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70af2f8a 1/*
c7114b4e 2 * Queued read/write locks
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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>
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23#include <asm/qrwlock.h>
24
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25/*
26 * This internal data structure is used for optimizing access to some of
27 * the subfields within the atomic_t cnts.
28 */
29struct __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
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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 */
53static __always_inline void
54rspin_until_writer_unlock(struct qrwlock *lock, u32 cnts)
55{
56 while ((cnts & _QW_WMASK) == _QW_LOCKED) {
3a6bfbc9 57 cpu_relax_lowlatency();
77e430e3 58 cnts = atomic_read_acquire(&lock->cnts);
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59 }
60}
61
62/**
f7d71f20 63 * queued_read_lock_slowpath - acquire read lock of a queue rwlock
70af2f8a 64 * @lock: Pointer to queue rwlock structure
0e06e5be 65 * @cnts: Current qrwlock lock value
70af2f8a 66 */
0e06e5be 67void queued_read_lock_slowpath(struct qrwlock *lock, u32 cnts)
70af2f8a 68{
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69 /*
70 * Readers come here when they cannot get the lock without waiting
71 */
72 if (unlikely(in_interrupt())) {
73 /*
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74 * Readers in interrupt context will get the lock immediately
75 * if the writer is just waiting (not holding the lock yet).
76 * The rspin_until_writer_unlock() function returns immediately
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77 * in this case. Otherwise, they will spin (with ACQUIRE
78 * semantics) until the lock is available without waiting in
79 * the queue.
70af2f8a 80 */
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81 rspin_until_writer_unlock(lock, cnts);
82 return;
83 }
84 atomic_sub(_QR_BIAS, &lock->cnts);
85
86 /*
87 * Put the reader into the wait queue
88 */
6e1e5196 89 arch_spin_lock(&lock->wait_lock);
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90
91 /*
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92 * The ACQUIRE semantics of the following spinning code ensure
93 * that accesses can't leak upwards out of our subsequent critical
94 * section in the case that the lock is currently held for write.
70af2f8a 95 */
77e430e3 96 cnts = atomic_add_return_acquire(_QR_BIAS, &lock->cnts) - _QR_BIAS;
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97 rspin_until_writer_unlock(lock, cnts);
98
99 /*
100 * Signal the next one in queue to become queue head
101 */
6e1e5196 102 arch_spin_unlock(&lock->wait_lock);
70af2f8a 103}
f7d71f20 104EXPORT_SYMBOL(queued_read_lock_slowpath);
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105
106/**
f7d71f20 107 * queued_write_lock_slowpath - acquire write lock of a queue rwlock
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108 * @lock : Pointer to queue rwlock structure
109 */
f7d71f20 110void queued_write_lock_slowpath(struct qrwlock *lock)
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111{
112 u32 cnts;
113
114 /* Put the writer into the wait queue */
6e1e5196 115 arch_spin_lock(&lock->wait_lock);
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116
117 /* Try to acquire the lock directly if no reader is present */
118 if (!atomic_read(&lock->cnts) &&
77e430e3 119 (atomic_cmpxchg_acquire(&lock->cnts, 0, _QW_LOCKED) == 0))
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120 goto unlock;
121
122 /*
123 * Set the waiting flag to notify readers that a writer is pending,
124 * or wait for a previous writer to go away.
125 */
126 for (;;) {
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127 struct __qrwlock *l = (struct __qrwlock *)lock;
128
129 if (!READ_ONCE(l->wmode) &&
77e430e3 130 (cmpxchg_relaxed(&l->wmode, 0, _QW_WAITING) == 0))
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131 break;
132
3a6bfbc9 133 cpu_relax_lowlatency();
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134 }
135
136 /* When no more readers, set the locked flag */
137 for (;;) {
138 cnts = atomic_read(&lock->cnts);
139 if ((cnts == _QW_WAITING) &&
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140 (atomic_cmpxchg_acquire(&lock->cnts, _QW_WAITING,
141 _QW_LOCKED) == _QW_WAITING))
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142 break;
143
3a6bfbc9 144 cpu_relax_lowlatency();
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145 }
146unlock:
6e1e5196 147 arch_spin_unlock(&lock->wait_lock);
70af2f8a 148}
f7d71f20 149EXPORT_SYMBOL(queued_write_lock_slowpath);