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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> | |
9ab6055f | 23 | #include <linux/spinlock.h> |
70af2f8a WL |
24 | #include <asm/qrwlock.h> |
25 | ||
26 | /** | |
27 | * rspin_until_writer_unlock - inc reader count & spin until writer is gone | |
28 | * @lock : Pointer to queue rwlock structure | |
29 | * @writer: Current queue rwlock writer status byte | |
30 | * | |
31 | * In interrupt context or at the head of the queue, the reader will just | |
32 | * increment the reader count & wait until the writer releases the lock. | |
33 | */ | |
34 | static __always_inline void | |
35 | rspin_until_writer_unlock(struct qrwlock *lock, u32 cnts) | |
36 | { | |
37 | while ((cnts & _QW_WMASK) == _QW_LOCKED) { | |
f2f09a4c | 38 | cpu_relax(); |
77e430e3 | 39 | cnts = atomic_read_acquire(&lock->cnts); |
70af2f8a WL |
40 | } |
41 | } | |
42 | ||
43 | /** | |
f7d71f20 | 44 | * queued_read_lock_slowpath - acquire read lock of a queue rwlock |
70af2f8a | 45 | * @lock: Pointer to queue rwlock structure |
0e06e5be | 46 | * @cnts: Current qrwlock lock value |
70af2f8a | 47 | */ |
0e06e5be | 48 | void queued_read_lock_slowpath(struct qrwlock *lock, u32 cnts) |
70af2f8a | 49 | { |
70af2f8a WL |
50 | /* |
51 | * Readers come here when they cannot get the lock without waiting | |
52 | */ | |
53 | if (unlikely(in_interrupt())) { | |
54 | /* | |
0e06e5be WL |
55 | * Readers in interrupt context will get the lock immediately |
56 | * if the writer is just waiting (not holding the lock yet). | |
57 | * The rspin_until_writer_unlock() function returns immediately | |
77e430e3 WD |
58 | * in this case. Otherwise, they will spin (with ACQUIRE |
59 | * semantics) until the lock is available without waiting in | |
60 | * the queue. | |
70af2f8a | 61 | */ |
70af2f8a WL |
62 | rspin_until_writer_unlock(lock, cnts); |
63 | return; | |
64 | } | |
65 | atomic_sub(_QR_BIAS, &lock->cnts); | |
66 | ||
67 | /* | |
68 | * Put the reader into the wait queue | |
69 | */ | |
6e1e5196 | 70 | arch_spin_lock(&lock->wait_lock); |
70af2f8a WL |
71 | |
72 | /* | |
77e430e3 WD |
73 | * The ACQUIRE semantics of the following spinning code ensure |
74 | * that accesses can't leak upwards out of our subsequent critical | |
75 | * section in the case that the lock is currently held for write. | |
70af2f8a | 76 | */ |
f9852b74 | 77 | cnts = atomic_fetch_add_acquire(_QR_BIAS, &lock->cnts); |
70af2f8a WL |
78 | rspin_until_writer_unlock(lock, cnts); |
79 | ||
80 | /* | |
81 | * Signal the next one in queue to become queue head | |
82 | */ | |
6e1e5196 | 83 | arch_spin_unlock(&lock->wait_lock); |
70af2f8a | 84 | } |
f7d71f20 | 85 | EXPORT_SYMBOL(queued_read_lock_slowpath); |
70af2f8a WL |
86 | |
87 | /** | |
f7d71f20 | 88 | * queued_write_lock_slowpath - acquire write lock of a queue rwlock |
70af2f8a WL |
89 | * @lock : Pointer to queue rwlock structure |
90 | */ | |
f7d71f20 | 91 | void queued_write_lock_slowpath(struct qrwlock *lock) |
70af2f8a WL |
92 | { |
93 | u32 cnts; | |
94 | ||
95 | /* Put the writer into the wait queue */ | |
6e1e5196 | 96 | arch_spin_lock(&lock->wait_lock); |
70af2f8a WL |
97 | |
98 | /* Try to acquire the lock directly if no reader is present */ | |
99 | if (!atomic_read(&lock->cnts) && | |
77e430e3 | 100 | (atomic_cmpxchg_acquire(&lock->cnts, 0, _QW_LOCKED) == 0)) |
70af2f8a WL |
101 | goto unlock; |
102 | ||
103 | /* | |
104 | * Set the waiting flag to notify readers that a writer is pending, | |
105 | * or wait for a previous writer to go away. | |
106 | */ | |
107 | for (;;) { | |
e0d02285 WD |
108 | if (!READ_ONCE(lock->wmode) && |
109 | (cmpxchg_relaxed(&lock->wmode, 0, _QW_WAITING) == 0)) | |
70af2f8a WL |
110 | break; |
111 | ||
f2f09a4c | 112 | cpu_relax(); |
70af2f8a WL |
113 | } |
114 | ||
115 | /* When no more readers, set the locked flag */ | |
116 | for (;;) { | |
117 | cnts = atomic_read(&lock->cnts); | |
118 | if ((cnts == _QW_WAITING) && | |
77e430e3 WD |
119 | (atomic_cmpxchg_acquire(&lock->cnts, _QW_WAITING, |
120 | _QW_LOCKED) == _QW_WAITING)) | |
70af2f8a WL |
121 | break; |
122 | ||
f2f09a4c | 123 | cpu_relax(); |
70af2f8a WL |
124 | } |
125 | unlock: | |
6e1e5196 | 126 | arch_spin_unlock(&lock->wait_lock); |
70af2f8a | 127 | } |
f7d71f20 | 128 | EXPORT_SYMBOL(queued_write_lock_slowpath); |