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1/*
2 * Copyright (C) 2008 Oracle. All rights reserved.
3 *
4 * This program is free software; you can redistribute it and/or
5 * modify it under the terms of the GNU General Public
6 * License v2 as published by the Free Software Foundation.
7 *
8 * This program is distributed in the hope that it will be useful,
9 * but WITHOUT ANY WARRANTY; without even the implied warranty of
10 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
11 * General Public License for more details.
12 *
13 * You should have received a copy of the GNU General Public
14 * License along with this program; if not, write to the
15 * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
16 * Boston, MA 021110-1307, USA.
17 */
18#include <linux/sched.h>
19#include <linux/pagemap.h>
20#include <linux/spinlock.h>
21#include <linux/page-flags.h>
22#include <asm/bug.h>
23#include "ctree.h"
24#include "extent_io.h"
25#include "locking.h"
26
27void btrfs_assert_tree_read_locked(struct extent_buffer *eb);
28
29/*
30 * if we currently have a spinning reader or writer lock
31 * (indicated by the rw flag) this will bump the count
32 * of blocking holders and drop the spinlock.
33 */
34void btrfs_set_lock_blocking_rw(struct extent_buffer *eb, int rw)
35{
36 if (rw == BTRFS_WRITE_LOCK) {
37 if (atomic_read(&eb->blocking_writers) == 0) {
38 WARN_ON(atomic_read(&eb->spinning_writers) != 1);
39 atomic_dec(&eb->spinning_writers);
40 btrfs_assert_tree_locked(eb);
41 atomic_inc(&eb->blocking_writers);
42 write_unlock(&eb->lock);
43 }
44 } else if (rw == BTRFS_READ_LOCK) {
45 btrfs_assert_tree_read_locked(eb);
46 atomic_inc(&eb->blocking_readers);
47 WARN_ON(atomic_read(&eb->spinning_readers) == 0);
48 atomic_dec(&eb->spinning_readers);
49 read_unlock(&eb->lock);
50 }
51 return;
52}
53
54/*
55 * if we currently have a blocking lock, take the spinlock
56 * and drop our blocking count
57 */
58void btrfs_clear_lock_blocking_rw(struct extent_buffer *eb, int rw)
59{
60 if (rw == BTRFS_WRITE_LOCK_BLOCKING) {
61 BUG_ON(atomic_read(&eb->blocking_writers) != 1);
62 write_lock(&eb->lock);
63 WARN_ON(atomic_read(&eb->spinning_writers));
64 atomic_inc(&eb->spinning_writers);
65 if (atomic_dec_and_test(&eb->blocking_writers))
66 wake_up(&eb->write_lock_wq);
67 } else if (rw == BTRFS_READ_LOCK_BLOCKING) {
68 BUG_ON(atomic_read(&eb->blocking_readers) == 0);
69 read_lock(&eb->lock);
70 atomic_inc(&eb->spinning_readers);
71 if (atomic_dec_and_test(&eb->blocking_readers))
72 wake_up(&eb->read_lock_wq);
73 }
74 return;
75}
76
77/*
78 * take a spinning read lock. This will wait for any blocking
79 * writers
80 */
81void btrfs_tree_read_lock(struct extent_buffer *eb)
82{
83again:
84 wait_event(eb->write_lock_wq, atomic_read(&eb->blocking_writers) == 0);
85 read_lock(&eb->lock);
86 if (atomic_read(&eb->blocking_writers)) {
87 read_unlock(&eb->lock);
88 wait_event(eb->write_lock_wq,
89 atomic_read(&eb->blocking_writers) == 0);
90 goto again;
91 }
92 atomic_inc(&eb->read_locks);
93 atomic_inc(&eb->spinning_readers);
94}
95
96/*
97 * returns 1 if we get the read lock and 0 if we don't
98 * this won't wait for blocking writers
99 */
100int btrfs_try_tree_read_lock(struct extent_buffer *eb)
101{
102 if (atomic_read(&eb->blocking_writers))
103 return 0;
104
105 read_lock(&eb->lock);
106 if (atomic_read(&eb->blocking_writers)) {
107 read_unlock(&eb->lock);
108 return 0;
109 }
110 atomic_inc(&eb->read_locks);
111 atomic_inc(&eb->spinning_readers);
112 return 1;
113}
114
115/*
116 * returns 1 if we get the read lock and 0 if we don't
117 * this won't wait for blocking writers or readers
118 */
119int btrfs_try_tree_write_lock(struct extent_buffer *eb)
120{
121 if (atomic_read(&eb->blocking_writers) ||
122 atomic_read(&eb->blocking_readers))
123 return 0;
124 write_lock(&eb->lock);
125 if (atomic_read(&eb->blocking_writers) ||
126 atomic_read(&eb->blocking_readers)) {
127 write_unlock(&eb->lock);
128 return 0;
129 }
130 atomic_inc(&eb->write_locks);
131 atomic_inc(&eb->spinning_writers);
132 return 1;
133}
134
135/*
136 * drop a spinning read lock
137 */
138void btrfs_tree_read_unlock(struct extent_buffer *eb)
139{
140 btrfs_assert_tree_read_locked(eb);
141 WARN_ON(atomic_read(&eb->spinning_readers) == 0);
142 atomic_dec(&eb->spinning_readers);
143 atomic_dec(&eb->read_locks);
144 read_unlock(&eb->lock);
145}
146
147/*
148 * drop a blocking read lock
149 */
150void btrfs_tree_read_unlock_blocking(struct extent_buffer *eb)
151{
152 btrfs_assert_tree_read_locked(eb);
153 WARN_ON(atomic_read(&eb->blocking_readers) == 0);
154 if (atomic_dec_and_test(&eb->blocking_readers))
155 wake_up(&eb->read_lock_wq);
156 atomic_dec(&eb->read_locks);
157}
158
159/*
160 * take a spinning write lock. This will wait for both
161 * blocking readers or writers
162 */
163int btrfs_tree_lock(struct extent_buffer *eb)
164{
165again:
166 wait_event(eb->read_lock_wq, atomic_read(&eb->blocking_readers) == 0);
167 wait_event(eb->write_lock_wq, atomic_read(&eb->blocking_writers) == 0);
168 write_lock(&eb->lock);
169 if (atomic_read(&eb->blocking_readers)) {
170 write_unlock(&eb->lock);
171 wait_event(eb->read_lock_wq,
172 atomic_read(&eb->blocking_readers) == 0);
173 goto again;
174 }
175 if (atomic_read(&eb->blocking_writers)) {
176 write_unlock(&eb->lock);
177 wait_event(eb->write_lock_wq,
178 atomic_read(&eb->blocking_writers) == 0);
179 goto again;
180 }
181 WARN_ON(atomic_read(&eb->spinning_writers));
182 atomic_inc(&eb->spinning_writers);
183 atomic_inc(&eb->write_locks);
184 return 0;
185}
186
187/*
188 * drop a spinning or a blocking write lock.
189 */
190int btrfs_tree_unlock(struct extent_buffer *eb)
191{
192 int blockers = atomic_read(&eb->blocking_writers);
193
194 BUG_ON(blockers > 1);
195
196 btrfs_assert_tree_locked(eb);
197 atomic_dec(&eb->write_locks);
198
199 if (blockers) {
200 WARN_ON(atomic_read(&eb->spinning_writers));
201 atomic_dec(&eb->blocking_writers);
202 smp_wmb();
203 wake_up(&eb->write_lock_wq);
204 } else {
205 WARN_ON(atomic_read(&eb->spinning_writers) != 1);
206 atomic_dec(&eb->spinning_writers);
207 write_unlock(&eb->lock);
208 }
209 return 0;
210}
211
212void btrfs_assert_tree_locked(struct extent_buffer *eb)
213{
214 BUG_ON(!atomic_read(&eb->write_locks));
215}
216
217void btrfs_assert_tree_read_locked(struct extent_buffer *eb)
218{
219 BUG_ON(!atomic_read(&eb->read_locks));
220}
1// SPDX-License-Identifier: GPL-2.0
2/*
3 * Copyright (C) 2008 Oracle. All rights reserved.
4 */
5
6#include <linux/sched.h>
7#include <linux/pagemap.h>
8#include <linux/spinlock.h>
9#include <linux/page-flags.h>
10#include <asm/bug.h>
11#include "misc.h"
12#include "ctree.h"
13#include "extent_io.h"
14#include "locking.h"
15
16/*
17 * Extent buffer locking
18 * =====================
19 *
20 * We use a rw_semaphore for tree locking, and the semantics are exactly the
21 * same:
22 *
23 * - reader/writer exclusion
24 * - writer/writer exclusion
25 * - reader/reader sharing
26 * - try-lock semantics for readers and writers
27 *
28 * The rwsem implementation does opportunistic spinning which reduces number of
29 * times the locking task needs to sleep.
30 */
31
32/*
33 * __btrfs_tree_read_lock - lock extent buffer for read
34 * @eb: the eb to be locked
35 * @nest: the nesting level to be used for lockdep
36 *
37 * This takes the read lock on the extent buffer, using the specified nesting
38 * level for lockdep purposes.
39 */
40void __btrfs_tree_read_lock(struct extent_buffer *eb, enum btrfs_lock_nesting nest)
41{
42 u64 start_ns = 0;
43
44 if (trace_btrfs_tree_read_lock_enabled())
45 start_ns = ktime_get_ns();
46
47 down_read_nested(&eb->lock, nest);
48 eb->lock_owner = current->pid;
49 trace_btrfs_tree_read_lock(eb, start_ns);
50}
51
52void btrfs_tree_read_lock(struct extent_buffer *eb)
53{
54 __btrfs_tree_read_lock(eb, BTRFS_NESTING_NORMAL);
55}
56
57/*
58 * Try-lock for read.
59 *
60 * Return 1 if the rwlock has been taken, 0 otherwise
61 */
62int btrfs_try_tree_read_lock(struct extent_buffer *eb)
63{
64 if (down_read_trylock(&eb->lock)) {
65 eb->lock_owner = current->pid;
66 trace_btrfs_try_tree_read_lock(eb);
67 return 1;
68 }
69 return 0;
70}
71
72/*
73 * Try-lock for write.
74 *
75 * Return 1 if the rwlock has been taken, 0 otherwise
76 */
77int btrfs_try_tree_write_lock(struct extent_buffer *eb)
78{
79 if (down_write_trylock(&eb->lock)) {
80 eb->lock_owner = current->pid;
81 trace_btrfs_try_tree_write_lock(eb);
82 return 1;
83 }
84 return 0;
85}
86
87/*
88 * Release read lock.
89 */
90void btrfs_tree_read_unlock(struct extent_buffer *eb)
91{
92 trace_btrfs_tree_read_unlock(eb);
93 eb->lock_owner = 0;
94 up_read(&eb->lock);
95}
96
97/*
98 * __btrfs_tree_lock - lock eb for write
99 * @eb: the eb to lock
100 * @nest: the nesting to use for the lock
101 *
102 * Returns with the eb->lock write locked.
103 */
104void __btrfs_tree_lock(struct extent_buffer *eb, enum btrfs_lock_nesting nest)
105 __acquires(&eb->lock)
106{
107 u64 start_ns = 0;
108
109 if (trace_btrfs_tree_lock_enabled())
110 start_ns = ktime_get_ns();
111
112 down_write_nested(&eb->lock, nest);
113 eb->lock_owner = current->pid;
114 trace_btrfs_tree_lock(eb, start_ns);
115}
116
117void btrfs_tree_lock(struct extent_buffer *eb)
118{
119 __btrfs_tree_lock(eb, BTRFS_NESTING_NORMAL);
120}
121
122/*
123 * Release the write lock.
124 */
125void btrfs_tree_unlock(struct extent_buffer *eb)
126{
127 trace_btrfs_tree_unlock(eb);
128 eb->lock_owner = 0;
129 up_write(&eb->lock);
130}
131
132/*
133 * This releases any locks held in the path starting at level and going all the
134 * way up to the root.
135 *
136 * btrfs_search_slot will keep the lock held on higher nodes in a few corner
137 * cases, such as COW of the block at slot zero in the node. This ignores
138 * those rules, and it should only be called when there are no more updates to
139 * be done higher up in the tree.
140 */
141void btrfs_unlock_up_safe(struct btrfs_path *path, int level)
142{
143 int i;
144
145 if (path->keep_locks)
146 return;
147
148 for (i = level; i < BTRFS_MAX_LEVEL; i++) {
149 if (!path->nodes[i])
150 continue;
151 if (!path->locks[i])
152 continue;
153 btrfs_tree_unlock_rw(path->nodes[i], path->locks[i]);
154 path->locks[i] = 0;
155 }
156}
157
158/*
159 * Loop around taking references on and locking the root node of the tree until
160 * we end up with a lock on the root node.
161 *
162 * Return: root extent buffer with write lock held
163 */
164struct extent_buffer *btrfs_lock_root_node(struct btrfs_root *root)
165{
166 struct extent_buffer *eb;
167
168 while (1) {
169 eb = btrfs_root_node(root);
170 btrfs_tree_lock(eb);
171 if (eb == root->node)
172 break;
173 btrfs_tree_unlock(eb);
174 free_extent_buffer(eb);
175 }
176 return eb;
177}
178
179/*
180 * Loop around taking references on and locking the root node of the tree until
181 * we end up with a lock on the root node.
182 *
183 * Return: root extent buffer with read lock held
184 */
185struct extent_buffer *btrfs_read_lock_root_node(struct btrfs_root *root)
186{
187 struct extent_buffer *eb;
188
189 while (1) {
190 eb = btrfs_root_node(root);
191 btrfs_tree_read_lock(eb);
192 if (eb == root->node)
193 break;
194 btrfs_tree_read_unlock(eb);
195 free_extent_buffer(eb);
196 }
197 return eb;
198}
199
200/*
201 * DREW locks
202 * ==========
203 *
204 * DREW stands for double-reader-writer-exclusion lock. It's used in situation
205 * where you want to provide A-B exclusion but not AA or BB.
206 *
207 * Currently implementation gives more priority to reader. If a reader and a
208 * writer both race to acquire their respective sides of the lock the writer
209 * would yield its lock as soon as it detects a concurrent reader. Additionally
210 * if there are pending readers no new writers would be allowed to come in and
211 * acquire the lock.
212 */
213
214int btrfs_drew_lock_init(struct btrfs_drew_lock *lock)
215{
216 int ret;
217
218 ret = percpu_counter_init(&lock->writers, 0, GFP_KERNEL);
219 if (ret)
220 return ret;
221
222 atomic_set(&lock->readers, 0);
223 init_waitqueue_head(&lock->pending_readers);
224 init_waitqueue_head(&lock->pending_writers);
225
226 return 0;
227}
228
229void btrfs_drew_lock_destroy(struct btrfs_drew_lock *lock)
230{
231 percpu_counter_destroy(&lock->writers);
232}
233
234/* Return true if acquisition is successful, false otherwise */
235bool btrfs_drew_try_write_lock(struct btrfs_drew_lock *lock)
236{
237 if (atomic_read(&lock->readers))
238 return false;
239
240 percpu_counter_inc(&lock->writers);
241
242 /* Ensure writers count is updated before we check for pending readers */
243 smp_mb();
244 if (atomic_read(&lock->readers)) {
245 btrfs_drew_write_unlock(lock);
246 return false;
247 }
248
249 return true;
250}
251
252void btrfs_drew_write_lock(struct btrfs_drew_lock *lock)
253{
254 while (true) {
255 if (btrfs_drew_try_write_lock(lock))
256 return;
257 wait_event(lock->pending_writers, !atomic_read(&lock->readers));
258 }
259}
260
261void btrfs_drew_write_unlock(struct btrfs_drew_lock *lock)
262{
263 percpu_counter_dec(&lock->writers);
264 cond_wake_up(&lock->pending_readers);
265}
266
267void btrfs_drew_read_lock(struct btrfs_drew_lock *lock)
268{
269 atomic_inc(&lock->readers);
270
271 /*
272 * Ensure the pending reader count is perceieved BEFORE this reader
273 * goes to sleep in case of active writers. This guarantees new writers
274 * won't be allowed and that the current reader will be woken up when
275 * the last active writer finishes its jobs.
276 */
277 smp_mb__after_atomic();
278
279 wait_event(lock->pending_readers,
280 percpu_counter_sum(&lock->writers) == 0);
281}
282
283void btrfs_drew_read_unlock(struct btrfs_drew_lock *lock)
284{
285 /*
286 * atomic_dec_and_test implies a full barrier, so woken up writers
287 * are guaranteed to see the decrement
288 */
289 if (atomic_dec_and_test(&lock->readers))
290 wake_up(&lock->pending_writers);
291}