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