1// SPDX-License-Identifier: GPL-2.0
  2#include <linux/percpu.h>
  3#include <linux/sched.h>
  4#include <linux/osq_lock.h>
  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 */
 14
 15struct optimistic_spin_node {
 16	struct optimistic_spin_node *next, *prev;
 17	int locked; /* 1 if lock acquired */
 18	int cpu; /* encoded CPU # + 1 value */
 19};
 20
 21static DEFINE_PER_CPU_SHARED_ALIGNED(struct optimistic_spin_node, osq_node);
 22
 23/*
 24 * We use the value 0 to represent "no CPU", thus the encoded value
 25 * will be the CPU number incremented by 1.
 26 */
 27static inline int encode_cpu(int cpu_nr)
 28{
 29	return cpu_nr + 1;
 30}
 31
 32static inline int node_cpu(struct optimistic_spin_node *node)
 33{
 34	return node->cpu - 1;
 35}
 36
 37static inline struct optimistic_spin_node *decode_cpu(int encoded_cpu_val)
 38{
 39	int cpu_nr = encoded_cpu_val - 1;
 40
 41	return per_cpu_ptr(&osq_node, cpu_nr);
 42}
 43
 44/*
 45 * Get a stable @node->next pointer, either for unlock() or unqueue() purposes.
 46 * Can return NULL in case we were the last queued and we updated @lock instead.
 47 *
 48 * If osq_lock() is being cancelled there must be a previous node
 49 * and 'old_cpu' is its CPU #.
 50 * For osq_unlock() there is never a previous node and old_cpu is
 51 * set to OSQ_UNLOCKED_VAL.
 52 */
 53static inline struct optimistic_spin_node *
 54osq_wait_next(struct optimistic_spin_queue *lock,
 55	      struct optimistic_spin_node *node,
 56	      int old_cpu)
 57{
 58	int curr = encode_cpu(smp_processor_id());
 59
 60	for (;;) {
 61		if (atomic_read(&lock->tail) == curr &&
 62		    atomic_cmpxchg_acquire(&lock->tail, curr, old_cpu) == curr) {
 63			/*
 64			 * We were the last queued, we moved @lock back. @prev
 65			 * will now observe @lock and will complete its
 66			 * unlock()/unqueue().
 67			 */
 68			return NULL;
 69		}
 70
 71		/*
 72		 * We must xchg() the @node->next value, because if we were to
 73		 * leave it in, a concurrent unlock()/unqueue() from
 74		 * @node->next might complete Step-A and think its @prev is
 75		 * still valid.
 76		 *
 77		 * If the concurrent unlock()/unqueue() wins the race, we'll
 78		 * wait for either @lock to point to us, through its Step-B, or
 79		 * wait for a new @node->next from its Step-C.
 80		 */
 81		if (node->next) {
 82			struct optimistic_spin_node *next;
 83
 84			next = xchg(&node->next, NULL);
 85			if (next)
 86				return next;
 87		}
 88
 89		cpu_relax();
 90	}
 91}
 92
 93bool osq_lock(struct optimistic_spin_queue *lock)
 94{
 95	struct optimistic_spin_node *node = this_cpu_ptr(&osq_node);
 96	struct optimistic_spin_node *prev, *next;
 97	int curr = encode_cpu(smp_processor_id());
 98	int old;
 99
100	node->locked = 0;
101	node->next = NULL;
102	node->cpu = curr;
103
104	/*
105	 * We need both ACQUIRE (pairs with corresponding RELEASE in
106	 * unlock() uncontended, or fastpath) and RELEASE (to publish
107	 * the node fields we just initialised) semantics when updating
108	 * the lock tail.
109	 */
110	old = atomic_xchg(&lock->tail, curr);
111	if (old == OSQ_UNLOCKED_VAL)
112		return true;
113
114	prev = decode_cpu(old);
115	node->prev = prev;
116
117	/*
118	 * osq_lock()			unqueue
119	 *
120	 * node->prev = prev		osq_wait_next()
121	 * WMB				MB
122	 * prev->next = node		next->prev = prev // unqueue-C
123	 *
124	 * Here 'node->prev' and 'next->prev' are the same variable and we need
125	 * to ensure these stores happen in-order to avoid corrupting the list.
126	 */
127	smp_wmb();
128
129	WRITE_ONCE(prev->next, node);
130
131	/*
132	 * Normally @prev is untouchable after the above store; because at that
133	 * moment unlock can proceed and wipe the node element from stack.
134	 *
135	 * However, since our nodes are static per-cpu storage, we're
136	 * guaranteed their existence -- this allows us to apply
137	 * cmpxchg in an attempt to undo our queueing.
138	 */
139
140	/*
141	 * Wait to acquire the lock or cancellation. Note that need_resched()
142	 * will come with an IPI, which will wake smp_cond_load_relaxed() if it
143	 * is implemented with a monitor-wait. vcpu_is_preempted() relies on
144	 * polling, be careful.
145	 */
146	if (smp_cond_load_relaxed(&node->locked, VAL || need_resched() ||
147				  vcpu_is_preempted(node_cpu(node->prev))))
148		return true;
149
150	/* unqueue */
151	/*
152	 * Step - A  -- stabilize @prev
153	 *
154	 * Undo our @prev->next assignment; this will make @prev's
155	 * unlock()/unqueue() wait for a next pointer since @lock points to us
156	 * (or later).
157	 */
158
159	for (;;) {
160		/*
161		 * cpu_relax() below implies a compiler barrier which would
162		 * prevent this comparison being optimized away.
163		 */
164		if (data_race(prev->next) == node &&
165		    cmpxchg(&prev->next, node, NULL) == node)
166			break;
167
168		/*
169		 * We can only fail the cmpxchg() racing against an unlock(),
170		 * in which case we should observe @node->locked becoming
171		 * true.
172		 */
173		if (smp_load_acquire(&node->locked))
174			return true;
175
176		cpu_relax();
177
178		/*
179		 * Or we race against a concurrent unqueue()'s step-B, in which
180		 * case its step-C will write us a new @node->prev pointer.
181		 */
182		prev = READ_ONCE(node->prev);
183	}
184
185	/*
186	 * Step - B -- stabilize @next
187	 *
188	 * Similar to unlock(), wait for @node->next or move @lock from @node
189	 * back to @prev.
190	 */
191
192	next = osq_wait_next(lock, node, prev->cpu);
193	if (!next)
194		return false;
195
196	/*
197	 * Step - C -- unlink
198	 *
199	 * @prev is stable because its still waiting for a new @prev->next
200	 * pointer, @next is stable because our @node->next pointer is NULL and
201	 * it will wait in Step-A.
202	 */
203
204	WRITE_ONCE(next->prev, prev);
205	WRITE_ONCE(prev->next, next);
206
207	return false;
208}
209
210void osq_unlock(struct optimistic_spin_queue *lock)
211{
212	struct optimistic_spin_node *node, *next;
213	int curr = encode_cpu(smp_processor_id());
214
215	/*
216	 * Fast path for the uncontended case.
217	 */
218	if (atomic_try_cmpxchg_release(&lock->tail, &curr, OSQ_UNLOCKED_VAL))
219		return;
220
221	/*
222	 * Second most likely case.
223	 */
224	node = this_cpu_ptr(&osq_node);
225	next = xchg(&node->next, NULL);
226	if (next) {
227		WRITE_ONCE(next->locked, 1);
228		return;
229	}
230
231	next = osq_wait_next(lock, node, OSQ_UNLOCKED_VAL);
232	if (next)
233		WRITE_ONCE(next->locked, 1);
234}