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Commit | Line | Data |
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70af2f8a | 1 | /* |
c7114b4e | 2 | * Queued read/write locks |
70af2f8a WL |
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 | #include <linux/smp.h> | |
19 | #include <linux/bug.h> | |
20 | #include <linux/cpumask.h> | |
21 | #include <linux/percpu.h> | |
22 | #include <linux/hardirq.h> | |
70af2f8a WL |
23 | #include <asm/qrwlock.h> |
24 | ||
405963b6 WL |
25 | /* |
26 | * This internal data structure is used for optimizing access to some of | |
27 | * the subfields within the atomic_t cnts. | |
28 | */ | |
29 | struct __qrwlock { | |
30 | union { | |
31 | atomic_t cnts; | |
32 | struct { | |
33 | #ifdef __LITTLE_ENDIAN | |
34 | u8 wmode; /* Writer mode */ | |
35 | u8 rcnts[3]; /* Reader counts */ | |
36 | #else | |
37 | u8 rcnts[3]; /* Reader counts */ | |
38 | u8 wmode; /* Writer mode */ | |
39 | #endif | |
40 | }; | |
41 | }; | |
42 | arch_spinlock_t lock; | |
43 | }; | |
44 | ||
70af2f8a WL |
45 | /** |
46 | * rspin_until_writer_unlock - inc reader count & spin until writer is gone | |
47 | * @lock : Pointer to queue rwlock structure | |
48 | * @writer: Current queue rwlock writer status byte | |
49 | * | |
50 | * In interrupt context or at the head of the queue, the reader will just | |
51 | * increment the reader count & wait until the writer releases the lock. | |
52 | */ | |
53 | static __always_inline void | |
54 | rspin_until_writer_unlock(struct qrwlock *lock, u32 cnts) | |
55 | { | |
56 | while ((cnts & _QW_WMASK) == _QW_LOCKED) { | |
3a6bfbc9 | 57 | cpu_relax_lowlatency(); |
70af2f8a WL |
58 | cnts = smp_load_acquire((u32 *)&lock->cnts); |
59 | } | |
60 | } | |
61 | ||
62 | /** | |
63 | * queue_read_lock_slowpath - acquire read lock of a queue rwlock | |
64 | * @lock: Pointer to queue rwlock structure | |
65 | */ | |
66 | void queue_read_lock_slowpath(struct qrwlock *lock) | |
67 | { | |
68 | u32 cnts; | |
69 | ||
70 | /* | |
71 | * Readers come here when they cannot get the lock without waiting | |
72 | */ | |
73 | if (unlikely(in_interrupt())) { | |
74 | /* | |
75 | * Readers in interrupt context will spin until the lock is | |
76 | * available without waiting in the queue. | |
77 | */ | |
78 | cnts = smp_load_acquire((u32 *)&lock->cnts); | |
79 | rspin_until_writer_unlock(lock, cnts); | |
80 | return; | |
81 | } | |
82 | atomic_sub(_QR_BIAS, &lock->cnts); | |
83 | ||
84 | /* | |
85 | * Put the reader into the wait queue | |
86 | */ | |
87 | arch_spin_lock(&lock->lock); | |
88 | ||
89 | /* | |
90 | * At the head of the wait queue now, wait until the writer state | |
91 | * goes to 0 and then try to increment the reader count and get | |
92 | * the lock. It is possible that an incoming writer may steal the | |
93 | * lock in the interim, so it is necessary to check the writer byte | |
94 | * to make sure that the write lock isn't taken. | |
95 | */ | |
96 | while (atomic_read(&lock->cnts) & _QW_WMASK) | |
3a6bfbc9 | 97 | cpu_relax_lowlatency(); |
70af2f8a WL |
98 | |
99 | cnts = atomic_add_return(_QR_BIAS, &lock->cnts) - _QR_BIAS; | |
100 | rspin_until_writer_unlock(lock, cnts); | |
101 | ||
102 | /* | |
103 | * Signal the next one in queue to become queue head | |
104 | */ | |
105 | arch_spin_unlock(&lock->lock); | |
106 | } | |
107 | EXPORT_SYMBOL(queue_read_lock_slowpath); | |
108 | ||
109 | /** | |
110 | * queue_write_lock_slowpath - acquire write lock of a queue rwlock | |
111 | * @lock : Pointer to queue rwlock structure | |
112 | */ | |
113 | void queue_write_lock_slowpath(struct qrwlock *lock) | |
114 | { | |
115 | u32 cnts; | |
116 | ||
117 | /* Put the writer into the wait queue */ | |
118 | arch_spin_lock(&lock->lock); | |
119 | ||
120 | /* Try to acquire the lock directly if no reader is present */ | |
121 | if (!atomic_read(&lock->cnts) && | |
122 | (atomic_cmpxchg(&lock->cnts, 0, _QW_LOCKED) == 0)) | |
123 | goto unlock; | |
124 | ||
125 | /* | |
126 | * Set the waiting flag to notify readers that a writer is pending, | |
127 | * or wait for a previous writer to go away. | |
128 | */ | |
129 | for (;;) { | |
405963b6 WL |
130 | struct __qrwlock *l = (struct __qrwlock *)lock; |
131 | ||
132 | if (!READ_ONCE(l->wmode) && | |
133 | (cmpxchg(&l->wmode, 0, _QW_WAITING) == 0)) | |
70af2f8a WL |
134 | break; |
135 | ||
3a6bfbc9 | 136 | cpu_relax_lowlatency(); |
70af2f8a WL |
137 | } |
138 | ||
139 | /* When no more readers, set the locked flag */ | |
140 | for (;;) { | |
141 | cnts = atomic_read(&lock->cnts); | |
142 | if ((cnts == _QW_WAITING) && | |
143 | (atomic_cmpxchg(&lock->cnts, _QW_WAITING, | |
144 | _QW_LOCKED) == _QW_WAITING)) | |
145 | break; | |
146 | ||
3a6bfbc9 | 147 | cpu_relax_lowlatency(); |
70af2f8a WL |
148 | } |
149 | unlock: | |
150 | arch_spin_unlock(&lock->lock); | |
151 | } | |
152 | EXPORT_SYMBOL(queue_write_lock_slowpath); |