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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/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 might_sleep();
227 rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
228 trace_contention_begin(sem, LCB_F_PERCPU | LCB_F_WRITE);
229
230 /* Notify readers to take the slow path. */
231 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 if (!__percpu_down_write_trylock(sem))
238 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 rcuwait_wait_event(&sem->writer, readers_active_check(sem), TASK_UNINTERRUPTIBLE);
250 trace_contention_end(sem, 0);
251}
252EXPORT_SYMBOL_GPL(percpu_down_write);
253
254void 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 atomic_set_release(&sem->block, 0);
269
270 /*
271 * Prod any pending reader/writer to make progress.
272 */
273 __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 rcu_sync_exit(&sem->rss);
281}
282EXPORT_SYMBOL_GPL(percpu_up_write);