1 /* SPDX-License-Identifier: GPL-2.0 */
2 #ifndef __LINUX_SEQLOCK_H
3 #define __LINUX_SEQLOCK_H
4 /*
5 * Reader/writer consistent mechanism without starving writers. This type of
6 * lock for data where the reader wants a consistent set of information
7 * and is willing to retry if the information changes. There are two types
8 * of readers:
9 * 1. Sequence readers which never block a writer but they may have to retry
10 * if a writer is in progress by detecting change in sequence number.
11 * Writers do not wait for a sequence reader.
12 * 2. Locking readers which will wait if a writer or another locking reader
13 * is in progress. A locking reader in progress will also block a writer
14 * from going forward. Unlike the regular rwlock, the read lock here is
15 * exclusive so that only one locking reader can get it.
16 *
17 * This is not as cache friendly as brlock. Also, this may not work well
18 * for data that contains pointers, because any writer could
19 * invalidate a pointer that a reader was following.
20 *
21 * Expected non-blocking reader usage:
22 * do {
23 * seq = read_seqbegin(&foo);
24 * ...
25 * } while (read_seqretry(&foo, seq));
26 *
27 *
28 * On non-SMP the spin locks disappear but the writer still needs
29 * to increment the sequence variables because an interrupt routine could
30 * change the state of the data.
31 *
32 * Based on x86_64 vsyscall gettimeofday
33 * by Keith Owens and Andrea Arcangeli
34 */
35
36 #include <linux/spinlock.h>
37 #include <linux/preempt.h>
38 #include <linux/lockdep.h>
39 #include <linux/compiler.h>
40 #include <asm/processor.h>
41
42 /*
43 * Version using sequence counter only.
44 * This can be used when code has its own mutex protecting the
45 * updating starting before the write_seqcountbeqin() and ending
46 * after the write_seqcount_end().
47 */
48 typedef struct seqcount {
49 unsigned sequence;
50 #ifdef CONFIG_DEBUG_LOCK_ALLOC
51 struct lockdep_map dep_map;
52 #endif
53 } seqcount_t;
54
55 static inline void __seqcount_init(seqcount_t *s, const char *name,
56 struct lock_class_key *key)
57 {
58 /*
59 * Make sure we are not reinitializing a held lock:
60 */
61 lockdep_init_map(&s->dep_map, name, key, 0);
62 s->sequence = 0;
63 }
64
65 #ifdef CONFIG_DEBUG_LOCK_ALLOC
66 # define SEQCOUNT_DEP_MAP_INIT(lockname) \
67 .dep_map = { .name = #lockname } \
68
69 # define seqcount_init(s) \
70 do { \
71 static struct lock_class_key __key; \
72 __seqcount_init((s), #s, &__key); \
73 } while (0)
74
75 static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
76 {
77 seqcount_t *l = (seqcount_t *)s;
78 unsigned long flags;
79
80 local_irq_save(flags);
81 seqcount_acquire_read(&l->dep_map, 0, 0, _RET_IP_);
82 seqcount_release(&l->dep_map, 1, _RET_IP_);
83 local_irq_restore(flags);
84 }
85
86 #else
87 # define SEQCOUNT_DEP_MAP_INIT(lockname)
88 # define seqcount_init(s) __seqcount_init(s, NULL, NULL)
89 # define seqcount_lockdep_reader_access(x)
90 #endif
91
92 #define SEQCNT_ZERO(lockname) { .sequence = 0, SEQCOUNT_DEP_MAP_INIT(lockname)}
93
94
95 /**
96 * __read_seqcount_begin - begin a seq-read critical section (without barrier)
97 * @s: pointer to seqcount_t
98 * Returns: count to be passed to read_seqcount_retry
99 *
100 * __read_seqcount_begin is like read_seqcount_begin, but has no smp_rmb()
101 * barrier. Callers should ensure that smp_rmb() or equivalent ordering is
102 * provided before actually loading any of the variables that are to be
103 * protected in this critical section.
104 *
105 * Use carefully, only in critical code, and comment how the barrier is
106 * provided.
107 */
108 static inline unsigned __read_seqcount_begin(const seqcount_t *s)
109 {
110 unsigned ret;
111
112 repeat:
113 ret = READ_ONCE(s->sequence);
114 if (unlikely(ret & 1)) {
115 cpu_relax();
116 goto repeat;
117 }
118 return ret;
119 }
120
121 /**
122 * raw_read_seqcount - Read the raw seqcount
123 * @s: pointer to seqcount_t
124 * Returns: count to be passed to read_seqcount_retry
125 *
126 * raw_read_seqcount opens a read critical section of the given
127 * seqcount without any lockdep checking and without checking or
128 * masking the LSB. Calling code is responsible for handling that.
129 */
130 static inline unsigned raw_read_seqcount(const seqcount_t *s)
131 {
132 unsigned ret = READ_ONCE(s->sequence);
133 smp_rmb();
134 return ret;
135 }
136
137 /**
138 * raw_read_seqcount_begin - start seq-read critical section w/o lockdep
139 * @s: pointer to seqcount_t
140 * Returns: count to be passed to read_seqcount_retry
141 *
142 * raw_read_seqcount_begin opens a read critical section of the given
143 * seqcount, but without any lockdep checking. Validity of the critical
144 * section is tested by checking read_seqcount_retry function.
145 */
146 static inline unsigned raw_read_seqcount_begin(const seqcount_t *s)
147 {
148 unsigned ret = __read_seqcount_begin(s);
149 smp_rmb();
150 return ret;
151 }
152
153 /**
154 * read_seqcount_begin - begin a seq-read critical section
155 * @s: pointer to seqcount_t
156 * Returns: count to be passed to read_seqcount_retry
157 *
158 * read_seqcount_begin opens a read critical section of the given seqcount.
159 * Validity of the critical section is tested by checking read_seqcount_retry
160 * function.
161 */
162 static inline unsigned read_seqcount_begin(const seqcount_t *s)
163 {
164 seqcount_lockdep_reader_access(s);
165 return raw_read_seqcount_begin(s);
166 }
167
168 /**
169 * raw_seqcount_begin - begin a seq-read critical section
170 * @s: pointer to seqcount_t
171 * Returns: count to be passed to read_seqcount_retry
172 *
173 * raw_seqcount_begin opens a read critical section of the given seqcount.
174 * Validity of the critical section is tested by checking read_seqcount_retry
175 * function.
176 *
177 * Unlike read_seqcount_begin(), this function will not wait for the count
178 * to stabilize. If a writer is active when we begin, we will fail the
179 * read_seqcount_retry() instead of stabilizing at the beginning of the
180 * critical section.
181 */
182 static inline unsigned raw_seqcount_begin(const seqcount_t *s)
183 {
184 unsigned ret = READ_ONCE(s->sequence);
185 smp_rmb();
186 return ret & ~1;
187 }
188
189 /**
190 * __read_seqcount_retry - end a seq-read critical section (without barrier)
191 * @s: pointer to seqcount_t
192 * @start: count, from read_seqcount_begin
193 * Returns: 1 if retry is required, else 0
194 *
195 * __read_seqcount_retry is like read_seqcount_retry, but has no smp_rmb()
196 * barrier. Callers should ensure that smp_rmb() or equivalent ordering is
197 * provided before actually loading any of the variables that are to be
198 * protected in this critical section.
199 *
200 * Use carefully, only in critical code, and comment how the barrier is
201 * provided.
202 */
203 static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start)
204 {
205 return unlikely(s->sequence != start);
206 }
207
208 /**
209 * read_seqcount_retry - end a seq-read critical section
210 * @s: pointer to seqcount_t
211 * @start: count, from read_seqcount_begin
212 * Returns: 1 if retry is required, else 0
213 *
214 * read_seqcount_retry closes a read critical section of the given seqcount.
215 * If the critical section was invalid, it must be ignored (and typically
216 * retried).
217 */
218 static inline int read_seqcount_retry(const seqcount_t *s, unsigned start)
219 {
220 smp_rmb();
221 return __read_seqcount_retry(s, start);
222 }
223
224
225
226 static inline void raw_write_seqcount_begin(seqcount_t *s)
227 {
228 s->sequence++;
229 smp_wmb();
230 }
231
232 static inline void raw_write_seqcount_end(seqcount_t *s)
233 {
234 smp_wmb();
235 s->sequence++;
236 }
237
238 /**
239 * raw_write_seqcount_barrier - do a seq write barrier
240 * @s: pointer to seqcount_t
241 *
242 * This can be used to provide an ordering guarantee instead of the
243 * usual consistency guarantee. It is one wmb cheaper, because we can
244 * collapse the two back-to-back wmb()s.
245 *
246 * seqcount_t seq;
247 * bool X = true, Y = false;
248 *
249 * void read(void)
250 * {
251 * bool x, y;
252 *
253 * do {
254 * int s = read_seqcount_begin(&seq);
255 *
256 * x = X; y = Y;
257 *
258 * } while (read_seqcount_retry(&seq, s));
259 *
260 * BUG_ON(!x && !y);
261 * }
262 *
263 * void write(void)
264 * {
265 * Y = true;
266 *
267 * raw_write_seqcount_barrier(seq);
268 *
269 * X = false;
270 * }
271 */
272 static inline void raw_write_seqcount_barrier(seqcount_t *s)
273 {
274 s->sequence++;
275 smp_wmb();
276 s->sequence++;
277 }
278
279 static inline int raw_read_seqcount_latch(seqcount_t *s)
280 {
281 /* Pairs with the first smp_wmb() in raw_write_seqcount_latch() */
282 int seq = READ_ONCE(s->sequence); /* ^^^ */
283 return seq;
284 }
285
286 /**
287 * raw_write_seqcount_latch - redirect readers to even/odd copy
288 * @s: pointer to seqcount_t
289 *
290 * The latch technique is a multiversion concurrency control method that allows
291 * queries during non-atomic modifications. If you can guarantee queries never
292 * interrupt the modification -- e.g. the concurrency is strictly between CPUs
293 * -- you most likely do not need this.
294 *
295 * Where the traditional RCU/lockless data structures rely on atomic
296 * modifications to ensure queries observe either the old or the new state the
297 * latch allows the same for non-atomic updates. The trade-off is doubling the
298 * cost of storage; we have to maintain two copies of the entire data
299 * structure.
300 *
301 * Very simply put: we first modify one copy and then the other. This ensures
302 * there is always one copy in a stable state, ready to give us an answer.
303 *
304 * The basic form is a data structure like:
305 *
306 * struct latch_struct {
307 * seqcount_t seq;
308 * struct data_struct data[2];
309 * };
310 *
311 * Where a modification, which is assumed to be externally serialized, does the
312 * following:
313 *
314 * void latch_modify(struct latch_struct *latch, ...)
315 * {
316 * smp_wmb(); <- Ensure that the last data[1] update is visible
317 * latch->seq++;
318 * smp_wmb(); <- Ensure that the seqcount update is visible
319 *
320 * modify(latch->data[0], ...);
321 *
322 * smp_wmb(); <- Ensure that the data[0] update is visible
323 * latch->seq++;
324 * smp_wmb(); <- Ensure that the seqcount update is visible
325 *
326 * modify(latch->data[1], ...);
327 * }
328 *
329 * The query will have a form like:
330 *
331 * struct entry *latch_query(struct latch_struct *latch, ...)
332 * {
333 * struct entry *entry;
334 * unsigned seq, idx;
335 *
336 * do {
337 * seq = raw_read_seqcount_latch(&latch->seq);
338 *
339 * idx = seq & 0x01;
340 * entry = data_query(latch->data[idx], ...);
341 *
342 * smp_rmb();
343 * } while (seq != latch->seq);
344 *
345 * return entry;
346 * }
347 *
348 * So during the modification, queries are first redirected to data[1]. Then we
349 * modify data[0]. When that is complete, we redirect queries back to data[0]
350 * and we can modify data[1].
351 *
352 * NOTE: The non-requirement for atomic modifications does _NOT_ include
353 * the publishing of new entries in the case where data is a dynamic
354 * data structure.
355 *
356 * An iteration might start in data[0] and get suspended long enough
357 * to miss an entire modification sequence, once it resumes it might
358 * observe the new entry.
359 *
360 * NOTE: When data is a dynamic data structure; one should use regular RCU
361 * patterns to manage the lifetimes of the objects within.
362 */
363 static inline void raw_write_seqcount_latch(seqcount_t *s)
364 {
365 smp_wmb(); /* prior stores before incrementing "sequence" */
366 s->sequence++;
367 smp_wmb(); /* increment "sequence" before following stores */
368 }
369
370 /*
371 * Sequence counter only version assumes that callers are using their
372 * own mutexing.
373 */
374 static inline void write_seqcount_begin_nested(seqcount_t *s, int subclass)
375 {
376 raw_write_seqcount_begin(s);
377 seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
378 }
379
380 static inline void write_seqcount_begin(seqcount_t *s)
381 {
382 write_seqcount_begin_nested(s, 0);
383 }
384
385 static inline void write_seqcount_end(seqcount_t *s)
386 {
387 seqcount_release(&s->dep_map, 1, _RET_IP_);
388 raw_write_seqcount_end(s);
389 }
390
391 /**
392 * write_seqcount_invalidate - invalidate in-progress read-side seq operations
393 * @s: pointer to seqcount_t
394 *
395 * After write_seqcount_invalidate, no read-side seq operations will complete
396 * successfully and see data older than this.
397 */
398 static inline void write_seqcount_invalidate(seqcount_t *s)
399 {
400 smp_wmb();
401 s->sequence+=2;
402 }
403
404 typedef struct {
405 struct seqcount seqcount;
406 spinlock_t lock;
407 } seqlock_t;
408
409 /*
410 * These macros triggered gcc-3.x compile-time problems. We think these are
411 * OK now. Be cautious.
412 */
413 #define __SEQLOCK_UNLOCKED(lockname) \
414 { \
415 .seqcount = SEQCNT_ZERO(lockname), \
416 .lock = __SPIN_LOCK_UNLOCKED(lockname) \
417 }
418
419 #define seqlock_init(x) \
420 do { \
421 seqcount_init(&(x)->seqcount); \
422 spin_lock_init(&(x)->lock); \
423 } while (0)
424
425 #define DEFINE_SEQLOCK(x) \
426 seqlock_t x = __SEQLOCK_UNLOCKED(x)
427
428 /*
429 * Read side functions for starting and finalizing a read side section.
430 */
431 static inline unsigned read_seqbegin(const seqlock_t *sl)
432 {
433 return read_seqcount_begin(&sl->seqcount);
434 }
435
436 static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
437 {
438 return read_seqcount_retry(&sl->seqcount, start);
439 }
440
441 /*
442 * Lock out other writers and update the count.
443 * Acts like a normal spin_lock/unlock.
444 * Don't need preempt_disable() because that is in the spin_lock already.
445 */
446 static inline void write_seqlock(seqlock_t *sl)
447 {
448 spin_lock(&sl->lock);
449 write_seqcount_begin(&sl->seqcount);
450 }
451
452 static inline void write_sequnlock(seqlock_t *sl)
453 {
454 write_seqcount_end(&sl->seqcount);
455 spin_unlock(&sl->lock);
456 }
457
458 static inline void write_seqlock_bh(seqlock_t *sl)
459 {
460 spin_lock_bh(&sl->lock);
461 write_seqcount_begin(&sl->seqcount);
462 }
463
464 static inline void write_sequnlock_bh(seqlock_t *sl)
465 {
466 write_seqcount_end(&sl->seqcount);
467 spin_unlock_bh(&sl->lock);
468 }
469
470 static inline void write_seqlock_irq(seqlock_t *sl)
471 {
472 spin_lock_irq(&sl->lock);
473 write_seqcount_begin(&sl->seqcount);
474 }
475
476 static inline void write_sequnlock_irq(seqlock_t *sl)
477 {
478 write_seqcount_end(&sl->seqcount);
479 spin_unlock_irq(&sl->lock);
480 }
481
482 static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
483 {
484 unsigned long flags;
485
486 spin_lock_irqsave(&sl->lock, flags);
487 write_seqcount_begin(&sl->seqcount);
488 return flags;
489 }
490
491 #define write_seqlock_irqsave(lock, flags) \
492 do { flags = __write_seqlock_irqsave(lock); } while (0)
493
494 static inline void
495 write_sequnlock_irqrestore(seqlock_t *sl, unsigned long flags)
496 {
497 write_seqcount_end(&sl->seqcount);
498 spin_unlock_irqrestore(&sl->lock, flags);
499 }
500
501 /*
502 * A locking reader exclusively locks out other writers and locking readers,
503 * but doesn't update the sequence number. Acts like a normal spin_lock/unlock.
504 * Don't need preempt_disable() because that is in the spin_lock already.
505 */
506 static inline void read_seqlock_excl(seqlock_t *sl)
507 {
508 spin_lock(&sl->lock);
509 }
510
511 static inline void read_sequnlock_excl(seqlock_t *sl)
512 {
513 spin_unlock(&sl->lock);
514 }
515
516 /**
517 * read_seqbegin_or_lock - begin a sequence number check or locking block
518 * @lock: sequence lock
519 * @seq : sequence number to be checked
520 *
521 * First try it once optimistically without taking the lock. If that fails,
522 * take the lock. The sequence number is also used as a marker for deciding
523 * whether to be a reader (even) or writer (odd).
524 * N.B. seq must be initialized to an even number to begin with.
525 */
526 static inline void read_seqbegin_or_lock(seqlock_t *lock, int *seq)
527 {
528 if (!(*seq & 1)) /* Even */
529 *seq = read_seqbegin(lock);
530 else /* Odd */
531 read_seqlock_excl(lock);
532 }
533
534 static inline int need_seqretry(seqlock_t *lock, int seq)
535 {
536 return !(seq & 1) && read_seqretry(lock, seq);
537 }
538
539 static inline void done_seqretry(seqlock_t *lock, int seq)
540 {
541 if (seq & 1)
542 read_sequnlock_excl(lock);
543 }
544
545 static inline void read_seqlock_excl_bh(seqlock_t *sl)
546 {
547 spin_lock_bh(&sl->lock);
548 }
549
550 static inline void read_sequnlock_excl_bh(seqlock_t *sl)
551 {
552 spin_unlock_bh(&sl->lock);
553 }
554
555 static inline void read_seqlock_excl_irq(seqlock_t *sl)
556 {
557 spin_lock_irq(&sl->lock);
558 }
559
560 static inline void read_sequnlock_excl_irq(seqlock_t *sl)
561 {
562 spin_unlock_irq(&sl->lock);
563 }
564
565 static inline unsigned long __read_seqlock_excl_irqsave(seqlock_t *sl)
566 {
567 unsigned long flags;
568
569 spin_lock_irqsave(&sl->lock, flags);
570 return flags;
571 }
572
573 #define read_seqlock_excl_irqsave(lock, flags) \
574 do { flags = __read_seqlock_excl_irqsave(lock); } while (0)
575
576 static inline void
577 read_sequnlock_excl_irqrestore(seqlock_t *sl, unsigned long flags)
578 {
579 spin_unlock_irqrestore(&sl->lock, flags);
580 }
581
582 static inline unsigned long
583 read_seqbegin_or_lock_irqsave(seqlock_t *lock, int *seq)
584 {
585 unsigned long flags = 0;
586
587 if (!(*seq & 1)) /* Even */
588 *seq = read_seqbegin(lock);
589 else /* Odd */
590 read_seqlock_excl_irqsave(lock, flags);
591
592 return flags;
593 }
594
595 static inline void
596 done_seqretry_irqrestore(seqlock_t *lock, int seq, unsigned long flags)
597 {
598 if (seq & 1)
599 read_sequnlock_excl_irqrestore(lock, flags);
600 }
601 #endif /* __LINUX_SEQLOCK_H */