1// SPDX-License-Identifier: GPL-2.0-only
2#include <linux/atomic.h>
3#include <linux/percpu.h>
4#include <linux/wait.h>
5#include <linux/lockdep.h>
6#include <linux/percpu-rwsem.h>
7#include <linux/rcupdate.h>
8#include <linux/sched.h>
9#include <linux/sched/task.h>
10#include <linux/sched/debug.h>
11#include <linux/errno.h>
12#include <trace/events/lock.h>
13
14int __percpu_init_rwsem(struct percpu_rw_semaphore *sem,
15 const char *name, struct lock_class_key *key)
16{
17 sem->read_count = alloc_percpu(int);
18 if (unlikely(!sem->read_count))
19 return -ENOMEM;
20
21 rcu_sync_init(&sem->rss);
22 rcuwait_init(w: &sem->writer);
23 init_waitqueue_head(&sem->waiters);
24 atomic_set(v: &sem->block, i: 0);
25#ifdef CONFIG_DEBUG_LOCK_ALLOC
26 debug_check_no_locks_freed((void *)sem, sizeof(*sem));
27 lockdep_init_map(&sem->dep_map, name, key, 0);
28#endif
29 return 0;
30}
31EXPORT_SYMBOL_GPL(__percpu_init_rwsem);
32
33void percpu_free_rwsem(struct percpu_rw_semaphore *sem)
34{
35 /*
36 * XXX: temporary kludge. The error path in alloc_super()
37 * assumes that percpu_free_rwsem() is safe after kzalloc().
38 */
39 if (!sem->read_count)
40 return;
41
42 rcu_sync_dtor(&sem->rss);
43 free_percpu(pdata: sem->read_count);
44 sem->read_count = NULL; /* catch use after free bugs */
45}
46EXPORT_SYMBOL_GPL(percpu_free_rwsem);
47
48static bool __percpu_down_read_trylock(struct percpu_rw_semaphore *sem)
49{
50 this_cpu_inc(*sem->read_count);
51
52 /*
53 * Due to having preemption disabled the decrement happens on
54 * the same CPU as the increment, avoiding the
55 * increment-on-one-CPU-and-decrement-on-another problem.
56 *
57 * If the reader misses the writer's assignment of sem->block, then the
58 * writer is guaranteed to see the reader's increment.
59 *
60 * Conversely, any readers that increment their sem->read_count after
61 * the writer looks are guaranteed to see the sem->block value, which
62 * in turn means that they are guaranteed to immediately decrement
63 * their sem->read_count, so that it doesn't matter that the writer
64 * missed them.
65 */
66
67 smp_mb(); /* A matches D */
68
69 /*
70 * If !sem->block the critical section starts here, matched by the
71 * release in percpu_up_write().
72 */
73 if (likely(!atomic_read_acquire(&sem->block)))
74 return true;
75
76 this_cpu_dec(*sem->read_count);
77
78 /* Prod writer to re-evaluate readers_active_check() */
79 rcuwait_wake_up(w: &sem->writer);
80
81 return false;
82}
83
84static inline bool __percpu_down_write_trylock(struct percpu_rw_semaphore *sem)
85{
86 if (atomic_read(v: &sem->block))
87 return false;
88
89 return atomic_xchg(v: &sem->block, new: 1) == 0;
90}
91
92static bool __percpu_rwsem_trylock(struct percpu_rw_semaphore *sem, bool reader)
93{
94 if (reader) {
95 bool ret;
96
97 preempt_disable();
98 ret = __percpu_down_read_trylock(sem);
99 preempt_enable();
100
101 return ret;
102 }
103 return __percpu_down_write_trylock(sem);
104}
105
106/*
107 * The return value of wait_queue_entry::func means:
108 *
109 * <0 - error, wakeup is terminated and the error is returned
110 * 0 - no wakeup, a next waiter is tried
111 * >0 - woken, if EXCLUSIVE, counted towards @nr_exclusive.
112 *
113 * We use EXCLUSIVE for both readers and writers to preserve FIFO order,
114 * and play games with the return value to allow waking multiple readers.
115 *
116 * Specifically, we wake readers until we've woken a single writer, or until a
117 * trylock fails.
118 */
119static int percpu_rwsem_wake_function(struct wait_queue_entry *wq_entry,
120 unsigned int mode, int wake_flags,
121 void *key)
122{
123 bool reader = wq_entry->flags & WQ_FLAG_CUSTOM;
124 struct percpu_rw_semaphore *sem = key;
125 struct task_struct *p;
126
127 /* concurrent against percpu_down_write(), can get stolen */
128 if (!__percpu_rwsem_trylock(sem, reader))
129 return 1;
130
131 p = get_task_struct(t: wq_entry->private);
132 list_del_init(entry: &wq_entry->entry);
133 smp_store_release(&wq_entry->private, NULL);
134
135 wake_up_process(tsk: p);
136 put_task_struct(t: p);
137
138 return !reader; /* wake (readers until) 1 writer */
139}
140
141static void percpu_rwsem_wait(struct percpu_rw_semaphore *sem, bool reader,
142 bool freeze)
143{
144 DEFINE_WAIT_FUNC(wq_entry, percpu_rwsem_wake_function);
145 bool wait;
146
147 spin_lock_irq(lock: &sem->waiters.lock);
148 /*
149 * Serialize against the wakeup in percpu_up_write(), if we fail
150 * the trylock, the wakeup must see us on the list.
151 */
152 wait = !__percpu_rwsem_trylock(sem, reader);
153 if (wait) {
154 wq_entry.flags |= WQ_FLAG_EXCLUSIVE | reader * WQ_FLAG_CUSTOM;
155 __add_wait_queue_entry_tail(wq_head: &sem->waiters, wq_entry: &wq_entry);
156 }
157 spin_unlock_irq(lock: &sem->waiters.lock);
158
159 while (wait) {
160 set_current_state(TASK_UNINTERRUPTIBLE |
161 (freeze ? TASK_FREEZABLE : 0));
162 if (!smp_load_acquire(&wq_entry.private))
163 break;
164 schedule();
165 }
166 __set_current_state(TASK_RUNNING);
167}
168
169bool __sched __percpu_down_read(struct percpu_rw_semaphore *sem, bool try,
170 bool freeze)
171{
172 if (__percpu_down_read_trylock(sem))
173 return true;
174
175 if (try)
176 return false;
177
178 trace_contention_begin(lock: sem, LCB_F_PERCPU | LCB_F_READ);
179 preempt_enable();
180 percpu_rwsem_wait(sem, /* .reader = */ true, freeze);
181 preempt_disable();
182 trace_contention_end(lock: sem, ret: 0);
183
184 return true;
185}
186EXPORT_SYMBOL_GPL(__percpu_down_read);
187
188#define per_cpu_sum(var) \
189({ \
190 TYPEOF_UNQUAL(var) __sum = 0; \
191 int cpu; \
192 compiletime_assert_atomic_type(__sum); \
193 for_each_possible_cpu(cpu) \
194 __sum += per_cpu(var, cpu); \
195 __sum; \
196})
197
198bool percpu_is_read_locked(struct percpu_rw_semaphore *sem)
199{
200 return per_cpu_sum(*sem->read_count) != 0 && !atomic_read(v: &sem->block);
201}
202EXPORT_SYMBOL_GPL(percpu_is_read_locked);
203
204/*
205 * Return true if the modular sum of the sem->read_count per-CPU variable is
206 * zero. If this sum is zero, then it is stable due to the fact that if any
207 * newly arriving readers increment a given counter, they will immediately
208 * decrement that same counter.
209 *
210 * Assumes sem->block is set.
211 */
212static bool readers_active_check(struct percpu_rw_semaphore *sem)
213{
214 if (per_cpu_sum(*sem->read_count) != 0)
215 return false;
216
217 /*
218 * If we observed the decrement; ensure we see the entire critical
219 * section.
220 */
221
222 smp_mb(); /* C matches B */
223
224 return true;
225}
226
227void __sched percpu_down_write(struct percpu_rw_semaphore *sem)
228{
229 bool contended = false;
230
231 might_sleep();
232 rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
233
234 /* Notify readers to take the slow path. */
235 rcu_sync_enter(&sem->rss);
236
237 /*
238 * Try set sem->block; this provides writer-writer exclusion.
239 * Having sem->block set makes new readers block.
240 */
241 if (!__percpu_down_write_trylock(sem)) {
242 trace_contention_begin(lock: sem, LCB_F_PERCPU | LCB_F_WRITE);
243 percpu_rwsem_wait(sem, /* .reader = */ false, freeze: false);
244 contended = true;
245 }
246
247 /* smp_mb() implied by __percpu_down_write_trylock() on success -- D matches A */
248
249 /*
250 * If they don't see our store of sem->block, then we are guaranteed to
251 * see their sem->read_count increment, and therefore will wait for
252 * them.
253 */
254
255 /* Wait for all active readers to complete. */
256 rcuwait_wait_event(&sem->writer, readers_active_check(sem), TASK_UNINTERRUPTIBLE);
257 if (contended)
258 trace_contention_end(lock: sem, ret: 0);
259}
260EXPORT_SYMBOL_GPL(percpu_down_write);
261
262void percpu_up_write(struct percpu_rw_semaphore *sem)
263{
264 rwsem_release(&sem->dep_map, _RET_IP_);
265
266 /*
267 * Signal the writer is done, no fast path yet.
268 *
269 * One reason that we cannot just immediately flip to readers_fast is
270 * that new readers might fail to see the results of this writer's
271 * critical section.
272 *
273 * Therefore we force it through the slow path which guarantees an
274 * acquire and thereby guarantees the critical section's consistency.
275 */
276 atomic_set_release(v: &sem->block, i: 0);
277
278 /*
279 * Prod any pending reader/writer to make progress.
280 */
281 __wake_up(wq_head: &sem->waiters, TASK_NORMAL, nr: 1, key: sem);
282
283 /*
284 * Once this completes (at least one RCU-sched grace period hence) the
285 * reader fast path will be available again. Safe to use outside the
286 * exclusive write lock because its counting.
287 */
288 rcu_sync_exit(&sem->rss);
289}
290EXPORT_SYMBOL_GPL(percpu_up_write);
291