1 /*
2  * Queued spinlock
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 2013-2015 Hewlett-Packard Development Company, L.P.
15  * (C) Copyright 2015 Hewlett-Packard Enterprise Development LP
16  *
17  * Authors: Waiman Long <waiman.long@hpe.com>
18  */
19 #ifndef __ASM_GENERIC_QSPINLOCK_H
20 #define __ASM_GENERIC_QSPINLOCK_H
21 
22 #include <asm-generic/qspinlock_types.h>
23 
24 /**
25  * queued_spin_is_locked - is the spinlock locked?
26  * @lock: Pointer to queued spinlock structure
27  * Return: 1 if it is locked, 0 otherwise
28  */
29 static __always_inline int queued_spin_is_locked(struct qspinlock *lock)
30 {
31 	/*
32 	 * queued_spin_lock_slowpath() can ACQUIRE the lock before
33 	 * issuing the unordered store that sets _Q_LOCKED_VAL.
34 	 *
35 	 * See both smp_cond_acquire() sites for more detail.
36 	 *
37 	 * This however means that in code like:
38 	 *
39 	 *   spin_lock(A)		spin_lock(B)
40 	 *   spin_unlock_wait(B)	spin_is_locked(A)
41 	 *   do_something()		do_something()
42 	 *
43 	 * Both CPUs can end up running do_something() because the store
44 	 * setting _Q_LOCKED_VAL will pass through the loads in
45 	 * spin_unlock_wait() and/or spin_is_locked().
46 	 *
47 	 * Avoid this by issuing a full memory barrier between the spin_lock()
48 	 * and the loads in spin_unlock_wait() and spin_is_locked().
49 	 *
50 	 * Note that regular mutual exclusion doesn't care about this
51 	 * delayed store.
52 	 */
53 	smp_mb();
54 	return atomic_read(&lock->val) & _Q_LOCKED_MASK;
55 }
56 
57 /**
58  * queued_spin_value_unlocked - is the spinlock structure unlocked?
59  * @lock: queued spinlock structure
60  * Return: 1 if it is unlocked, 0 otherwise
61  *
62  * N.B. Whenever there are tasks waiting for the lock, it is considered
63  *      locked wrt the lockref code to avoid lock stealing by the lockref
64  *      code and change things underneath the lock. This also allows some
65  *      optimizations to be applied without conflict with lockref.
66  */
67 static __always_inline int queued_spin_value_unlocked(struct qspinlock lock)
68 {
69 	return !atomic_read(&lock.val);
70 }
71 
72 /**
73  * queued_spin_is_contended - check if the lock is contended
74  * @lock : Pointer to queued spinlock structure
75  * Return: 1 if lock contended, 0 otherwise
76  */
77 static __always_inline int queued_spin_is_contended(struct qspinlock *lock)
78 {
79 	return atomic_read(&lock->val) & ~_Q_LOCKED_MASK;
80 }
81 /**
82  * queued_spin_trylock - try to acquire the queued spinlock
83  * @lock : Pointer to queued spinlock structure
84  * Return: 1 if lock acquired, 0 if failed
85  */
86 static __always_inline int queued_spin_trylock(struct qspinlock *lock)
87 {
88 	if (!atomic_read(&lock->val) &&
89 	   (atomic_cmpxchg_acquire(&lock->val, 0, _Q_LOCKED_VAL) == 0))
90 		return 1;
91 	return 0;
92 }
93 
94 extern void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val);
95 
96 /**
97  * queued_spin_lock - acquire a queued spinlock
98  * @lock: Pointer to queued spinlock structure
99  */
100 static __always_inline void queued_spin_lock(struct qspinlock *lock)
101 {
102 	u32 val;
103 
104 	val = atomic_cmpxchg_acquire(&lock->val, 0, _Q_LOCKED_VAL);
105 	if (likely(val == 0))
106 		return;
107 	queued_spin_lock_slowpath(lock, val);
108 }
109 
110 #ifndef queued_spin_unlock
111 /**
112  * queued_spin_unlock - release a queued spinlock
113  * @lock : Pointer to queued spinlock structure
114  */
115 static __always_inline void queued_spin_unlock(struct qspinlock *lock)
116 {
117 	/*
118 	 * smp_mb__before_atomic() in order to guarantee release semantics
119 	 */
120 	smp_mb__before_atomic();
121 	atomic_sub(_Q_LOCKED_VAL, &lock->val);
122 }
123 #endif
124 
125 /**
126  * queued_spin_unlock_wait - wait until current lock holder releases the lock
127  * @lock : Pointer to queued spinlock structure
128  *
129  * There is a very slight possibility of live-lock if the lockers keep coming
130  * and the waiter is just unfortunate enough to not see any unlock state.
131  */
132 static inline void queued_spin_unlock_wait(struct qspinlock *lock)
133 {
134 	/* See queued_spin_is_locked() */
135 	smp_mb();
136 	while (atomic_read(&lock->val) & _Q_LOCKED_MASK)
137 		cpu_relax();
138 }
139 
140 #ifndef virt_spin_lock
141 static __always_inline bool virt_spin_lock(struct qspinlock *lock)
142 {
143 	return false;
144 }
145 #endif
146 
147 /*
148  * Remapping spinlock architecture specific functions to the corresponding
149  * queued spinlock functions.
150  */
151 #define arch_spin_is_locked(l)		queued_spin_is_locked(l)
152 #define arch_spin_is_contended(l)	queued_spin_is_contended(l)
153 #define arch_spin_value_unlocked(l)	queued_spin_value_unlocked(l)
154 #define arch_spin_lock(l)		queued_spin_lock(l)
155 #define arch_spin_trylock(l)		queued_spin_trylock(l)
156 #define arch_spin_unlock(l)		queued_spin_unlock(l)
157 #define arch_spin_lock_flags(l, f)	queued_spin_lock(l)
158 #define arch_spin_unlock_wait(l)	queued_spin_unlock_wait(l)
159 
160 #endif /* __ASM_GENERIC_QSPINLOCK_H */
161