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	WRITE_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 (!READ_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 = READ_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	WRITE_ONCE(next->prev, prev);
174	WRITE_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		WRITE_ONCE(next->locked, 1);
197		return;
198	}
199
200	next = osq_wait_next(lock, node, NULL);
201	if (next)
202		WRITE_ONCE(next->locked, 1);
203}
204