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b2441318 | 1 | // SPDX-License-Identifier: GPL-2.0 |
fb0527bd | 2 | #include <linux/percpu.h> |
fb0527bd | 3 | #include <linux/sched.h> |
d84b6728 | 4 | #include <linux/osq_lock.h> |
fb0527bd PZ |
5 | |
6 | /* | |
7 | * An MCS like lock especially tailored for optimistic spinning for sleeping | |
8 | * lock implementations (mutex, rwsem, etc). | |
9 | * | |
10 | * Using a single mcs node per CPU is safe because sleeping locks should not be | |
11 | * called from interrupt context and we have preemption disabled while | |
12 | * spinning. | |
13 | */ | |
046a619d | 14 | static DEFINE_PER_CPU_SHARED_ALIGNED(struct optimistic_spin_node, osq_node); |
fb0527bd | 15 | |
90631822 JL |
16 | /* |
17 | * We use the value 0 to represent "no CPU", thus the encoded value | |
18 | * will be the CPU number incremented by 1. | |
19 | */ | |
20 | static inline int encode_cpu(int cpu_nr) | |
21 | { | |
22 | return cpu_nr + 1; | |
23 | } | |
24 | ||
5aff60a1 PX |
25 | static inline int node_cpu(struct optimistic_spin_node *node) |
26 | { | |
27 | return node->cpu - 1; | |
28 | } | |
29 | ||
90631822 JL |
30 | static inline struct optimistic_spin_node *decode_cpu(int encoded_cpu_val) |
31 | { | |
32 | int cpu_nr = encoded_cpu_val - 1; | |
33 | ||
34 | return per_cpu_ptr(&osq_node, cpu_nr); | |
35 | } | |
36 | ||
fb0527bd PZ |
37 | /* |
38 | * Get a stable @node->next pointer, either for unlock() or unqueue() purposes. | |
39 | * Can return NULL in case we were the last queued and we updated @lock instead. | |
40 | */ | |
046a619d | 41 | static inline struct optimistic_spin_node * |
90631822 | 42 | osq_wait_next(struct optimistic_spin_queue *lock, |
046a619d JL |
43 | struct optimistic_spin_node *node, |
44 | struct optimistic_spin_node *prev) | |
fb0527bd | 45 | { |
046a619d | 46 | struct optimistic_spin_node *next = NULL; |
90631822 JL |
47 | int curr = encode_cpu(smp_processor_id()); |
48 | int old; | |
49 | ||
50 | /* | |
51 | * If there is a prev node in queue, then the 'old' value will be | |
52 | * the prev node's CPU #, else it's set to OSQ_UNLOCKED_VAL since if | |
53 | * we're currently last in queue, then the queue will then become empty. | |
54 | */ | |
55 | old = prev ? prev->cpu : OSQ_UNLOCKED_VAL; | |
fb0527bd PZ |
56 | |
57 | for (;;) { | |
90631822 | 58 | if (atomic_read(&lock->tail) == curr && |
c55a6ffa | 59 | atomic_cmpxchg_acquire(&lock->tail, curr, old) == curr) { |
fb0527bd PZ |
60 | /* |
61 | * We were the last queued, we moved @lock back. @prev | |
62 | * will now observe @lock and will complete its | |
63 | * unlock()/unqueue(). | |
64 | */ | |
65 | break; | |
66 | } | |
67 | ||
68 | /* | |
69 | * We must xchg() the @node->next value, because if we were to | |
70 | * leave it in, a concurrent unlock()/unqueue() from | |
71 | * @node->next might complete Step-A and think its @prev is | |
72 | * still valid. | |
73 | * | |
74 | * If the concurrent unlock()/unqueue() wins the race, we'll | |
75 | * wait for either @lock to point to us, through its Step-B, or | |
76 | * wait for a new @node->next from its Step-C. | |
77 | */ | |
78 | if (node->next) { | |
79 | next = xchg(&node->next, NULL); | |
80 | if (next) | |
81 | break; | |
82 | } | |
83 | ||
f2f09a4c | 84 | cpu_relax(); |
fb0527bd PZ |
85 | } |
86 | ||
87 | return next; | |
88 | } | |
89 | ||
90631822 | 90 | bool osq_lock(struct optimistic_spin_queue *lock) |
fb0527bd | 91 | { |
046a619d JL |
92 | struct optimistic_spin_node *node = this_cpu_ptr(&osq_node); |
93 | struct optimistic_spin_node *prev, *next; | |
90631822 JL |
94 | int curr = encode_cpu(smp_processor_id()); |
95 | int old; | |
fb0527bd PZ |
96 | |
97 | node->locked = 0; | |
98 | node->next = NULL; | |
90631822 | 99 | node->cpu = curr; |
fb0527bd | 100 | |
c55a6ffa | 101 | /* |
b4b29f94 WD |
102 | * We need both ACQUIRE (pairs with corresponding RELEASE in |
103 | * unlock() uncontended, or fastpath) and RELEASE (to publish | |
104 | * the node fields we just initialised) semantics when updating | |
105 | * the lock tail. | |
c55a6ffa | 106 | */ |
b4b29f94 | 107 | old = atomic_xchg(&lock->tail, curr); |
90631822 | 108 | if (old == OSQ_UNLOCKED_VAL) |
fb0527bd PZ |
109 | return true; |
110 | ||
90631822 JL |
111 | prev = decode_cpu(old); |
112 | node->prev = prev; | |
50972fe7 PS |
113 | |
114 | /* | |
115 | * osq_lock() unqueue | |
116 | * | |
117 | * node->prev = prev osq_wait_next() | |
118 | * WMB MB | |
119 | * prev->next = node next->prev = prev // unqueue-C | |
120 | * | |
121 | * Here 'node->prev' and 'next->prev' are the same variable and we need | |
122 | * to ensure these stores happen in-order to avoid corrupting the list. | |
123 | */ | |
124 | smp_wmb(); | |
125 | ||
4d3199e4 | 126 | WRITE_ONCE(prev->next, node); |
fb0527bd PZ |
127 | |
128 | /* | |
129 | * Normally @prev is untouchable after the above store; because at that | |
130 | * moment unlock can proceed and wipe the node element from stack. | |
131 | * | |
132 | * However, since our nodes are static per-cpu storage, we're | |
133 | * guaranteed their existence -- this allows us to apply | |
134 | * cmpxchg in an attempt to undo our queueing. | |
135 | */ | |
136 | ||
f5bfdc8e | 137 | /* |
e2db7592 | 138 | * Wait to acquire the lock or cancellation. Note that need_resched() |
f5bfdc8e WL |
139 | * will come with an IPI, which will wake smp_cond_load_relaxed() if it |
140 | * is implemented with a monitor-wait. vcpu_is_preempted() relies on | |
141 | * polling, be careful. | |
142 | */ | |
143 | if (smp_cond_load_relaxed(&node->locked, VAL || need_resched() || | |
144 | vcpu_is_preempted(node_cpu(node->prev)))) | |
145 | return true; | |
fb0527bd | 146 | |
f5bfdc8e | 147 | /* unqueue */ |
fb0527bd PZ |
148 | /* |
149 | * Step - A -- stabilize @prev | |
150 | * | |
151 | * Undo our @prev->next assignment; this will make @prev's | |
152 | * unlock()/unqueue() wait for a next pointer since @lock points to us | |
153 | * (or later). | |
154 | */ | |
155 | ||
156 | for (;;) { | |
33190b67 QC |
157 | /* |
158 | * cpu_relax() below implies a compiler barrier which would | |
159 | * prevent this comparison being optimized away. | |
160 | */ | |
161 | if (data_race(prev->next) == node && | |
fb0527bd PZ |
162 | cmpxchg(&prev->next, node, NULL) == node) |
163 | break; | |
164 | ||
165 | /* | |
166 | * We can only fail the cmpxchg() racing against an unlock(), | |
e2db7592 | 167 | * in which case we should observe @node->locked becoming |
fb0527bd PZ |
168 | * true. |
169 | */ | |
170 | if (smp_load_acquire(&node->locked)) | |
171 | return true; | |
172 | ||
f2f09a4c | 173 | cpu_relax(); |
fb0527bd PZ |
174 | |
175 | /* | |
176 | * Or we race against a concurrent unqueue()'s step-B, in which | |
177 | * case its step-C will write us a new @node->prev pointer. | |
178 | */ | |
4d3199e4 | 179 | prev = READ_ONCE(node->prev); |
fb0527bd PZ |
180 | } |
181 | ||
182 | /* | |
183 | * Step - B -- stabilize @next | |
184 | * | |
185 | * Similar to unlock(), wait for @node->next or move @lock from @node | |
186 | * back to @prev. | |
187 | */ | |
188 | ||
189 | next = osq_wait_next(lock, node, prev); | |
190 | if (!next) | |
191 | return false; | |
192 | ||
193 | /* | |
194 | * Step - C -- unlink | |
195 | * | |
196 | * @prev is stable because its still waiting for a new @prev->next | |
197 | * pointer, @next is stable because our @node->next pointer is NULL and | |
198 | * it will wait in Step-A. | |
199 | */ | |
200 | ||
4d3199e4 DB |
201 | WRITE_ONCE(next->prev, prev); |
202 | WRITE_ONCE(prev->next, next); | |
fb0527bd PZ |
203 | |
204 | return false; | |
205 | } | |
206 | ||
90631822 | 207 | void osq_unlock(struct optimistic_spin_queue *lock) |
fb0527bd | 208 | { |
33ecd208 | 209 | struct optimistic_spin_node *node, *next; |
90631822 | 210 | int curr = encode_cpu(smp_processor_id()); |
fb0527bd PZ |
211 | |
212 | /* | |
213 | * Fast path for the uncontended case. | |
214 | */ | |
c55a6ffa DB |
215 | if (likely(atomic_cmpxchg_release(&lock->tail, curr, |
216 | OSQ_UNLOCKED_VAL) == curr)) | |
fb0527bd PZ |
217 | return; |
218 | ||
219 | /* | |
220 | * Second most likely case. | |
221 | */ | |
33ecd208 | 222 | node = this_cpu_ptr(&osq_node); |
fb0527bd PZ |
223 | next = xchg(&node->next, NULL); |
224 | if (next) { | |
4d3199e4 | 225 | WRITE_ONCE(next->locked, 1); |
fb0527bd PZ |
226 | return; |
227 | } | |
228 | ||
229 | next = osq_wait_next(lock, node, NULL); | |
230 | if (next) | |
4d3199e4 | 231 | WRITE_ONCE(next->locked, 1); |
fb0527bd | 232 | } |