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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 atomic_set(&brw->write_ctr, 0);
21 atomic_set(&brw->slow_read_ctr, 0);
22 init_waitqueue_head(&brw->write_waitq);
23 return 0;
24}
25
26void percpu_free_rwsem(struct percpu_rw_semaphore *brw)
27{
28 free_percpu(brw->fast_read_ctr);
29 brw->fast_read_ctr = NULL; /* catch use after free bugs */
30}
31
32/*
33 * This is the fast-path for down_read/up_read, it only needs to ensure
34 * there is no pending writer (atomic_read(write_ctr) == 0) and inc/dec the
35 * fast per-cpu counter. The writer uses synchronize_sched_expedited() to
36 * serialize with the preempt-disabled section below.
37 *
38 * The nontrivial part is that we should guarantee acquire/release semantics
39 * in case when
40 *
41 * R_W: down_write() comes after up_read(), the writer should see all
42 * changes done by the reader
43 * or
44 * W_R: down_read() comes after up_write(), the reader should see all
45 * changes done by the writer
46 *
47 * If this helper fails the callers rely on the normal rw_semaphore and
48 * atomic_dec_and_test(), so in this case we have the necessary barriers.
49 *
50 * But if it succeeds we do not have any barriers, atomic_read(write_ctr) or
51 * __this_cpu_add() below can be reordered with any LOAD/STORE done by the
52 * reader inside the critical section. See the comments in down_write and
53 * up_write below.
54 */
55static bool update_fast_ctr(struct percpu_rw_semaphore *brw, unsigned int val)
56{
57 bool success = false;
58
59 preempt_disable();
60 if (likely(!atomic_read(&brw->write_ctr))) {
61 __this_cpu_add(*brw->fast_read_ctr, val);
62 success = true;
63 }
64 preempt_enable();
65
66 return success;
67}
68
69/*
70 * Like the normal down_read() this is not recursive, the writer can
71 * come after the first percpu_down_read() and create the deadlock.
72 *
73 * Note: returns with lock_is_held(brw->rw_sem) == T for lockdep,
74 * percpu_up_read() does rwsem_release(). This pairs with the usage
75 * of ->rw_sem in percpu_down/up_write().
76 */
77void percpu_down_read(struct percpu_rw_semaphore *brw)
78{
79 might_sleep();
80 if (likely(update_fast_ctr(brw, +1))) {
81 rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 0, _RET_IP_);
82 return;
83 }
84
85 down_read(&brw->rw_sem);
86 atomic_inc(&brw->slow_read_ctr);
87 /* avoid up_read()->rwsem_release() */
88 __up_read(&brw->rw_sem);
89}
90
91void percpu_up_read(struct percpu_rw_semaphore *brw)
92{
93 rwsem_release(&brw->rw_sem.dep_map, 1, _RET_IP_);
94
95 if (likely(update_fast_ctr(brw, -1)))
96 return;
97
98 /* false-positive is possible but harmless */
99 if (atomic_dec_and_test(&brw->slow_read_ctr))
100 wake_up_all(&brw->write_waitq);
101}
102
103static int clear_fast_ctr(struct percpu_rw_semaphore *brw)
104{
105 unsigned int sum = 0;
106 int cpu;
107
108 for_each_possible_cpu(cpu) {
109 sum += per_cpu(*brw->fast_read_ctr, cpu);
110 per_cpu(*brw->fast_read_ctr, cpu) = 0;
111 }
112
113 return sum;
114}
115
116/*
117 * A writer increments ->write_ctr to force the readers to switch to the
118 * slow mode, note the atomic_read() check in update_fast_ctr().
119 *
120 * After that the readers can only inc/dec the slow ->slow_read_ctr counter,
121 * ->fast_read_ctr is stable. Once the writer moves its sum into the slow
122 * counter it represents the number of active readers.
123 *
124 * Finally the writer takes ->rw_sem for writing and blocks the new readers,
125 * then waits until the slow counter becomes zero.
126 */
127void percpu_down_write(struct percpu_rw_semaphore *brw)
128{
129 /* tell update_fast_ctr() there is a pending writer */
130 atomic_inc(&brw->write_ctr);
131 /*
132 * 1. Ensures that write_ctr != 0 is visible to any down_read/up_read
133 * so that update_fast_ctr() can't succeed.
134 *
135 * 2. Ensures we see the result of every previous this_cpu_add() in
136 * update_fast_ctr().
137 *
138 * 3. Ensures that if any reader has exited its critical section via
139 * fast-path, it executes a full memory barrier before we return.
140 * See R_W case in the comment above update_fast_ctr().
141 */
142 synchronize_sched_expedited();
143
144 /* exclude other writers, and block the new readers completely */
145 down_write(&brw->rw_sem);
146
147 /* nobody can use fast_read_ctr, move its sum into slow_read_ctr */
148 atomic_add(clear_fast_ctr(brw), &brw->slow_read_ctr);
149
150 /* wait for all readers to complete their percpu_up_read() */
151 wait_event(brw->write_waitq, !atomic_read(&brw->slow_read_ctr));
152}
153
154void percpu_up_write(struct percpu_rw_semaphore *brw)
155{
156 /* release the lock, but the readers can't use the fast-path */
157 up_write(&brw->rw_sem);
158 /*
159 * Insert the barrier before the next fast-path in down_read,
160 * see W_R case in the comment above update_fast_ctr().
161 */
162 synchronize_sched_expedited();
163 /* the last writer unblocks update_fast_ctr() */
164 atomic_dec(&brw->write_ctr);
165}
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);