1 /* SPDX-License-Identifier: GPL-2.0-or-later */
2 /*
3 * Queue read/write lock
4 *
5 * These use generic atomic and locking routines, but depend on a fair spinlock
6 * implementation in order to be fair themselves. The implementation in
7 * asm-generic/spinlock.h meets these requirements.
8 *
9 * (C) Copyright 2013-2014 Hewlett-Packard Development Company, L.P.
10 *
11 * Authors: Waiman Long <waiman.long@hp.com>
12 */
13 #ifndef __ASM_GENERIC_QRWLOCK_H
14 #define __ASM_GENERIC_QRWLOCK_H
15
16 #include <linux/atomic.h>
17 #include <asm/barrier.h>
18 #include <asm/processor.h>
19
20 #include <asm-generic/qrwlock_types.h>
21
22 /* Must be included from asm/spinlock.h after defining arch_spin_is_locked. */
23
24 /*
25 * Writer states & reader shift and bias.
26 */
27 #define _QW_WAITING 0x100 /* A writer is waiting */
28 #define _QW_LOCKED 0x0ff /* A writer holds the lock */
29 #define _QW_WMASK 0x1ff /* Writer mask */
30 #define _QR_SHIFT 9 /* Reader count shift */
31 #define _QR_BIAS (1U << _QR_SHIFT)
32
33 /*
34 * External function declarations
35 */
36 extern void queued_read_lock_slowpath(struct qrwlock *lock);
37 extern void queued_write_lock_slowpath(struct qrwlock *lock);
38
39 /**
40 * queued_read_trylock - try to acquire read lock of a queued rwlock
41 * @lock : Pointer to queued rwlock structure
42 * Return: 1 if lock acquired, 0 if failed
43 */
44 static inline int queued_read_trylock(struct qrwlock *lock)
45 {
46 int cnts;
47
48 cnts = atomic_read(&lock->cnts);
49 if (likely(!(cnts & _QW_WMASK))) {
50 cnts = (u32)atomic_add_return_acquire(_QR_BIAS, &lock->cnts);
51 if (likely(!(cnts & _QW_WMASK)))
52 return 1;
53 atomic_sub(_QR_BIAS, &lock->cnts);
54 }
55 return 0;
56 }
57
58 /**
59 * queued_write_trylock - try to acquire write lock of a queued rwlock
60 * @lock : Pointer to queued rwlock structure
61 * Return: 1 if lock acquired, 0 if failed
62 */
63 static inline int queued_write_trylock(struct qrwlock *lock)
64 {
65 int cnts;
66
67 cnts = atomic_read(&lock->cnts);
68 if (unlikely(cnts))
69 return 0;
70
71 return likely(atomic_try_cmpxchg_acquire(&lock->cnts, &cnts,
72 _QW_LOCKED));
73 }
74 /**
75 * queued_read_lock - acquire read lock of a queued rwlock
76 * @lock: Pointer to queued rwlock structure
77 */
78 static inline void queued_read_lock(struct qrwlock *lock)
79 {
80 int cnts;
81
82 cnts = atomic_add_return_acquire(_QR_BIAS, &lock->cnts);
83 if (likely(!(cnts & _QW_WMASK)))
84 return;
85
86 /* The slowpath will decrement the reader count, if necessary. */
87 queued_read_lock_slowpath(lock);
88 }
89
90 /**
91 * queued_write_lock - acquire write lock of a queued rwlock
92 * @lock : Pointer to queued rwlock structure
93 */
94 static inline void queued_write_lock(struct qrwlock *lock)
95 {
96 int cnts = 0;
97 /* Optimize for the unfair lock case where the fair flag is 0. */
98 if (likely(atomic_try_cmpxchg_acquire(&lock->cnts, &cnts, _QW_LOCKED)))
99 return;
100
101 queued_write_lock_slowpath(lock);
102 }
103
104 /**
105 * queued_read_unlock - release read lock of a queued rwlock
106 * @lock : Pointer to queued rwlock structure
107 */
108 static inline void queued_read_unlock(struct qrwlock *lock)
109 {
110 /*
111 * Atomically decrement the reader count
112 */
113 (void)atomic_sub_return_release(_QR_BIAS, &lock->cnts);
114 }
115
116 /**
117 * queued_write_unlock - release write lock of a queued rwlock
118 * @lock : Pointer to queued rwlock structure
119 */
120 static inline void queued_write_unlock(struct qrwlock *lock)
121 {
122 smp_store_release(&lock->wlocked, 0);
123 }
124
125 /**
126 * queued_rwlock_is_contended - check if the lock is contended
127 * @lock : Pointer to queued rwlock structure
128 * Return: 1 if lock contended, 0 otherwise
129 */
130 static inline int queued_rwlock_is_contended(struct qrwlock *lock)
131 {
132 return arch_spin_is_locked(&lock->wait_lock);
133 }
134
135 /*
136 * Remapping rwlock architecture specific functions to the corresponding
137 * queued rwlock functions.
138 */
139 #define arch_read_lock(l) queued_read_lock(l)
140 #define arch_write_lock(l) queued_write_lock(l)
141 #define arch_read_trylock(l) queued_read_trylock(l)
142 #define arch_write_trylock(l) queued_write_trylock(l)
143 #define arch_read_unlock(l) queued_read_unlock(l)
144 #define arch_write_unlock(l) queued_write_unlock(l)
145 #define arch_rwlock_is_contended(l) queued_rwlock_is_contended(l)
146
147 #endif /* __ASM_GENERIC_QRWLOCK_H */
148
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