Line data Source code
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 :
14 45 : int __percpu_init_rwsem(struct percpu_rw_semaphore *sem,
15 : const char *name, struct lock_class_key *key)
16 : {
17 45 : sem->read_count = alloc_percpu(int);
18 45 : if (unlikely(!sem->read_count))
19 : return -ENOMEM;
20 :
21 45 : rcu_sync_init(&sem->rss);
22 90 : rcuwait_init(&sem->writer);
23 45 : init_waitqueue_head(&sem->waiters);
24 90 : atomic_set(&sem->block, 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 45 : return 0;
30 : }
31 : EXPORT_SYMBOL_GPL(__percpu_init_rwsem);
32 :
33 15 : void 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 15 : if (!sem->read_count)
40 : return;
41 :
42 15 : rcu_sync_dtor(&sem->rss);
43 15 : free_percpu(sem->read_count);
44 15 : sem->read_count = NULL; /* catch use after free bugs */
45 : }
46 : EXPORT_SYMBOL_GPL(percpu_free_rwsem);
47 :
48 0 : static bool __percpu_down_read_trylock(struct percpu_rw_semaphore *sem)
49 : {
50 0 : 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 0 : 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 0 : if (likely(!atomic_read_acquire(&sem->block)))
74 : return true;
75 :
76 0 : this_cpu_dec(*sem->read_count);
77 :
78 : /* Prod writer to re-evaluate readers_active_check() */
79 0 : rcuwait_wake_up(&sem->writer);
80 :
81 0 : return false;
82 : }
83 :
84 : static inline bool __percpu_down_write_trylock(struct percpu_rw_semaphore *sem)
85 : {
86 0 : if (atomic_read(&sem->block))
87 : return false;
88 :
89 0 : return atomic_xchg(&sem->block, 1) == 0;
90 : }
91 :
92 : static bool __percpu_rwsem_trylock(struct percpu_rw_semaphore *sem, bool reader)
93 : {
94 0 : if (reader) {
95 : bool ret;
96 :
97 0 : preempt_disable();
98 0 : ret = __percpu_down_read_trylock(sem);
99 0 : 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 : */
119 0 : static int percpu_rwsem_wake_function(struct wait_queue_entry *wq_entry,
120 : unsigned int mode, int wake_flags,
121 : void *key)
122 : {
123 0 : bool reader = wq_entry->flags & WQ_FLAG_CUSTOM;
124 0 : struct percpu_rw_semaphore *sem = key;
125 : struct task_struct *p;
126 :
127 : /* concurrent against percpu_down_write(), can get stolen */
128 0 : if (!__percpu_rwsem_trylock(sem, reader))
129 : return 1;
130 :
131 0 : p = get_task_struct(wq_entry->private);
132 0 : list_del_init(&wq_entry->entry);
133 0 : smp_store_release(&wq_entry->private, NULL);
134 :
135 0 : wake_up_process(p);
136 0 : put_task_struct(p);
137 :
138 0 : return !reader; /* wake (readers until) 1 writer */
139 : }
140 :
141 0 : static void percpu_rwsem_wait(struct percpu_rw_semaphore *sem, bool reader)
142 : {
143 0 : DEFINE_WAIT_FUNC(wq_entry, percpu_rwsem_wake_function);
144 : bool wait;
145 :
146 0 : spin_lock_irq(&sem->waiters.lock);
147 : /*
148 : * Serialize against the wakeup in percpu_up_write(), if we fail
149 : * the trylock, the wakeup must see us on the list.
150 : */
151 0 : wait = !__percpu_rwsem_trylock(sem, reader);
152 0 : if (wait) {
153 0 : wq_entry.flags |= WQ_FLAG_EXCLUSIVE | reader * WQ_FLAG_CUSTOM;
154 0 : __add_wait_queue_entry_tail(&sem->waiters, &wq_entry);
155 : }
156 0 : spin_unlock_irq(&sem->waiters.lock);
157 :
158 0 : while (wait) {
159 0 : set_current_state(TASK_UNINTERRUPTIBLE);
160 0 : if (!smp_load_acquire(&wq_entry.private))
161 : break;
162 0 : schedule();
163 : }
164 0 : __set_current_state(TASK_RUNNING);
165 0 : }
166 :
167 0 : bool __sched __percpu_down_read(struct percpu_rw_semaphore *sem, bool try)
168 : {
169 0 : if (__percpu_down_read_trylock(sem))
170 : return true;
171 :
172 0 : if (try)
173 : return false;
174 :
175 0 : trace_contention_begin(sem, LCB_F_PERCPU | LCB_F_READ);
176 0 : preempt_enable();
177 0 : percpu_rwsem_wait(sem, /* .reader = */ true);
178 0 : preempt_disable();
179 0 : trace_contention_end(sem, 0);
180 :
181 0 : return true;
182 : }
183 : EXPORT_SYMBOL_GPL(__percpu_down_read);
184 :
185 : #define per_cpu_sum(var) \
186 : ({ \
187 : typeof(var) __sum = 0; \
188 : int cpu; \
189 : compiletime_assert_atomic_type(__sum); \
190 : for_each_possible_cpu(cpu) \
191 : __sum += per_cpu(var, cpu); \
192 : __sum; \
193 : })
194 :
195 0 : bool percpu_is_read_locked(struct percpu_rw_semaphore *sem)
196 : {
197 0 : return per_cpu_sum(*sem->read_count) != 0 && !atomic_read(&sem->block);
198 : }
199 : EXPORT_SYMBOL_GPL(percpu_is_read_locked);
200 :
201 : /*
202 : * Return true if the modular sum of the sem->read_count per-CPU variable is
203 : * zero. If this sum is zero, then it is stable due to the fact that if any
204 : * newly arriving readers increment a given counter, they will immediately
205 : * decrement that same counter.
206 : *
207 : * Assumes sem->block is set.
208 : */
209 : static bool readers_active_check(struct percpu_rw_semaphore *sem)
210 : {
211 0 : if (per_cpu_sum(*sem->read_count) != 0)
212 : return false;
213 :
214 : /*
215 : * If we observed the decrement; ensure we see the entire critical
216 : * section.
217 : */
218 :
219 0 : smp_mb(); /* C matches B */
220 :
221 : return true;
222 : }
223 :
224 0 : void __sched percpu_down_write(struct percpu_rw_semaphore *sem)
225 : {
226 : might_sleep();
227 : rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
228 0 : trace_contention_begin(sem, LCB_F_PERCPU | LCB_F_WRITE);
229 :
230 : /* Notify readers to take the slow path. */
231 0 : rcu_sync_enter(&sem->rss);
232 :
233 : /*
234 : * Try set sem->block; this provides writer-writer exclusion.
235 : * Having sem->block set makes new readers block.
236 : */
237 0 : if (!__percpu_down_write_trylock(sem))
238 0 : percpu_rwsem_wait(sem, /* .reader = */ false);
239 :
240 : /* smp_mb() implied by __percpu_down_write_trylock() on success -- D matches A */
241 :
242 : /*
243 : * If they don't see our store of sem->block, then we are guaranteed to
244 : * see their sem->read_count increment, and therefore will wait for
245 : * them.
246 : */
247 :
248 : /* Wait for all active readers to complete. */
249 0 : rcuwait_wait_event(&sem->writer, readers_active_check(sem), TASK_UNINTERRUPTIBLE);
250 0 : trace_contention_end(sem, 0);
251 0 : }
252 : EXPORT_SYMBOL_GPL(percpu_down_write);
253 :
254 0 : void percpu_up_write(struct percpu_rw_semaphore *sem)
255 : {
256 : rwsem_release(&sem->dep_map, _RET_IP_);
257 :
258 : /*
259 : * Signal the writer is done, no fast path yet.
260 : *
261 : * One reason that we cannot just immediately flip to readers_fast is
262 : * that new readers might fail to see the results of this writer's
263 : * critical section.
264 : *
265 : * Therefore we force it through the slow path which guarantees an
266 : * acquire and thereby guarantees the critical section's consistency.
267 : */
268 0 : atomic_set_release(&sem->block, 0);
269 :
270 : /*
271 : * Prod any pending reader/writer to make progress.
272 : */
273 0 : __wake_up(&sem->waiters, TASK_NORMAL, 1, sem);
274 :
275 : /*
276 : * Once this completes (at least one RCU-sched grace period hence) the
277 : * reader fast path will be available again. Safe to use outside the
278 : * exclusive write lock because its counting.
279 : */
280 0 : rcu_sync_exit(&sem->rss);
281 0 : }
282 : EXPORT_SYMBOL_GPL(percpu_up_write);
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