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1#include <linux/atomic.h>
2#include <linux/rwsem.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/errno.h>
10
11int __percpu_init_rwsem(struct percpu_rw_semaphore *brw,
12 const char *name, struct lock_class_key *rwsem_key)
13{
14 brw->fast_read_ctr = alloc_percpu(int);
15 if (unlikely(!brw->fast_read_ctr))
16 return -ENOMEM;
17
18 /* ->rw_sem represents the whole percpu_rw_semaphore for lockdep */
19 __init_rwsem(&brw->rw_sem, name, rwsem_key);
20 rcu_sync_init(&brw->rss, RCU_SCHED_SYNC);
21 atomic_set(&brw->slow_read_ctr, 0);
22 init_waitqueue_head(&brw->write_waitq);
23 return 0;
24}
25EXPORT_SYMBOL_GPL(__percpu_init_rwsem);
26
27void percpu_free_rwsem(struct percpu_rw_semaphore *brw)
28{
29 /*
30 * XXX: temporary kludge. The error path in alloc_super()
31 * assumes that percpu_free_rwsem() is safe after kzalloc().
32 */
33 if (!brw->fast_read_ctr)
34 return;
35
36 rcu_sync_dtor(&brw->rss);
37 free_percpu(brw->fast_read_ctr);
38 brw->fast_read_ctr = NULL; /* catch use after free bugs */
39}
40
41/*
42 * This is the fast-path for down_read/up_read. If it succeeds we rely
43 * on the barriers provided by rcu_sync_enter/exit; see the comments in
44 * percpu_down_write() and percpu_up_write().
45 *
46 * If this helper fails the callers rely on the normal rw_semaphore and
47 * atomic_dec_and_test(), so in this case we have the necessary barriers.
48 */
49static bool update_fast_ctr(struct percpu_rw_semaphore *brw, unsigned int val)
50{
51 bool success;
52
53 preempt_disable();
54 success = rcu_sync_is_idle(&brw->rss);
55 if (likely(success))
56 __this_cpu_add(*brw->fast_read_ctr, val);
57 preempt_enable();
58
59 return success;
60}
61
62/*
63 * Like the normal down_read() this is not recursive, the writer can
64 * come after the first percpu_down_read() and create the deadlock.
65 *
66 * Note: returns with lock_is_held(brw->rw_sem) == T for lockdep,
67 * percpu_up_read() does rwsem_release(). This pairs with the usage
68 * of ->rw_sem in percpu_down/up_write().
69 */
70void percpu_down_read(struct percpu_rw_semaphore *brw)
71{
72 might_sleep();
73 rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 0, _RET_IP_);
74
75 if (likely(update_fast_ctr(brw, +1)))
76 return;
77
78 /* Avoid rwsem_acquire_read() and rwsem_release() */
79 __down_read(&brw->rw_sem);
80 atomic_inc(&brw->slow_read_ctr);
81 __up_read(&brw->rw_sem);
82}
83EXPORT_SYMBOL_GPL(percpu_down_read);
84
85int percpu_down_read_trylock(struct percpu_rw_semaphore *brw)
86{
87 if (unlikely(!update_fast_ctr(brw, +1))) {
88 if (!__down_read_trylock(&brw->rw_sem))
89 return 0;
90 atomic_inc(&brw->slow_read_ctr);
91 __up_read(&brw->rw_sem);
92 }
93
94 rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 1, _RET_IP_);
95 return 1;
96}
97
98void percpu_up_read(struct percpu_rw_semaphore *brw)
99{
100 rwsem_release(&brw->rw_sem.dep_map, 1, _RET_IP_);
101
102 if (likely(update_fast_ctr(brw, -1)))
103 return;
104
105 /* false-positive is possible but harmless */
106 if (atomic_dec_and_test(&brw->slow_read_ctr))
107 wake_up_all(&brw->write_waitq);
108}
109EXPORT_SYMBOL_GPL(percpu_up_read);
110
111static int clear_fast_ctr(struct percpu_rw_semaphore *brw)
112{
113 unsigned int sum = 0;
114 int cpu;
115
116 for_each_possible_cpu(cpu) {
117 sum += per_cpu(*brw->fast_read_ctr, cpu);
118 per_cpu(*brw->fast_read_ctr, cpu) = 0;
119 }
120
121 return sum;
122}
123
124void percpu_down_write(struct percpu_rw_semaphore *brw)
125{
126 /*
127 * Make rcu_sync_is_idle() == F and thus disable the fast-path in
128 * percpu_down_read() and percpu_up_read(), and wait for gp pass.
129 *
130 * The latter synchronises us with the preceding readers which used
131 * the fast-past, so we can not miss the result of __this_cpu_add()
132 * or anything else inside their criticial sections.
133 */
134 rcu_sync_enter(&brw->rss);
135
136 /* exclude other writers, and block the new readers completely */
137 down_write(&brw->rw_sem);
138
139 /* nobody can use fast_read_ctr, move its sum into slow_read_ctr */
140 atomic_add(clear_fast_ctr(brw), &brw->slow_read_ctr);
141
142 /* wait for all readers to complete their percpu_up_read() */
143 wait_event(brw->write_waitq, !atomic_read(&brw->slow_read_ctr));
144}
145EXPORT_SYMBOL_GPL(percpu_down_write);
146
147void percpu_up_write(struct percpu_rw_semaphore *brw)
148{
149 /* release the lock, but the readers can't use the fast-path */
150 up_write(&brw->rw_sem);
151 /*
152 * Enable the fast-path in percpu_down_read() and percpu_up_read()
153 * but only after another gp pass; this adds the necessary barrier
154 * to ensure the reader can't miss the changes done by us.
155 */
156 rcu_sync_exit(&brw->rss);
157}
158EXPORT_SYMBOL_GPL(percpu_up_write);
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(&sem->writer);
23 init_waitqueue_head(&sem->waiters);
24 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 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(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(&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(&sem->block))
87 return false;
88
89 return atomic_xchg(&sem->block, 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(wq_entry->private);
132 list_del_init(&wq_entry->entry);
133 smp_store_release(&wq_entry->private, NULL);
134
135 wake_up_process(p);
136 put_task_struct(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{
143 DEFINE_WAIT_FUNC(wq_entry, percpu_rwsem_wake_function);
144 bool wait;
145
146 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 wait = !__percpu_rwsem_trylock(sem, reader);
152 if (wait) {
153 wq_entry.flags |= WQ_FLAG_EXCLUSIVE | reader * WQ_FLAG_CUSTOM;
154 __add_wait_queue_entry_tail(&sem->waiters, &wq_entry);
155 }
156 spin_unlock_irq(&sem->waiters.lock);
157
158 while (wait) {
159 set_current_state(TASK_UNINTERRUPTIBLE);
160 if (!smp_load_acquire(&wq_entry.private))
161 break;
162 schedule();
163 }
164 __set_current_state(TASK_RUNNING);
165}
166
167bool __sched __percpu_down_read(struct percpu_rw_semaphore *sem, bool try)
168{
169 if (__percpu_down_read_trylock(sem))
170 return true;
171
172 if (try)
173 return false;
174
175 trace_contention_begin(sem, LCB_F_PERCPU | LCB_F_READ);
176 preempt_enable();
177 percpu_rwsem_wait(sem, /* .reader = */ true);
178 preempt_disable();
179 trace_contention_end(sem, 0);
180
181 return true;
182}
183EXPORT_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
195bool percpu_is_read_locked(struct percpu_rw_semaphore *sem)
196{
197 return per_cpu_sum(*sem->read_count) != 0 && !atomic_read(&sem->block);
198}
199EXPORT_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 */
209static bool readers_active_check(struct percpu_rw_semaphore *sem)
210{
211 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 smp_mb(); /* C matches B */
220
221 return true;
222}
223
224void __sched percpu_down_write(struct percpu_rw_semaphore *sem)
225{
226 bool contended = false;
227
228 might_sleep();
229 rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
230
231 /* Notify readers to take the slow path. */
232 rcu_sync_enter(&sem->rss);
233
234 /*
235 * Try set sem->block; this provides writer-writer exclusion.
236 * Having sem->block set makes new readers block.
237 */
238 if (!__percpu_down_write_trylock(sem)) {
239 trace_contention_begin(sem, LCB_F_PERCPU | LCB_F_WRITE);
240 percpu_rwsem_wait(sem, /* .reader = */ false);
241 contended = true;
242 }
243
244 /* smp_mb() implied by __percpu_down_write_trylock() on success -- D matches A */
245
246 /*
247 * If they don't see our store of sem->block, then we are guaranteed to
248 * see their sem->read_count increment, and therefore will wait for
249 * them.
250 */
251
252 /* Wait for all active readers to complete. */
253 rcuwait_wait_event(&sem->writer, readers_active_check(sem), TASK_UNINTERRUPTIBLE);
254 if (contended)
255 trace_contention_end(sem, 0);
256}
257EXPORT_SYMBOL_GPL(percpu_down_write);
258
259void percpu_up_write(struct percpu_rw_semaphore *sem)
260{
261 rwsem_release(&sem->dep_map, _RET_IP_);
262
263 /*
264 * Signal the writer is done, no fast path yet.
265 *
266 * One reason that we cannot just immediately flip to readers_fast is
267 * that new readers might fail to see the results of this writer's
268 * critical section.
269 *
270 * Therefore we force it through the slow path which guarantees an
271 * acquire and thereby guarantees the critical section's consistency.
272 */
273 atomic_set_release(&sem->block, 0);
274
275 /*
276 * Prod any pending reader/writer to make progress.
277 */
278 __wake_up(&sem->waiters, TASK_NORMAL, 1, sem);
279
280 /*
281 * Once this completes (at least one RCU-sched grace period hence) the
282 * reader fast path will be available again. Safe to use outside the
283 * exclusive write lock because its counting.
284 */
285 rcu_sync_exit(&sem->rss);
286}
287EXPORT_SYMBOL_GPL(percpu_up_write);