1 /* SPDX-License-Identifier: GPL-2.0-or-later */
2 /*
3 * Queued spinlock
4 *
5 * A 'generic' spinlock implementation that is based on MCS locks. For an
6 * architecture that's looking for a 'generic' spinlock, please first consider
7 * ticket-lock.h and only come looking here when you've considered all the
8 * constraints below and can show your hardware does actually perform better
9 * with qspinlock.
10 *
11 * qspinlock relies on atomic_*_release()/atomic_*_acquire() to be RCsc (or no
12 * weaker than RCtso if you're power), where regular code only expects atomic_t
13 * to be RCpc.
14 *
15 * qspinlock relies on a far greater (compared to asm-generic/spinlock.h) set
16 * of atomic operations to behave well together, please audit them carefully to
17 * ensure they all have forward progress. Many atomic operations may default to
18 * cmpxchg() loops which will not have good forward progress properties on
19 * LL/SC architectures.
20 *
21 * One notable example is atomic_fetch_or_acquire(), which x86 cannot (cheaply)
22 * do. Carefully read the patches that introduced
23 * queued_fetch_set_pending_acquire().
24 *
25 * qspinlock also heavily relies on mixed size atomic operations, in specific
26 * it requires architectures to have xchg16; something which many LL/SC
27 * architectures need to implement as a 32bit and+or in order to satisfy the
28 * forward progress guarantees mentioned above.
29 *
30 * Further reading on mixed size atomics that might be relevant:
31 *
32 * http://www.cl.cam.ac.uk/~pes20/popl17/mixed-size.pdf
33 *
34 * (C) Copyright 2013-2015 Hewlett-Packard Development Company, L.P.
35 * (C) Copyright 2015 Hewlett-Packard Enterprise Development LP
36 *
37 * Authors: Waiman Long <waiman.long@hpe.com>
38 */
39 #ifndef __ASM_GENERIC_QSPINLOCK_H
40 #define __ASM_GENERIC_QSPINLOCK_H
41
42 #include <asm-generic/qspinlock_types.h>
43 #include <linux/atomic.h>
44
45 #ifndef queued_spin_is_locked
46 /**
47 * queued_spin_is_locked - is the spinlock locked?
48 * @lock: Pointer to queued spinlock structure
49 * Return: 1 if it is locked, 0 otherwise
50 */
51 static __always_inline int queued_spin_is_locked(struct qspinlock *lock)
52 {
53 /*
54 * Any !0 state indicates it is locked, even if _Q_LOCKED_VAL
55 * isn't immediately observable.
56 */
57 return atomic_read(&lock->val);
58 }
59 #endif
60
61 /**
62 * queued_spin_value_unlocked - is the spinlock structure unlocked?
63 * @lock: queued spinlock structure
64 * Return: 1 if it is unlocked, 0 otherwise
65 *
66 * N.B. Whenever there are tasks waiting for the lock, it is considered
67 * locked wrt the lockref code to avoid lock stealing by the lockref
68 * code and change things underneath the lock. This also allows some
69 * optimizations to be applied without conflict with lockref.
70 */
71 static __always_inline int queued_spin_value_unlocked(struct qspinlock lock)
72 {
73 return !lock.val.counter;
74 }
75
76 /**
77 * queued_spin_is_contended - check if the lock is contended
78 * @lock : Pointer to queued spinlock structure
79 * Return: 1 if lock contended, 0 otherwise
80 */
81 static __always_inline int queued_spin_is_contended(struct qspinlock *lock)
82 {
83 return atomic_read(&lock->val) & ~_Q_LOCKED_MASK;
84 }
85 /**
86 * queued_spin_trylock - try to acquire the queued spinlock
87 * @lock : Pointer to queued spinlock structure
88 * Return: 1 if lock acquired, 0 if failed
89 */
90 static __always_inline int queued_spin_trylock(struct qspinlock *lock)
91 {
92 int val = atomic_read(&lock->val);
93
94 if (unlikely(val))
95 return 0;
96
97 return likely(atomic_try_cmpxchg_acquire(&lock->val, &val, _Q_LOCKED_VAL));
98 }
99
100 extern void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val);
101
102 #ifndef queued_spin_lock
103 /**
104 * queued_spin_lock - acquire a queued spinlock
105 * @lock: Pointer to queued spinlock structure
106 */
107 static __always_inline void queued_spin_lock(struct qspinlock *lock)
108 {
109 int val = 0;
110
111 if (likely(atomic_try_cmpxchg_acquire(&lock->val, &val, _Q_LOCKED_VAL)))
112 return;
113
114 queued_spin_lock_slowpath(lock, val);
115 }
116 #endif
117
118 #ifndef queued_spin_unlock
119 /**
120 * queued_spin_unlock - release a queued spinlock
121 * @lock : Pointer to queued spinlock structure
122 */
123 static __always_inline void queued_spin_unlock(struct qspinlock *lock)
124 {
125 /*
126 * unlock() needs release semantics:
127 */
128 smp_store_release(&lock->locked, 0);
129 }
130 #endif
131
132 #ifndef virt_spin_lock
133 static __always_inline bool virt_spin_lock(struct qspinlock *lock)
134 {
135 return false;
136 }
137 #endif
138
139 /*
140 * Remapping spinlock architecture specific functions to the corresponding
141 * queued spinlock functions.
142 */
143 #define arch_spin_is_locked(l) queued_spin_is_locked(l)
144 #define arch_spin_is_contended(l) queued_spin_is_contended(l)
145 #define arch_spin_value_unlocked(l) queued_spin_value_unlocked(l)
146 #define arch_spin_lock(l) queued_spin_lock(l)
147 #define arch_spin_trylock(l) queued_spin_trylock(l)
148 #define arch_spin_unlock(l) queued_spin_unlock(l)
149
150 #endif /* __ASM_GENERIC_QSPINLOCK_H */
151
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