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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 <trace/events/btrfs.h>
12#include "misc.h"
13#include "ctree.h"
14#include "extent_io.h"
15#include "locking.h"
16#include "accessors.h"
17
18/*
19 * Lockdep class keys for extent_buffer->lock's in this root. For a given
20 * eb, the lockdep key is determined by the btrfs_root it belongs to and
21 * the level the eb occupies in the tree.
22 *
23 * Different roots are used for different purposes and may nest inside each
24 * other and they require separate keysets. As lockdep keys should be
25 * static, assign keysets according to the purpose of the root as indicated
26 * by btrfs_root->root_key.objectid. This ensures that all special purpose
27 * roots have separate keysets.
28 *
29 * Lock-nesting across peer nodes is always done with the immediate parent
30 * node locked thus preventing deadlock. As lockdep doesn't know this, use
31 * subclass to avoid triggering lockdep warning in such cases.
32 *
33 * The key is set by the readpage_end_io_hook after the buffer has passed
34 * csum validation but before the pages are unlocked. It is also set by
35 * btrfs_init_new_buffer on freshly allocated blocks.
36 *
37 * We also add a check to make sure the highest level of the tree is the
38 * same as our lockdep setup here. If BTRFS_MAX_LEVEL changes, this code
39 * needs update as well.
40 */
41#ifdef CONFIG_DEBUG_LOCK_ALLOC
42#if BTRFS_MAX_LEVEL != 8
43#error
44#endif
45
46#define DEFINE_LEVEL(stem, level) \
47 .names[level] = "btrfs-" stem "-0" #level,
48
49#define DEFINE_NAME(stem) \
50 DEFINE_LEVEL(stem, 0) \
51 DEFINE_LEVEL(stem, 1) \
52 DEFINE_LEVEL(stem, 2) \
53 DEFINE_LEVEL(stem, 3) \
54 DEFINE_LEVEL(stem, 4) \
55 DEFINE_LEVEL(stem, 5) \
56 DEFINE_LEVEL(stem, 6) \
57 DEFINE_LEVEL(stem, 7)
58
59static struct btrfs_lockdep_keyset {
60 u64 id; /* root objectid */
61 /* Longest entry: btrfs-block-group-00 */
62 char names[BTRFS_MAX_LEVEL][24];
63 struct lock_class_key keys[BTRFS_MAX_LEVEL];
64} btrfs_lockdep_keysets[] = {
65 { .id = BTRFS_ROOT_TREE_OBJECTID, DEFINE_NAME("root") },
66 { .id = BTRFS_EXTENT_TREE_OBJECTID, DEFINE_NAME("extent") },
67 { .id = BTRFS_CHUNK_TREE_OBJECTID, DEFINE_NAME("chunk") },
68 { .id = BTRFS_DEV_TREE_OBJECTID, DEFINE_NAME("dev") },
69 { .id = BTRFS_CSUM_TREE_OBJECTID, DEFINE_NAME("csum") },
70 { .id = BTRFS_QUOTA_TREE_OBJECTID, DEFINE_NAME("quota") },
71 { .id = BTRFS_TREE_LOG_OBJECTID, DEFINE_NAME("log") },
72 { .id = BTRFS_TREE_RELOC_OBJECTID, DEFINE_NAME("treloc") },
73 { .id = BTRFS_DATA_RELOC_TREE_OBJECTID, DEFINE_NAME("dreloc") },
74 { .id = BTRFS_UUID_TREE_OBJECTID, DEFINE_NAME("uuid") },
75 { .id = BTRFS_FREE_SPACE_TREE_OBJECTID, DEFINE_NAME("free-space") },
76 { .id = BTRFS_BLOCK_GROUP_TREE_OBJECTID, DEFINE_NAME("block-group") },
77 { .id = BTRFS_RAID_STRIPE_TREE_OBJECTID, DEFINE_NAME("raid-stripe") },
78 { .id = 0, DEFINE_NAME("tree") },
79};
80
81#undef DEFINE_LEVEL
82#undef DEFINE_NAME
83
84void btrfs_set_buffer_lockdep_class(u64 objectid, struct extent_buffer *eb, int level)
85{
86 struct btrfs_lockdep_keyset *ks;
87
88 BUG_ON(level >= ARRAY_SIZE(ks->keys));
89
90 /* Find the matching keyset, id 0 is the default entry */
91 for (ks = btrfs_lockdep_keysets; ks->id; ks++)
92 if (ks->id == objectid)
93 break;
94
95 lockdep_set_class_and_name(&eb->lock, &ks->keys[level], ks->names[level]);
96}
97
98void btrfs_maybe_reset_lockdep_class(struct btrfs_root *root, struct extent_buffer *eb)
99{
100 if (test_bit(BTRFS_ROOT_RESET_LOCKDEP_CLASS, &root->state))
101 btrfs_set_buffer_lockdep_class(root->root_key.objectid,
102 eb, btrfs_header_level(eb));
103}
104
105#endif
106
107#ifdef CONFIG_BTRFS_DEBUG
108static void btrfs_set_eb_lock_owner(struct extent_buffer *eb, pid_t owner)
109{
110 eb->lock_owner = owner;
111}
112#else
113static void btrfs_set_eb_lock_owner(struct extent_buffer *eb, pid_t owner) { }
114#endif
115
116/*
117 * Extent buffer locking
118 * =====================
119 *
120 * We use a rw_semaphore for tree locking, and the semantics are exactly the
121 * same:
122 *
123 * - reader/writer exclusion
124 * - writer/writer exclusion
125 * - reader/reader sharing
126 * - try-lock semantics for readers and writers
127 *
128 * The rwsem implementation does opportunistic spinning which reduces number of
129 * times the locking task needs to sleep.
130 */
131
132/*
133 * __btrfs_tree_read_lock - lock extent buffer for read
134 * @eb: the eb to be locked
135 * @nest: the nesting level to be used for lockdep
136 *
137 * This takes the read lock on the extent buffer, using the specified nesting
138 * level for lockdep purposes.
139 */
140void __btrfs_tree_read_lock(struct extent_buffer *eb, enum btrfs_lock_nesting nest)
141{
142 u64 start_ns = 0;
143
144 if (trace_btrfs_tree_read_lock_enabled())
145 start_ns = ktime_get_ns();
146
147 down_read_nested(&eb->lock, nest);
148 trace_btrfs_tree_read_lock(eb, start_ns);
149}
150
151void btrfs_tree_read_lock(struct extent_buffer *eb)
152{
153 __btrfs_tree_read_lock(eb, BTRFS_NESTING_NORMAL);
154}
155
156/*
157 * Try-lock for read.
158 *
159 * Return 1 if the rwlock has been taken, 0 otherwise
160 */
161int btrfs_try_tree_read_lock(struct extent_buffer *eb)
162{
163 if (down_read_trylock(&eb->lock)) {
164 trace_btrfs_try_tree_read_lock(eb);
165 return 1;
166 }
167 return 0;
168}
169
170/*
171 * Try-lock for write.
172 *
173 * Return 1 if the rwlock has been taken, 0 otherwise
174 */
175int btrfs_try_tree_write_lock(struct extent_buffer *eb)
176{
177 if (down_write_trylock(&eb->lock)) {
178 btrfs_set_eb_lock_owner(eb, current->pid);
179 trace_btrfs_try_tree_write_lock(eb);
180 return 1;
181 }
182 return 0;
183}
184
185/*
186 * Release read lock.
187 */
188void btrfs_tree_read_unlock(struct extent_buffer *eb)
189{
190 trace_btrfs_tree_read_unlock(eb);
191 up_read(&eb->lock);
192}
193
194/*
195 * Lock eb for write.
196 *
197 * @eb: the eb to lock
198 * @nest: the nesting to use for the lock
199 *
200 * Returns with the eb->lock write locked.
201 */
202void __btrfs_tree_lock(struct extent_buffer *eb, enum btrfs_lock_nesting nest)
203 __acquires(&eb->lock)
204{
205 u64 start_ns = 0;
206
207 if (trace_btrfs_tree_lock_enabled())
208 start_ns = ktime_get_ns();
209
210 down_write_nested(&eb->lock, nest);
211 btrfs_set_eb_lock_owner(eb, current->pid);
212 trace_btrfs_tree_lock(eb, start_ns);
213}
214
215void btrfs_tree_lock(struct extent_buffer *eb)
216{
217 __btrfs_tree_lock(eb, BTRFS_NESTING_NORMAL);
218}
219
220/*
221 * Release the write lock.
222 */
223void btrfs_tree_unlock(struct extent_buffer *eb)
224{
225 trace_btrfs_tree_unlock(eb);
226 btrfs_set_eb_lock_owner(eb, 0);
227 up_write(&eb->lock);
228}
229
230/*
231 * This releases any locks held in the path starting at level and going all the
232 * way up to the root.
233 *
234 * btrfs_search_slot will keep the lock held on higher nodes in a few corner
235 * cases, such as COW of the block at slot zero in the node. This ignores
236 * those rules, and it should only be called when there are no more updates to
237 * be done higher up in the tree.
238 */
239void btrfs_unlock_up_safe(struct btrfs_path *path, int level)
240{
241 int i;
242
243 if (path->keep_locks)
244 return;
245
246 for (i = level; i < BTRFS_MAX_LEVEL; i++) {
247 if (!path->nodes[i])
248 continue;
249 if (!path->locks[i])
250 continue;
251 btrfs_tree_unlock_rw(path->nodes[i], path->locks[i]);
252 path->locks[i] = 0;
253 }
254}
255
256/*
257 * Loop around taking references on and locking the root node of the tree until
258 * we end up with a lock on the root node.
259 *
260 * Return: root extent buffer with write lock held
261 */
262struct extent_buffer *btrfs_lock_root_node(struct btrfs_root *root)
263{
264 struct extent_buffer *eb;
265
266 while (1) {
267 eb = btrfs_root_node(root);
268
269 btrfs_maybe_reset_lockdep_class(root, eb);
270 btrfs_tree_lock(eb);
271 if (eb == root->node)
272 break;
273 btrfs_tree_unlock(eb);
274 free_extent_buffer(eb);
275 }
276 return eb;
277}
278
279/*
280 * Loop around taking references on and locking the root node of the tree until
281 * we end up with a lock on the root node.
282 *
283 * Return: root extent buffer with read lock held
284 */
285struct extent_buffer *btrfs_read_lock_root_node(struct btrfs_root *root)
286{
287 struct extent_buffer *eb;
288
289 while (1) {
290 eb = btrfs_root_node(root);
291
292 btrfs_maybe_reset_lockdep_class(root, eb);
293 btrfs_tree_read_lock(eb);
294 if (eb == root->node)
295 break;
296 btrfs_tree_read_unlock(eb);
297 free_extent_buffer(eb);
298 }
299 return eb;
300}
301
302/*
303 * Loop around taking references on and locking the root node of the tree in
304 * nowait mode until we end up with a lock on the root node or returning to
305 * avoid blocking.
306 *
307 * Return: root extent buffer with read lock held or -EAGAIN.
308 */
309struct extent_buffer *btrfs_try_read_lock_root_node(struct btrfs_root *root)
310{
311 struct extent_buffer *eb;
312
313 while (1) {
314 eb = btrfs_root_node(root);
315 if (!btrfs_try_tree_read_lock(eb)) {
316 free_extent_buffer(eb);
317 return ERR_PTR(-EAGAIN);
318 }
319 if (eb == root->node)
320 break;
321 btrfs_tree_read_unlock(eb);
322 free_extent_buffer(eb);
323 }
324 return eb;
325}
326
327/*
328 * DREW locks
329 * ==========
330 *
331 * DREW stands for double-reader-writer-exclusion lock. It's used in situation
332 * where you want to provide A-B exclusion but not AA or BB.
333 *
334 * Currently implementation gives more priority to reader. If a reader and a
335 * writer both race to acquire their respective sides of the lock the writer
336 * would yield its lock as soon as it detects a concurrent reader. Additionally
337 * if there are pending readers no new writers would be allowed to come in and
338 * acquire the lock.
339 */
340
341void btrfs_drew_lock_init(struct btrfs_drew_lock *lock)
342{
343 atomic_set(&lock->readers, 0);
344 atomic_set(&lock->writers, 0);
345 init_waitqueue_head(&lock->pending_readers);
346 init_waitqueue_head(&lock->pending_writers);
347}
348
349/* Return true if acquisition is successful, false otherwise */
350bool btrfs_drew_try_write_lock(struct btrfs_drew_lock *lock)
351{
352 if (atomic_read(&lock->readers))
353 return false;
354
355 atomic_inc(&lock->writers);
356
357 /* Ensure writers count is updated before we check for pending readers */
358 smp_mb__after_atomic();
359 if (atomic_read(&lock->readers)) {
360 btrfs_drew_write_unlock(lock);
361 return false;
362 }
363
364 return true;
365}
366
367void btrfs_drew_write_lock(struct btrfs_drew_lock *lock)
368{
369 while (true) {
370 if (btrfs_drew_try_write_lock(lock))
371 return;
372 wait_event(lock->pending_writers, !atomic_read(&lock->readers));
373 }
374}
375
376void btrfs_drew_write_unlock(struct btrfs_drew_lock *lock)
377{
378 atomic_dec(&lock->writers);
379 cond_wake_up(&lock->pending_readers);
380}
381
382void btrfs_drew_read_lock(struct btrfs_drew_lock *lock)
383{
384 atomic_inc(&lock->readers);
385
386 /*
387 * Ensure the pending reader count is perceieved BEFORE this reader
388 * goes to sleep in case of active writers. This guarantees new writers
389 * won't be allowed and that the current reader will be woken up when
390 * the last active writer finishes its jobs.
391 */
392 smp_mb__after_atomic();
393
394 wait_event(lock->pending_readers, atomic_read(&lock->writers) == 0);
395}
396
397void btrfs_drew_read_unlock(struct btrfs_drew_lock *lock)
398{
399 /*
400 * atomic_dec_and_test implies a full barrier, so woken up writers
401 * are guaranteed to see the decrement
402 */
403 if (atomic_dec_and_test(&lock->readers))
404 wake_up(&lock->pending_writers);
405}
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#include "accessors.h"
16
17/*
18 * Lockdep class keys for extent_buffer->lock's in this root. For a given
19 * eb, the lockdep key is determined by the btrfs_root it belongs to and
20 * the level the eb occupies in the tree.
21 *
22 * Different roots are used for different purposes and may nest inside each
23 * other and they require separate keysets. As lockdep keys should be
24 * static, assign keysets according to the purpose of the root as indicated
25 * by btrfs_root->root_key.objectid. This ensures that all special purpose
26 * roots have separate keysets.
27 *
28 * Lock-nesting across peer nodes is always done with the immediate parent
29 * node locked thus preventing deadlock. As lockdep doesn't know this, use
30 * subclass to avoid triggering lockdep warning in such cases.
31 *
32 * The key is set by the readpage_end_io_hook after the buffer has passed
33 * csum validation but before the pages are unlocked. It is also set by
34 * btrfs_init_new_buffer on freshly allocated blocks.
35 *
36 * We also add a check to make sure the highest level of the tree is the
37 * same as our lockdep setup here. If BTRFS_MAX_LEVEL changes, this code
38 * needs update as well.
39 */
40#ifdef CONFIG_DEBUG_LOCK_ALLOC
41#if BTRFS_MAX_LEVEL != 8
42#error
43#endif
44
45#define DEFINE_LEVEL(stem, level) \
46 .names[level] = "btrfs-" stem "-0" #level,
47
48#define DEFINE_NAME(stem) \
49 DEFINE_LEVEL(stem, 0) \
50 DEFINE_LEVEL(stem, 1) \
51 DEFINE_LEVEL(stem, 2) \
52 DEFINE_LEVEL(stem, 3) \
53 DEFINE_LEVEL(stem, 4) \
54 DEFINE_LEVEL(stem, 5) \
55 DEFINE_LEVEL(stem, 6) \
56 DEFINE_LEVEL(stem, 7)
57
58static struct btrfs_lockdep_keyset {
59 u64 id; /* root objectid */
60 /* Longest entry: btrfs-free-space-00 */
61 char names[BTRFS_MAX_LEVEL][20];
62 struct lock_class_key keys[BTRFS_MAX_LEVEL];
63} btrfs_lockdep_keysets[] = {
64 { .id = BTRFS_ROOT_TREE_OBJECTID, DEFINE_NAME("root") },
65 { .id = BTRFS_EXTENT_TREE_OBJECTID, DEFINE_NAME("extent") },
66 { .id = BTRFS_CHUNK_TREE_OBJECTID, DEFINE_NAME("chunk") },
67 { .id = BTRFS_DEV_TREE_OBJECTID, DEFINE_NAME("dev") },
68 { .id = BTRFS_CSUM_TREE_OBJECTID, DEFINE_NAME("csum") },
69 { .id = BTRFS_QUOTA_TREE_OBJECTID, DEFINE_NAME("quota") },
70 { .id = BTRFS_TREE_LOG_OBJECTID, DEFINE_NAME("log") },
71 { .id = BTRFS_TREE_RELOC_OBJECTID, DEFINE_NAME("treloc") },
72 { .id = BTRFS_DATA_RELOC_TREE_OBJECTID, DEFINE_NAME("dreloc") },
73 { .id = BTRFS_UUID_TREE_OBJECTID, DEFINE_NAME("uuid") },
74 { .id = BTRFS_FREE_SPACE_TREE_OBJECTID, DEFINE_NAME("free-space") },
75 { .id = 0, DEFINE_NAME("tree") },
76};
77
78#undef DEFINE_LEVEL
79#undef DEFINE_NAME
80
81void btrfs_set_buffer_lockdep_class(u64 objectid, struct extent_buffer *eb, int level)
82{
83 struct btrfs_lockdep_keyset *ks;
84
85 BUG_ON(level >= ARRAY_SIZE(ks->keys));
86
87 /* Find the matching keyset, id 0 is the default entry */
88 for (ks = btrfs_lockdep_keysets; ks->id; ks++)
89 if (ks->id == objectid)
90 break;
91
92 lockdep_set_class_and_name(&eb->lock, &ks->keys[level], ks->names[level]);
93}
94
95void btrfs_maybe_reset_lockdep_class(struct btrfs_root *root, struct extent_buffer *eb)
96{
97 if (test_bit(BTRFS_ROOT_RESET_LOCKDEP_CLASS, &root->state))
98 btrfs_set_buffer_lockdep_class(root->root_key.objectid,
99 eb, btrfs_header_level(eb));
100}
101
102#endif
103
104/*
105 * Extent buffer locking
106 * =====================
107 *
108 * We use a rw_semaphore for tree locking, and the semantics are exactly the
109 * same:
110 *
111 * - reader/writer exclusion
112 * - writer/writer exclusion
113 * - reader/reader sharing
114 * - try-lock semantics for readers and writers
115 *
116 * The rwsem implementation does opportunistic spinning which reduces number of
117 * times the locking task needs to sleep.
118 */
119
120/*
121 * __btrfs_tree_read_lock - lock extent buffer for read
122 * @eb: the eb to be locked
123 * @nest: the nesting level to be used for lockdep
124 *
125 * This takes the read lock on the extent buffer, using the specified nesting
126 * level for lockdep purposes.
127 */
128void __btrfs_tree_read_lock(struct extent_buffer *eb, enum btrfs_lock_nesting nest)
129{
130 u64 start_ns = 0;
131
132 if (trace_btrfs_tree_read_lock_enabled())
133 start_ns = ktime_get_ns();
134
135 down_read_nested(&eb->lock, nest);
136 trace_btrfs_tree_read_lock(eb, start_ns);
137}
138
139void btrfs_tree_read_lock(struct extent_buffer *eb)
140{
141 __btrfs_tree_read_lock(eb, BTRFS_NESTING_NORMAL);
142}
143
144/*
145 * Try-lock for read.
146 *
147 * Return 1 if the rwlock has been taken, 0 otherwise
148 */
149int btrfs_try_tree_read_lock(struct extent_buffer *eb)
150{
151 if (down_read_trylock(&eb->lock)) {
152 trace_btrfs_try_tree_read_lock(eb);
153 return 1;
154 }
155 return 0;
156}
157
158/*
159 * Try-lock for write.
160 *
161 * Return 1 if the rwlock has been taken, 0 otherwise
162 */
163int btrfs_try_tree_write_lock(struct extent_buffer *eb)
164{
165 if (down_write_trylock(&eb->lock)) {
166 eb->lock_owner = current->pid;
167 trace_btrfs_try_tree_write_lock(eb);
168 return 1;
169 }
170 return 0;
171}
172
173/*
174 * Release read lock.
175 */
176void btrfs_tree_read_unlock(struct extent_buffer *eb)
177{
178 trace_btrfs_tree_read_unlock(eb);
179 up_read(&eb->lock);
180}
181
182/*
183 * __btrfs_tree_lock - lock eb for write
184 * @eb: the eb to lock
185 * @nest: the nesting to use for the lock
186 *
187 * Returns with the eb->lock write locked.
188 */
189void __btrfs_tree_lock(struct extent_buffer *eb, enum btrfs_lock_nesting nest)
190 __acquires(&eb->lock)
191{
192 u64 start_ns = 0;
193
194 if (trace_btrfs_tree_lock_enabled())
195 start_ns = ktime_get_ns();
196
197 down_write_nested(&eb->lock, nest);
198 eb->lock_owner = current->pid;
199 trace_btrfs_tree_lock(eb, start_ns);
200}
201
202void btrfs_tree_lock(struct extent_buffer *eb)
203{
204 __btrfs_tree_lock(eb, BTRFS_NESTING_NORMAL);
205}
206
207/*
208 * Release the write lock.
209 */
210void btrfs_tree_unlock(struct extent_buffer *eb)
211{
212 trace_btrfs_tree_unlock(eb);
213 eb->lock_owner = 0;
214 up_write(&eb->lock);
215}
216
217/*
218 * This releases any locks held in the path starting at level and going all the
219 * way up to the root.
220 *
221 * btrfs_search_slot will keep the lock held on higher nodes in a few corner
222 * cases, such as COW of the block at slot zero in the node. This ignores
223 * those rules, and it should only be called when there are no more updates to
224 * be done higher up in the tree.
225 */
226void btrfs_unlock_up_safe(struct btrfs_path *path, int level)
227{
228 int i;
229
230 if (path->keep_locks)
231 return;
232
233 for (i = level; i < BTRFS_MAX_LEVEL; i++) {
234 if (!path->nodes[i])
235 continue;
236 if (!path->locks[i])
237 continue;
238 btrfs_tree_unlock_rw(path->nodes[i], path->locks[i]);
239 path->locks[i] = 0;
240 }
241}
242
243/*
244 * Loop around taking references on and locking the root node of the tree until
245 * we end up with a lock on the root node.
246 *
247 * Return: root extent buffer with write lock held
248 */
249struct extent_buffer *btrfs_lock_root_node(struct btrfs_root *root)
250{
251 struct extent_buffer *eb;
252
253 while (1) {
254 eb = btrfs_root_node(root);
255
256 btrfs_maybe_reset_lockdep_class(root, eb);
257 btrfs_tree_lock(eb);
258 if (eb == root->node)
259 break;
260 btrfs_tree_unlock(eb);
261 free_extent_buffer(eb);
262 }
263 return eb;
264}
265
266/*
267 * Loop around taking references on and locking the root node of the tree until
268 * we end up with a lock on the root node.
269 *
270 * Return: root extent buffer with read lock held
271 */
272struct extent_buffer *btrfs_read_lock_root_node(struct btrfs_root *root)
273{
274 struct extent_buffer *eb;
275
276 while (1) {
277 eb = btrfs_root_node(root);
278
279 btrfs_maybe_reset_lockdep_class(root, eb);
280 btrfs_tree_read_lock(eb);
281 if (eb == root->node)
282 break;
283 btrfs_tree_read_unlock(eb);
284 free_extent_buffer(eb);
285 }
286 return eb;
287}
288
289/*
290 * Loop around taking references on and locking the root node of the tree in
291 * nowait mode until we end up with a lock on the root node or returning to
292 * avoid blocking.
293 *
294 * Return: root extent buffer with read lock held or -EAGAIN.
295 */
296struct extent_buffer *btrfs_try_read_lock_root_node(struct btrfs_root *root)
297{
298 struct extent_buffer *eb;
299
300 while (1) {
301 eb = btrfs_root_node(root);
302 if (!btrfs_try_tree_read_lock(eb)) {
303 free_extent_buffer(eb);
304 return ERR_PTR(-EAGAIN);
305 }
306 if (eb == root->node)
307 break;
308 btrfs_tree_read_unlock(eb);
309 free_extent_buffer(eb);
310 }
311 return eb;
312}
313
314/*
315 * DREW locks
316 * ==========
317 *
318 * DREW stands for double-reader-writer-exclusion lock. It's used in situation
319 * where you want to provide A-B exclusion but not AA or BB.
320 *
321 * Currently implementation gives more priority to reader. If a reader and a
322 * writer both race to acquire their respective sides of the lock the writer
323 * would yield its lock as soon as it detects a concurrent reader. Additionally
324 * if there are pending readers no new writers would be allowed to come in and
325 * acquire the lock.
326 */
327
328int btrfs_drew_lock_init(struct btrfs_drew_lock *lock)
329{
330 int ret;
331
332 ret = percpu_counter_init(&lock->writers, 0, GFP_KERNEL);
333 if (ret)
334 return ret;
335
336 atomic_set(&lock->readers, 0);
337 init_waitqueue_head(&lock->pending_readers);
338 init_waitqueue_head(&lock->pending_writers);
339
340 return 0;
341}
342
343void btrfs_drew_lock_destroy(struct btrfs_drew_lock *lock)
344{
345 percpu_counter_destroy(&lock->writers);
346}
347
348/* Return true if acquisition is successful, false otherwise */
349bool btrfs_drew_try_write_lock(struct btrfs_drew_lock *lock)
350{
351 if (atomic_read(&lock->readers))
352 return false;
353
354 percpu_counter_inc(&lock->writers);
355
356 /* Ensure writers count is updated before we check for pending readers */
357 smp_mb();
358 if (atomic_read(&lock->readers)) {
359 btrfs_drew_write_unlock(lock);
360 return false;
361 }
362
363 return true;
364}
365
366void btrfs_drew_write_lock(struct btrfs_drew_lock *lock)
367{
368 while (true) {
369 if (btrfs_drew_try_write_lock(lock))
370 return;
371 wait_event(lock->pending_writers, !atomic_read(&lock->readers));
372 }
373}
374
375void btrfs_drew_write_unlock(struct btrfs_drew_lock *lock)
376{
377 percpu_counter_dec(&lock->writers);
378 cond_wake_up(&lock->pending_readers);
379}
380
381void btrfs_drew_read_lock(struct btrfs_drew_lock *lock)
382{
383 atomic_inc(&lock->readers);
384
385 /*
386 * Ensure the pending reader count is perceieved BEFORE this reader
387 * goes to sleep in case of active writers. This guarantees new writers
388 * won't be allowed and that the current reader will be woken up when
389 * the last active writer finishes its jobs.
390 */
391 smp_mb__after_atomic();
392
393 wait_event(lock->pending_readers,
394 percpu_counter_sum(&lock->writers) == 0);
395}
396
397void btrfs_drew_read_unlock(struct btrfs_drew_lock *lock)
398{
399 /*
400 * atomic_dec_and_test implies a full barrier, so woken up writers
401 * are guaranteed to see the decrement
402 */
403 if (atomic_dec_and_test(&lock->readers))
404 wake_up(&lock->pending_writers);
405}