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