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 queue_read_lock_slowpath(struct qrwlock *lock);
40extern 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 */
46static 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 */
55static 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 */
65static 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 */
84static 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 */
99static 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 */
115static 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 */
128static 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 */
142static 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 */
167