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70af2f8a WL |
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 | #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 | ||
25 | /** | |
26 | * rspin_until_writer_unlock - inc reader count & spin until writer is gone | |
27 | * @lock : Pointer to queue rwlock structure | |
28 | * @writer: Current queue rwlock writer status byte | |
29 | * | |
30 | * In interrupt context or at the head of the queue, the reader will just | |
31 | * increment the reader count & wait until the writer releases the lock. | |
32 | */ | |
33 | static __always_inline void | |
34 | rspin_until_writer_unlock(struct qrwlock *lock, u32 cnts) | |
35 | { | |
36 | while ((cnts & _QW_WMASK) == _QW_LOCKED) { | |
3a6bfbc9 | 37 | cpu_relax_lowlatency(); |
70af2f8a WL |
38 | cnts = smp_load_acquire((u32 *)&lock->cnts); |
39 | } | |
40 | } | |
41 | ||
42 | /** | |
43 | * queue_read_lock_slowpath - acquire read lock of a queue rwlock | |
44 | * @lock: Pointer to queue rwlock structure | |
45 | */ | |
46 | void queue_read_lock_slowpath(struct qrwlock *lock) | |
47 | { | |
48 | u32 cnts; | |
49 | ||
50 | /* | |
51 | * Readers come here when they cannot get the lock without waiting | |
52 | */ | |
53 | if (unlikely(in_interrupt())) { | |
54 | /* | |
55 | * Readers in interrupt context will spin until the lock is | |
56 | * available without waiting in the queue. | |
57 | */ | |
58 | cnts = smp_load_acquire((u32 *)&lock->cnts); | |
59 | rspin_until_writer_unlock(lock, cnts); | |
60 | return; | |
61 | } | |
62 | atomic_sub(_QR_BIAS, &lock->cnts); | |
63 | ||
64 | /* | |
65 | * Put the reader into the wait queue | |
66 | */ | |
67 | arch_spin_lock(&lock->lock); | |
68 | ||
69 | /* | |
70 | * At the head of the wait queue now, wait until the writer state | |
71 | * goes to 0 and then try to increment the reader count and get | |
72 | * the lock. It is possible that an incoming writer may steal the | |
73 | * lock in the interim, so it is necessary to check the writer byte | |
74 | * to make sure that the write lock isn't taken. | |
75 | */ | |
76 | while (atomic_read(&lock->cnts) & _QW_WMASK) | |
3a6bfbc9 | 77 | cpu_relax_lowlatency(); |
70af2f8a WL |
78 | |
79 | cnts = atomic_add_return(_QR_BIAS, &lock->cnts) - _QR_BIAS; | |
80 | rspin_until_writer_unlock(lock, cnts); | |
81 | ||
82 | /* | |
83 | * Signal the next one in queue to become queue head | |
84 | */ | |
85 | arch_spin_unlock(&lock->lock); | |
86 | } | |
87 | EXPORT_SYMBOL(queue_read_lock_slowpath); | |
88 | ||
89 | /** | |
90 | * queue_write_lock_slowpath - acquire write lock of a queue rwlock | |
91 | * @lock : Pointer to queue rwlock structure | |
92 | */ | |
93 | void queue_write_lock_slowpath(struct qrwlock *lock) | |
94 | { | |
95 | u32 cnts; | |
96 | ||
97 | /* Put the writer into the wait queue */ | |
98 | arch_spin_lock(&lock->lock); | |
99 | ||
100 | /* Try to acquire the lock directly if no reader is present */ | |
101 | if (!atomic_read(&lock->cnts) && | |
102 | (atomic_cmpxchg(&lock->cnts, 0, _QW_LOCKED) == 0)) | |
103 | goto unlock; | |
104 | ||
105 | /* | |
106 | * Set the waiting flag to notify readers that a writer is pending, | |
107 | * or wait for a previous writer to go away. | |
108 | */ | |
109 | for (;;) { | |
110 | cnts = atomic_read(&lock->cnts); | |
111 | if (!(cnts & _QW_WMASK) && | |
112 | (atomic_cmpxchg(&lock->cnts, cnts, | |
113 | cnts | _QW_WAITING) == cnts)) | |
114 | break; | |
115 | ||
3a6bfbc9 | 116 | cpu_relax_lowlatency(); |
70af2f8a WL |
117 | } |
118 | ||
119 | /* When no more readers, set the locked flag */ | |
120 | for (;;) { | |
121 | cnts = atomic_read(&lock->cnts); | |
122 | if ((cnts == _QW_WAITING) && | |
123 | (atomic_cmpxchg(&lock->cnts, _QW_WAITING, | |
124 | _QW_LOCKED) == _QW_WAITING)) | |
125 | break; | |
126 | ||
3a6bfbc9 | 127 | cpu_relax_lowlatency(); |
70af2f8a WL |
128 | } |
129 | unlock: | |
130 | arch_spin_unlock(&lock->lock); | |
131 | } | |
132 | EXPORT_SYMBOL(queue_write_lock_slowpath); |