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-rw-r--r--kernel/locking/osq_lock.c203
1 files changed, 203 insertions, 0 deletions
diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
new file mode 100644
index 000000000000..c112d00341b0
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+++ b/kernel/locking/osq_lock.c
@@ -0,0 +1,203 @@
1#include <linux/percpu.h>
2#include <linux/sched.h>
3#include <linux/osq_lock.h>
4
5/*
6 * An MCS like lock especially tailored for optimistic spinning for sleeping
7 * lock implementations (mutex, rwsem, etc).
8 *
9 * Using a single mcs node per CPU is safe because sleeping locks should not be
10 * called from interrupt context and we have preemption disabled while
11 * spinning.
12 */
13static DEFINE_PER_CPU_SHARED_ALIGNED(struct optimistic_spin_node, osq_node);
14
15/*
16 * We use the value 0 to represent "no CPU", thus the encoded value
17 * will be the CPU number incremented by 1.
18 */
19static inline int encode_cpu(int cpu_nr)
20{
21 return cpu_nr + 1;
22}
23
24static inline struct optimistic_spin_node *decode_cpu(int encoded_cpu_val)
25{
26 int cpu_nr = encoded_cpu_val - 1;
27
28 return per_cpu_ptr(&osq_node, cpu_nr);
29}
30
31/*
32 * Get a stable @node->next pointer, either for unlock() or unqueue() purposes.
33 * Can return NULL in case we were the last queued and we updated @lock instead.
34 */
35static inline struct optimistic_spin_node *
36osq_wait_next(struct optimistic_spin_queue *lock,
37 struct optimistic_spin_node *node,
38 struct optimistic_spin_node *prev)
39{
40 struct optimistic_spin_node *next = NULL;
41 int curr = encode_cpu(smp_processor_id());
42 int old;
43
44 /*
45 * If there is a prev node in queue, then the 'old' value will be
46 * the prev node's CPU #, else it's set to OSQ_UNLOCKED_VAL since if
47 * we're currently last in queue, then the queue will then become empty.
48 */
49 old = prev ? prev->cpu : OSQ_UNLOCKED_VAL;
50
51 for (;;) {
52 if (atomic_read(&lock->tail) == curr &&
53 atomic_cmpxchg(&lock->tail, curr, old) == curr) {
54 /*
55 * We were the last queued, we moved @lock back. @prev
56 * will now observe @lock and will complete its
57 * unlock()/unqueue().
58 */
59 break;
60 }
61
62 /*
63 * We must xchg() the @node->next value, because if we were to
64 * leave it in, a concurrent unlock()/unqueue() from
65 * @node->next might complete Step-A and think its @prev is
66 * still valid.
67 *
68 * If the concurrent unlock()/unqueue() wins the race, we'll
69 * wait for either @lock to point to us, through its Step-B, or
70 * wait for a new @node->next from its Step-C.
71 */
72 if (node->next) {
73 next = xchg(&node->next, NULL);
74 if (next)
75 break;
76 }
77
78 cpu_relax_lowlatency();
79 }
80
81 return next;
82}
83
84bool osq_lock(struct optimistic_spin_queue *lock)
85{
86 struct optimistic_spin_node *node = this_cpu_ptr(&osq_node);
87 struct optimistic_spin_node *prev, *next;
88 int curr = encode_cpu(smp_processor_id());
89 int old;
90
91 node->locked = 0;
92 node->next = NULL;
93 node->cpu = curr;
94
95 old = atomic_xchg(&lock->tail, curr);
96 if (old == OSQ_UNLOCKED_VAL)
97 return true;
98
99 prev = decode_cpu(old);
100 node->prev = prev;
101 ACCESS_ONCE(prev->next) = node;
102
103 /*
104 * Normally @prev is untouchable after the above store; because at that
105 * moment unlock can proceed and wipe the node element from stack.
106 *
107 * However, since our nodes are static per-cpu storage, we're
108 * guaranteed their existence -- this allows us to apply
109 * cmpxchg in an attempt to undo our queueing.
110 */
111
112 while (!ACCESS_ONCE(node->locked)) {
113 /*
114 * If we need to reschedule bail... so we can block.
115 */
116 if (need_resched())
117 goto unqueue;
118
119 cpu_relax_lowlatency();
120 }
121 return true;
122
123unqueue:
124 /*
125 * Step - A -- stabilize @prev
126 *
127 * Undo our @prev->next assignment; this will make @prev's
128 * unlock()/unqueue() wait for a next pointer since @lock points to us
129 * (or later).
130 */
131
132 for (;;) {
133 if (prev->next == node &&
134 cmpxchg(&prev->next, node, NULL) == node)
135 break;
136
137 /*
138 * We can only fail the cmpxchg() racing against an unlock(),
139 * in which case we should observe @node->locked becomming
140 * true.
141 */
142 if (smp_load_acquire(&node->locked))
143 return true;
144
145 cpu_relax_lowlatency();
146
147 /*
148 * Or we race against a concurrent unqueue()'s step-B, in which
149 * case its step-C will write us a new @node->prev pointer.
150 */
151 prev = ACCESS_ONCE(node->prev);
152 }
153
154 /*
155 * Step - B -- stabilize @next
156 *
157 * Similar to unlock(), wait for @node->next or move @lock from @node
158 * back to @prev.
159 */
160
161 next = osq_wait_next(lock, node, prev);
162 if (!next)
163 return false;
164
165 /*
166 * Step - C -- unlink
167 *
168 * @prev is stable because its still waiting for a new @prev->next
169 * pointer, @next is stable because our @node->next pointer is NULL and
170 * it will wait in Step-A.
171 */
172
173 ACCESS_ONCE(next->prev) = prev;
174 ACCESS_ONCE(prev->next) = next;
175
176 return false;
177}
178
179void osq_unlock(struct optimistic_spin_queue *lock)
180{
181 struct optimistic_spin_node *node, *next;
182 int curr = encode_cpu(smp_processor_id());
183
184 /*
185 * Fast path for the uncontended case.
186 */
187 if (likely(atomic_cmpxchg(&lock->tail, curr, OSQ_UNLOCKED_VAL) == curr))
188 return;
189
190 /*
191 * Second most likely case.
192 */
193 node = this_cpu_ptr(&osq_node);
194 next = xchg(&node->next, NULL);
195 if (next) {
196 ACCESS_ONCE(next->locked) = 1;
197 return;
198 }
199
200 next = osq_wait_next(lock, node, NULL);
201 if (next)
202 ACCESS_ONCE(next->locked) = 1;
203}
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