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_acquire(&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	/*
 96	 * We need both ACQUIRE (pairs with corresponding RELEASE in
 97	 * unlock() uncontended, or fastpath) and RELEASE (to publish
 98	 * the node fields we just initialised) semantics when updating
 99	 * the lock tail.
100	 */
101	old = atomic_xchg(&lock->tail, curr);
102	if (old == OSQ_UNLOCKED_VAL)
103		return true;
104
105	prev = decode_cpu(old);
106	node->prev = prev;
107	WRITE_ONCE(prev->next, node);
108
109	/*
110	 * Normally @prev is untouchable after the above store; because at that
111	 * moment unlock can proceed and wipe the node element from stack.
112	 *
113	 * However, since our nodes are static per-cpu storage, we're
114	 * guaranteed their existence -- this allows us to apply
115	 * cmpxchg in an attempt to undo our queueing.
116	 */
117
118	while (!READ_ONCE(node->locked)) {
119		/*
120		 * If we need to reschedule bail... so we can block.
121		 */
122		if (need_resched())
123			goto unqueue;
124
125		cpu_relax_lowlatency();
126	}
127	return true;
128
129unqueue:
130	/*
131	 * Step - A  -- stabilize @prev
132	 *
133	 * Undo our @prev->next assignment; this will make @prev's
134	 * unlock()/unqueue() wait for a next pointer since @lock points to us
135	 * (or later).
136	 */
137
138	for (;;) {
139		if (prev->next == node &&
140		    cmpxchg(&prev->next, node, NULL) == node)
141			break;
142
143		/*
144		 * We can only fail the cmpxchg() racing against an unlock(),
145		 * in which case we should observe @node->locked becomming
146		 * true.
147		 */
148		if (smp_load_acquire(&node->locked))
149			return true;
150
151		cpu_relax_lowlatency();
152
153		/*
154		 * Or we race against a concurrent unqueue()'s step-B, in which
155		 * case its step-C will write us a new @node->prev pointer.
156		 */
157		prev = READ_ONCE(node->prev);
158	}
159
160	/*
161	 * Step - B -- stabilize @next
162	 *
163	 * Similar to unlock(), wait for @node->next or move @lock from @node
164	 * back to @prev.
165	 */
166
167	next = osq_wait_next(lock, node, prev);
168	if (!next)
169		return false;
170
171	/*
172	 * Step - C -- unlink
173	 *
174	 * @prev is stable because its still waiting for a new @prev->next
175	 * pointer, @next is stable because our @node->next pointer is NULL and
176	 * it will wait in Step-A.
177	 */
178
179	WRITE_ONCE(next->prev, prev);
180	WRITE_ONCE(prev->next, next);
181
182	return false;
183}
184
185void osq_unlock(struct optimistic_spin_queue *lock)
186{
187	struct optimistic_spin_node *node, *next;
188	int curr = encode_cpu(smp_processor_id());
189
190	/*
191	 * Fast path for the uncontended case.
192	 */
193	if (likely(atomic_cmpxchg_release(&lock->tail, curr,
194					  OSQ_UNLOCKED_VAL) == curr))
195		return;
196
197	/*
198	 * Second most likely case.
199	 */
200	node = this_cpu_ptr(&osq_node);
201	next = xchg(&node->next, NULL);
202	if (next) {
203		WRITE_ONCE(next->locked, 1);
204		return;
205	}
206
207	next = osq_wait_next(lock, node, NULL);
208	if (next)
209		WRITE_ONCE(next->locked, 1);
210}