  93                                             cnts, cnts | _QW_LOCKED) == cnts);
  94}
  95/**
  96 * queued_read_lock - acquire read lock of a queue rwlock
  97 * @lock: Pointer to queue rwlock structure
  98 */
  99static inline void queued_read_lock(struct qrwlock *lock)
 100{
 101        u32 cnts;
 102
 103        cnts = atomic_add_return_acquire(_QR_BIAS, &lock->cnts);
 104        if (likely(!(cnts & _QW_WMASK)))
 105                return;
 106
 107        /* The slowpath will decrement the reader count, if necessary. */
 108        queued_read_lock_slowpath(lock, cnts);
 109}
 110
 111/**
 112 * queued_write_lock - acquire write lock of a queue rwlock
 113 * @lock : Pointer to queue rwlock structure
 114 */
 115static inline void queued_write_lock(struct qrwlock *lock)
 116{
 117        /* Optimize for the unfair lock case where the fair flag is 0. */
 118        if (atomic_cmpxchg_acquire(&lock->cnts, 0, _QW_LOCKED) == 0)
 119                return;
 120
 121        queued_write_lock_slowpath(lock);
 122}
 123
 124/**
 125 * queued_read_unlock - release read lock of a queue rwlock
 126 * @lock : Pointer to queue rwlock structure
 127 */
 128static inline void queued_read_unlock(struct qrwlock *lock)
 129{
 130        /*
 131         * Atomically decrement the reader count
 132         */
 133        (void)atomic_sub_return_release(_QR_BIAS, &lock->cnts);
 134}
 135
 136/**
 137 * queued_write_unlock - release write lock of a queue rwlock
 138 * @lock : Pointer to queue rwlock structure
 139 */
 140static inline void queued_write_unlock(struct qrwlock *lock)
 141{
 142        smp_store_release((u8 *)&lock->cnts, 0);
 143}
 144
 145/*
 146 * Remapping rwlock architecture specific functions to the corresponding
 147 * queue rwlock functions.
 148 */
 149#define arch_read_can_lock(l)   queued_read_can_lock(l)
 150#define arch_write_can_lock(l)  queued_write_can_lock(l)
 151#define arch_read_lock(l)       queued_read_lock(l)
 152#define arch_write_lock(l)      queued_write_lock(l)
 153#define arch_read_trylock(l)    queued_read_trylock(l)
 154#define arch_write_trylock(l)   queued_write_trylock(l)
 155#define arch_read_unlock(l)     queued_read_unlock(l)
 156#define arch_write_unlock(l)    queued_write_unlock(l)
 157
 158#endif /* __ASM_GENERIC_QRWLOCK_H */
 159