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