1/* SPDX-License-Identifier: GPL-2.0 */
2#ifndef __LINUX_SEQLOCK_H
3#define __LINUX_SEQLOCK_H
4
5/*
6 * seqcount_t / seqlock_t - a reader-writer consistency mechanism with
7 * lockless readers (read-only retry loops), and no writer starvation.
8 *
9 * See Documentation/locking/seqlock.rst
10 *
11 * Copyrights:
12 * - Based on x86_64 vsyscall gettimeofday: Keith Owens, Andrea Arcangeli
13 * - Sequence counters with associated locks, (C) 2020 Linutronix GmbH
14 */
15
16#include <linux/compiler.h>
17#include <linux/kcsan-checks.h>
18#include <linux/lockdep.h>
19#include <linux/mutex.h>
20#include <linux/preempt.h>
21#include <linux/seqlock_types.h>
22#include <linux/spinlock.h>
23
24#include <asm/processor.h>
25
26/*
27 * The seqlock seqcount_t interface does not prescribe a precise sequence of
28 * read begin/retry/end. For readers, typically there is a call to
29 * read_seqcount_begin() and read_seqcount_retry(), however, there are more
30 * esoteric cases which do not follow this pattern.
31 *
32 * As a consequence, we take the following best-effort approach for raw usage
33 * via seqcount_t under KCSAN: upon beginning a seq-reader critical section,
34 * pessimistically mark the next KCSAN_SEQLOCK_REGION_MAX memory accesses as
35 * atomics; if there is a matching read_seqcount_retry() call, no following
36 * memory operations are considered atomic. Usage of the seqlock_t interface
37 * is not affected.
38 */
39#define KCSAN_SEQLOCK_REGION_MAX 1000
40
41static inline void __seqcount_init(seqcount_t *s, const char *name,
42 struct lock_class_key *key)
43{
44 /*
45 * Make sure we are not reinitializing a held lock:
46 */
47 lockdep_init_map(lock: &s->dep_map, name, key, subclass: 0);
48 s->sequence = 0;
49}
50
51#ifdef CONFIG_DEBUG_LOCK_ALLOC
52
53# define SEQCOUNT_DEP_MAP_INIT(lockname) \
54 .dep_map = { .name = #lockname }
55
56/**
57 * seqcount_init() - runtime initializer for seqcount_t
58 * @s: Pointer to the seqcount_t instance
59 */
60# define seqcount_init(s) \
61 do { \
62 static struct lock_class_key __key; \
63 __seqcount_init((s), #s, &__key); \
64 } while (0)
65
66static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
67{
68 seqcount_t *l = (seqcount_t *)s;
69 unsigned long flags;
70
71 local_irq_save(flags);
72 seqcount_acquire_read(&l->dep_map, 0, 0, _RET_IP_);
73 seqcount_release(&l->dep_map, _RET_IP_);
74 local_irq_restore(flags);
75}
76
77#else
78# define SEQCOUNT_DEP_MAP_INIT(lockname)
79# define seqcount_init(s) __seqcount_init(s, NULL, NULL)
80# define seqcount_lockdep_reader_access(x)
81#endif
82
83/**
84 * SEQCNT_ZERO() - static initializer for seqcount_t
85 * @name: Name of the seqcount_t instance
86 */
87#define SEQCNT_ZERO(name) { .sequence = 0, SEQCOUNT_DEP_MAP_INIT(name) }
88
89/*
90 * Sequence counters with associated locks (seqcount_LOCKNAME_t)
91 *
92 * A sequence counter which associates the lock used for writer
93 * serialization at initialization time. This enables lockdep to validate
94 * that the write side critical section is properly serialized.
95 *
96 * For associated locks which do not implicitly disable preemption,
97 * preemption protection is enforced in the write side function.
98 *
99 * Lockdep is never used in any for the raw write variants.
100 *
101 * See Documentation/locking/seqlock.rst
102 */
103
104/*
105 * typedef seqcount_LOCKNAME_t - sequence counter with LOCKNAME associated
106 * @seqcount: The real sequence counter
107 * @lock: Pointer to the associated lock
108 *
109 * A plain sequence counter with external writer synchronization by
110 * LOCKNAME @lock. The lock is associated to the sequence counter in the
111 * static initializer or init function. This enables lockdep to validate
112 * that the write side critical section is properly serialized.
113 *
114 * LOCKNAME: raw_spinlock, spinlock, rwlock or mutex
115 */
116
117/*
118 * seqcount_LOCKNAME_init() - runtime initializer for seqcount_LOCKNAME_t
119 * @s: Pointer to the seqcount_LOCKNAME_t instance
120 * @lock: Pointer to the associated lock
121 */
122
123#define seqcount_LOCKNAME_init(s, _lock, lockname) \
124 do { \
125 seqcount_##lockname##_t *____s = (s); \
126 seqcount_init(&____s->seqcount); \
127 __SEQ_LOCK(____s->lock = (_lock)); \
128 } while (0)
129
130#define seqcount_raw_spinlock_init(s, lock) seqcount_LOCKNAME_init(s, lock, raw_spinlock)
131#define seqcount_spinlock_init(s, lock) seqcount_LOCKNAME_init(s, lock, spinlock)
132#define seqcount_rwlock_init(s, lock) seqcount_LOCKNAME_init(s, lock, rwlock)
133#define seqcount_mutex_init(s, lock) seqcount_LOCKNAME_init(s, lock, mutex)
134
135/*
136 * SEQCOUNT_LOCKNAME() - Instantiate seqcount_LOCKNAME_t and helpers
137 * seqprop_LOCKNAME_*() - Property accessors for seqcount_LOCKNAME_t
138 *
139 * @lockname: "LOCKNAME" part of seqcount_LOCKNAME_t
140 * @locktype: LOCKNAME canonical C data type
141 * @preemptible: preemptibility of above locktype
142 * @lockbase: prefix for associated lock/unlock
143 */
144#define SEQCOUNT_LOCKNAME(lockname, locktype, preemptible, lockbase) \
145static __always_inline seqcount_t * \
146__seqprop_##lockname##_ptr(seqcount_##lockname##_t *s) \
147{ \
148 return &s->seqcount; \
149} \
150 \
151static __always_inline const seqcount_t * \
152__seqprop_##lockname##_const_ptr(const seqcount_##lockname##_t *s) \
153{ \
154 return &s->seqcount; \
155} \
156 \
157static __always_inline unsigned \
158__seqprop_##lockname##_sequence(const seqcount_##lockname##_t *s) \
159{ \
160 unsigned seq = smp_load_acquire(&s->seqcount.sequence); \
161 \
162 if (!IS_ENABLED(CONFIG_PREEMPT_RT)) \
163 return seq; \
164 \
165 if (preemptible && unlikely(seq & 1)) { \
166 __SEQ_LOCK(lockbase##_lock(s->lock)); \
167 __SEQ_LOCK(lockbase##_unlock(s->lock)); \
168 \
169 /* \
170 * Re-read the sequence counter since the (possibly \
171 * preempted) writer made progress. \
172 */ \
173 seq = smp_load_acquire(&s->seqcount.sequence); \
174 } \
175 \
176 return seq; \
177} \
178 \
179static __always_inline bool \
180__seqprop_##lockname##_preemptible(const seqcount_##lockname##_t *s) \
181{ \
182 if (!IS_ENABLED(CONFIG_PREEMPT_RT)) \
183 return preemptible; \
184 \
185 /* PREEMPT_RT relies on the above LOCK+UNLOCK */ \
186 return false; \
187} \
188 \
189static __always_inline void \
190__seqprop_##lockname##_assert(const seqcount_##lockname##_t *s) \
191{ \
192 __SEQ_LOCK(lockdep_assert_held(s->lock)); \
193}
194
195/*
196 * __seqprop() for seqcount_t
197 */
198
199static inline seqcount_t *__seqprop_ptr(seqcount_t *s)
200{
201 return s;
202}
203
204static inline const seqcount_t *__seqprop_const_ptr(const seqcount_t *s)
205{
206 return s;
207}
208
209static inline unsigned __seqprop_sequence(const seqcount_t *s)
210{
211 return smp_load_acquire(&s->sequence);
212}
213
214static inline bool __seqprop_preemptible(const seqcount_t *s)
215{
216 return false;
217}
218
219static inline void __seqprop_assert(const seqcount_t *s)
220{
221 lockdep_assert_preemption_disabled();
222}
223
224#define __SEQ_RT IS_ENABLED(CONFIG_PREEMPT_RT)
225
226SEQCOUNT_LOCKNAME(raw_spinlock, raw_spinlock_t, false, raw_spin)
227SEQCOUNT_LOCKNAME(spinlock, spinlock_t, __SEQ_RT, spin)
228SEQCOUNT_LOCKNAME(rwlock, rwlock_t, __SEQ_RT, read)
229SEQCOUNT_LOCKNAME(mutex, struct mutex, true, mutex)
230#undef SEQCOUNT_LOCKNAME
231
232/*
233 * SEQCNT_LOCKNAME_ZERO - static initializer for seqcount_LOCKNAME_t
234 * @name: Name of the seqcount_LOCKNAME_t instance
235 * @lock: Pointer to the associated LOCKNAME
236 */
237
238#define SEQCOUNT_LOCKNAME_ZERO(seq_name, assoc_lock) { \
239 .seqcount = SEQCNT_ZERO(seq_name.seqcount), \
240 __SEQ_LOCK(.lock = (assoc_lock)) \
241}
242
243#define SEQCNT_RAW_SPINLOCK_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(name, lock)
244#define SEQCNT_SPINLOCK_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(name, lock)
245#define SEQCNT_RWLOCK_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(name, lock)
246#define SEQCNT_MUTEX_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(name, lock)
247#define SEQCNT_WW_MUTEX_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(name, lock)
248
249#define __seqprop_case(s, lockname, prop) \
250 seqcount_##lockname##_t: __seqprop_##lockname##_##prop
251
252#define __seqprop(s, prop) _Generic(*(s), \
253 seqcount_t: __seqprop_##prop, \
254 __seqprop_case((s), raw_spinlock, prop), \
255 __seqprop_case((s), spinlock, prop), \
256 __seqprop_case((s), rwlock, prop), \
257 __seqprop_case((s), mutex, prop))
258
259#define seqprop_ptr(s) __seqprop(s, ptr)(s)
260#define seqprop_const_ptr(s) __seqprop(s, const_ptr)(s)
261#define seqprop_sequence(s) __seqprop(s, sequence)(s)
262#define seqprop_preemptible(s) __seqprop(s, preemptible)(s)
263#define seqprop_assert(s) __seqprop(s, assert)(s)
264
265/**
266 * __read_seqcount_begin() - begin a seqcount_t read section
267 * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
268 *
269 * Return: count to be passed to read_seqcount_retry()
270 */
271#define __read_seqcount_begin(s) \
272({ \
273 unsigned __seq; \
274 \
275 while (unlikely((__seq = seqprop_sequence(s)) & 1)) \
276 cpu_relax(); \
277 \
278 kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX); \
279 __seq; \
280})
281
282/**
283 * raw_read_seqcount_begin() - begin a seqcount_t read section w/o lockdep
284 * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
285 *
286 * Return: count to be passed to read_seqcount_retry()
287 */
288#define raw_read_seqcount_begin(s) __read_seqcount_begin(s)
289
290/**
291 * read_seqcount_begin() - begin a seqcount_t read critical section
292 * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
293 *
294 * Return: count to be passed to read_seqcount_retry()
295 */
296#define read_seqcount_begin(s) \
297({ \
298 seqcount_lockdep_reader_access(seqprop_const_ptr(s)); \
299 raw_read_seqcount_begin(s); \
300})
301
302/**
303 * raw_read_seqcount() - read the raw seqcount_t counter value
304 * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
305 *
306 * raw_read_seqcount opens a read critical section of the given
307 * seqcount_t, without any lockdep checking, and without checking or
308 * masking the sequence counter LSB. Calling code is responsible for
309 * handling that.
310 *
311 * Return: count to be passed to read_seqcount_retry()
312 */
313#define raw_read_seqcount(s) \
314({ \
315 unsigned __seq = seqprop_sequence(s); \
316 \
317 kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX); \
318 __seq; \
319})
320
321/**
322 * raw_seqcount_try_begin() - begin a seqcount_t read critical section
323 * w/o lockdep and w/o counter stabilization
324 * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
325 * @start: count to be passed to read_seqcount_retry()
326 *
327 * Similar to raw_seqcount_begin(), except it enables eliding the critical
328 * section entirely if odd, instead of doing the speculation knowing it will
329 * fail.
330 *
331 * Useful when counter stabilization is more or less equivalent to taking
332 * the lock and there is a slowpath that does that.
333 *
334 * If true, start will be set to the (even) sequence count read.
335 *
336 * Return: true when a read critical section is started.
337 */
338#define raw_seqcount_try_begin(s, start) \
339({ \
340 start = raw_read_seqcount(s); \
341 !(start & 1); \
342})
343
344/**
345 * raw_seqcount_begin() - begin a seqcount_t read critical section w/o
346 * lockdep and w/o counter stabilization
347 * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
348 *
349 * raw_seqcount_begin opens a read critical section of the given
350 * seqcount_t. Unlike read_seqcount_begin(), this function will not wait
351 * for the count to stabilize. If a writer is active when it begins, it
352 * will fail the read_seqcount_retry() at the end of the read critical
353 * section instead of stabilizing at the beginning of it.
354 *
355 * Use this only in special kernel hot paths where the read section is
356 * small and has a high probability of success through other external
357 * means. It will save a single branching instruction.
358 *
359 * Return: count to be passed to read_seqcount_retry()
360 */
361#define raw_seqcount_begin(s) \
362({ \
363 /* \
364 * If the counter is odd, let read_seqcount_retry() fail \
365 * by decrementing the counter. \
366 */ \
367 raw_read_seqcount(s) & ~1; \
368})
369
370/**
371 * __read_seqcount_retry() - end a seqcount_t read section w/o barrier
372 * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
373 * @start: count, from read_seqcount_begin()
374 *
375 * __read_seqcount_retry is like read_seqcount_retry, but has no smp_rmb()
376 * barrier. Callers should ensure that smp_rmb() or equivalent ordering is
377 * provided before actually loading any of the variables that are to be
378 * protected in this critical section.
379 *
380 * Use carefully, only in critical code, and comment how the barrier is
381 * provided.
382 *
383 * Return: true if a read section retry is required, else false
384 */
385#define __read_seqcount_retry(s, start) \
386 do___read_seqcount_retry(seqprop_const_ptr(s), start)
387
388static inline int do___read_seqcount_retry(const seqcount_t *s, unsigned start)
389{
390 kcsan_atomic_next(n: 0);
391 return unlikely(READ_ONCE(s->sequence) != start);
392}
393
394/**
395 * read_seqcount_retry() - end a seqcount_t read critical section
396 * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
397 * @start: count, from read_seqcount_begin()
398 *
399 * read_seqcount_retry closes the read critical section of given
400 * seqcount_t. If the critical section was invalid, it must be ignored
401 * (and typically retried).
402 *
403 * Return: true if a read section retry is required, else false
404 */
405#define read_seqcount_retry(s, start) \
406 do_read_seqcount_retry(seqprop_const_ptr(s), start)
407
408static inline int do_read_seqcount_retry(const seqcount_t *s, unsigned start)
409{
410 smp_rmb();
411 return do___read_seqcount_retry(s, start);
412}
413
414/**
415 * raw_write_seqcount_begin() - start a seqcount_t write section w/o lockdep
416 * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
417 *
418 * Context: check write_seqcount_begin()
419 */
420#define raw_write_seqcount_begin(s) \
421do { \
422 if (seqprop_preemptible(s)) \
423 preempt_disable(); \
424 \
425 do_raw_write_seqcount_begin(seqprop_ptr(s)); \
426} while (0)
427
428static inline void do_raw_write_seqcount_begin(seqcount_t *s)
429{
430 kcsan_nestable_atomic_begin();
431 s->sequence++;
432 smp_wmb();
433}
434
435/**
436 * raw_write_seqcount_end() - end a seqcount_t write section w/o lockdep
437 * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
438 *
439 * Context: check write_seqcount_end()
440 */
441#define raw_write_seqcount_end(s) \
442do { \
443 do_raw_write_seqcount_end(seqprop_ptr(s)); \
444 \
445 if (seqprop_preemptible(s)) \
446 preempt_enable(); \
447} while (0)
448
449static inline void do_raw_write_seqcount_end(seqcount_t *s)
450{
451 smp_wmb();
452 s->sequence++;
453 kcsan_nestable_atomic_end();
454}
455
456/**
457 * write_seqcount_begin_nested() - start a seqcount_t write section with
458 * custom lockdep nesting level
459 * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
460 * @subclass: lockdep nesting level
461 *
462 * See Documentation/locking/lockdep-design.rst
463 * Context: check write_seqcount_begin()
464 */
465#define write_seqcount_begin_nested(s, subclass) \
466do { \
467 seqprop_assert(s); \
468 \
469 if (seqprop_preemptible(s)) \
470 preempt_disable(); \
471 \
472 do_write_seqcount_begin_nested(seqprop_ptr(s), subclass); \
473} while (0)
474
475static inline void do_write_seqcount_begin_nested(seqcount_t *s, int subclass)
476{
477 seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
478 do_raw_write_seqcount_begin(s);
479}
480
481/**
482 * write_seqcount_begin() - start a seqcount_t write side critical section
483 * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
484 *
485 * Context: sequence counter write side sections must be serialized and
486 * non-preemptible. Preemption will be automatically disabled if and
487 * only if the seqcount write serialization lock is associated, and
488 * preemptible. If readers can be invoked from hardirq or softirq
489 * context, interrupts or bottom halves must be respectively disabled.
490 */
491#define write_seqcount_begin(s) \
492do { \
493 seqprop_assert(s); \
494 \
495 if (seqprop_preemptible(s)) \
496 preempt_disable(); \
497 \
498 do_write_seqcount_begin(seqprop_ptr(s)); \
499} while (0)
500
501static inline void do_write_seqcount_begin(seqcount_t *s)
502{
503 do_write_seqcount_begin_nested(s, subclass: 0);
504}
505
506/**
507 * write_seqcount_end() - end a seqcount_t write side critical section
508 * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
509 *
510 * Context: Preemption will be automatically re-enabled if and only if
511 * the seqcount write serialization lock is associated, and preemptible.
512 */
513#define write_seqcount_end(s) \
514do { \
515 do_write_seqcount_end(seqprop_ptr(s)); \
516 \
517 if (seqprop_preemptible(s)) \
518 preempt_enable(); \
519} while (0)
520
521static inline void do_write_seqcount_end(seqcount_t *s)
522{
523 seqcount_release(&s->dep_map, _RET_IP_);
524 do_raw_write_seqcount_end(s);
525}
526
527/**
528 * raw_write_seqcount_barrier() - do a seqcount_t write barrier
529 * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
530 *
531 * This can be used to provide an ordering guarantee instead of the usual
532 * consistency guarantee. It is one wmb cheaper, because it can collapse
533 * the two back-to-back wmb()s.
534 *
535 * Note that writes surrounding the barrier should be declared atomic (e.g.
536 * via WRITE_ONCE): a) to ensure the writes become visible to other threads
537 * atomically, avoiding compiler optimizations; b) to document which writes are
538 * meant to propagate to the reader critical section. This is necessary because
539 * neither writes before nor after the barrier are enclosed in a seq-writer
540 * critical section that would ensure readers are aware of ongoing writes::
541 *
542 * seqcount_t seq;
543 * bool X = true, Y = false;
544 *
545 * void read(void)
546 * {
547 * bool x, y;
548 *
549 * do {
550 * int s = read_seqcount_begin(&seq);
551 *
552 * x = X; y = Y;
553 *
554 * } while (read_seqcount_retry(&seq, s));
555 *
556 * BUG_ON(!x && !y);
557 * }
558 *
559 * void write(void)
560 * {
561 * WRITE_ONCE(Y, true);
562 *
563 * raw_write_seqcount_barrier(seq);
564 *
565 * WRITE_ONCE(X, false);
566 * }
567 */
568#define raw_write_seqcount_barrier(s) \
569 do_raw_write_seqcount_barrier(seqprop_ptr(s))
570
571static inline void do_raw_write_seqcount_barrier(seqcount_t *s)
572{
573 kcsan_nestable_atomic_begin();
574 s->sequence++;
575 smp_wmb();
576 s->sequence++;
577 kcsan_nestable_atomic_end();
578}
579
580/**
581 * write_seqcount_invalidate() - invalidate in-progress seqcount_t read
582 * side operations
583 * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
584 *
585 * After write_seqcount_invalidate, no seqcount_t read side operations
586 * will complete successfully and see data older than this.
587 */
588#define write_seqcount_invalidate(s) \
589 do_write_seqcount_invalidate(seqprop_ptr(s))
590
591static inline void do_write_seqcount_invalidate(seqcount_t *s)
592{
593 smp_wmb();
594 kcsan_nestable_atomic_begin();
595 s->sequence+=2;
596 kcsan_nestable_atomic_end();
597}
598
599/*
600 * Latch sequence counters (seqcount_latch_t)
601 *
602 * A sequence counter variant where the counter even/odd value is used to
603 * switch between two copies of protected data. This allows the read path,
604 * typically NMIs, to safely interrupt the write side critical section.
605 *
606 * As the write sections are fully preemptible, no special handling for
607 * PREEMPT_RT is needed.
608 */
609typedef struct {
610 seqcount_t seqcount;
611} seqcount_latch_t;
612
613/**
614 * SEQCNT_LATCH_ZERO() - static initializer for seqcount_latch_t
615 * @seq_name: Name of the seqcount_latch_t instance
616 */
617#define SEQCNT_LATCH_ZERO(seq_name) { \
618 .seqcount = SEQCNT_ZERO(seq_name.seqcount), \
619}
620
621/**
622 * seqcount_latch_init() - runtime initializer for seqcount_latch_t
623 * @s: Pointer to the seqcount_latch_t instance
624 */
625#define seqcount_latch_init(s) seqcount_init(&(s)->seqcount)
626
627/**
628 * raw_read_seqcount_latch() - pick even/odd latch data copy
629 * @s: Pointer to seqcount_latch_t
630 *
631 * See raw_write_seqcount_latch() for details and a full reader/writer
632 * usage example.
633 *
634 * Return: sequence counter raw value. Use the lowest bit as an index for
635 * picking which data copy to read. The full counter must then be checked
636 * with raw_read_seqcount_latch_retry().
637 */
638static __always_inline unsigned raw_read_seqcount_latch(const seqcount_latch_t *s)
639{
640 /*
641 * Pairs with the first smp_wmb() in raw_write_seqcount_latch().
642 * Due to the dependent load, a full smp_rmb() is not needed.
643 */
644 return READ_ONCE(s->seqcount.sequence);
645}
646
647/**
648 * read_seqcount_latch() - pick even/odd latch data copy
649 * @s: Pointer to seqcount_latch_t
650 *
651 * See write_seqcount_latch() for details and a full reader/writer usage
652 * example.
653 *
654 * Return: sequence counter raw value. Use the lowest bit as an index for
655 * picking which data copy to read. The full counter must then be checked
656 * with read_seqcount_latch_retry().
657 */
658static __always_inline unsigned read_seqcount_latch(const seqcount_latch_t *s)
659{
660 kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);
661 return raw_read_seqcount_latch(s);
662}
663
664/**
665 * raw_read_seqcount_latch_retry() - end a seqcount_latch_t read section
666 * @s: Pointer to seqcount_latch_t
667 * @start: count, from raw_read_seqcount_latch()
668 *
669 * Return: true if a read section retry is required, else false
670 */
671static __always_inline int
672raw_read_seqcount_latch_retry(const seqcount_latch_t *s, unsigned start)
673{
674 smp_rmb();
675 return unlikely(READ_ONCE(s->seqcount.sequence) != start);
676}
677
678/**
679 * read_seqcount_latch_retry() - end a seqcount_latch_t read section
680 * @s: Pointer to seqcount_latch_t
681 * @start: count, from read_seqcount_latch()
682 *
683 * Return: true if a read section retry is required, else false
684 */
685static __always_inline int
686read_seqcount_latch_retry(const seqcount_latch_t *s, unsigned start)
687{
688 kcsan_atomic_next(n: 0);
689 return raw_read_seqcount_latch_retry(s, start);
690}
691
692/**
693 * raw_write_seqcount_latch() - redirect latch readers to even/odd copy
694 * @s: Pointer to seqcount_latch_t
695 */
696static __always_inline void raw_write_seqcount_latch(seqcount_latch_t *s)
697{
698 smp_wmb(); /* prior stores before incrementing "sequence" */
699 s->seqcount.sequence++;
700 smp_wmb(); /* increment "sequence" before following stores */
701}
702
703/**
704 * write_seqcount_latch_begin() - redirect latch readers to odd copy
705 * @s: Pointer to seqcount_latch_t
706 *
707 * The latch technique is a multiversion concurrency control method that allows
708 * queries during non-atomic modifications. If you can guarantee queries never
709 * interrupt the modification -- e.g. the concurrency is strictly between CPUs
710 * -- you most likely do not need this.
711 *
712 * Where the traditional RCU/lockless data structures rely on atomic
713 * modifications to ensure queries observe either the old or the new state the
714 * latch allows the same for non-atomic updates. The trade-off is doubling the
715 * cost of storage; we have to maintain two copies of the entire data
716 * structure.
717 *
718 * Very simply put: we first modify one copy and then the other. This ensures
719 * there is always one copy in a stable state, ready to give us an answer.
720 *
721 * The basic form is a data structure like::
722 *
723 * struct latch_struct {
724 * seqcount_latch_t seq;
725 * struct data_struct data[2];
726 * };
727 *
728 * Where a modification, which is assumed to be externally serialized, does the
729 * following::
730 *
731 * void latch_modify(struct latch_struct *latch, ...)
732 * {
733 * write_seqcount_latch_begin(&latch->seq);
734 * modify(latch->data[0], ...);
735 * write_seqcount_latch(&latch->seq);
736 * modify(latch->data[1], ...);
737 * write_seqcount_latch_end(&latch->seq);
738 * }
739 *
740 * The query will have a form like::
741 *
742 * struct entry *latch_query(struct latch_struct *latch, ...)
743 * {
744 * struct entry *entry;
745 * unsigned seq, idx;
746 *
747 * do {
748 * seq = read_seqcount_latch(&latch->seq);
749 *
750 * idx = seq & 0x01;
751 * entry = data_query(latch->data[idx], ...);
752 *
753 * // This includes needed smp_rmb()
754 * } while (read_seqcount_latch_retry(&latch->seq, seq));
755 *
756 * return entry;
757 * }
758 *
759 * So during the modification, queries are first redirected to data[1]. Then we
760 * modify data[0]. When that is complete, we redirect queries back to data[0]
761 * and we can modify data[1].
762 *
763 * NOTE:
764 *
765 * The non-requirement for atomic modifications does _NOT_ include
766 * the publishing of new entries in the case where data is a dynamic
767 * data structure.
768 *
769 * An iteration might start in data[0] and get suspended long enough
770 * to miss an entire modification sequence, once it resumes it might
771 * observe the new entry.
772 *
773 * NOTE2:
774 *
775 * When data is a dynamic data structure; one should use regular RCU
776 * patterns to manage the lifetimes of the objects within.
777 */
778static __always_inline void write_seqcount_latch_begin(seqcount_latch_t *s)
779{
780 kcsan_nestable_atomic_begin();
781 raw_write_seqcount_latch(s);
782}
783
784/**
785 * write_seqcount_latch() - redirect latch readers to even copy
786 * @s: Pointer to seqcount_latch_t
787 */
788static __always_inline void write_seqcount_latch(seqcount_latch_t *s)
789{
790 raw_write_seqcount_latch(s);
791}
792
793/**
794 * write_seqcount_latch_end() - end a seqcount_latch_t write section
795 * @s: Pointer to seqcount_latch_t
796 *
797 * Marks the end of a seqcount_latch_t writer section, after all copies of the
798 * latch-protected data have been updated.
799 */
800static __always_inline void write_seqcount_latch_end(seqcount_latch_t *s)
801{
802 kcsan_nestable_atomic_end();
803}
804
805#define __SEQLOCK_UNLOCKED(lockname) \
806 { \
807 .seqcount = SEQCNT_SPINLOCK_ZERO(lockname, &(lockname).lock), \
808 .lock = __SPIN_LOCK_UNLOCKED(lockname) \
809 }
810
811/**
812 * seqlock_init() - dynamic initializer for seqlock_t
813 * @sl: Pointer to the seqlock_t instance
814 */
815#define seqlock_init(sl) \
816 do { \
817 spin_lock_init(&(sl)->lock); \
818 seqcount_spinlock_init(&(sl)->seqcount, &(sl)->lock); \
819 } while (0)
820
821/**
822 * DEFINE_SEQLOCK(sl) - Define a statically allocated seqlock_t
823 * @sl: Name of the seqlock_t instance
824 */
825#define DEFINE_SEQLOCK(sl) \
826 seqlock_t sl = __SEQLOCK_UNLOCKED(sl)
827
828/**
829 * read_seqbegin() - start a seqlock_t read side critical section
830 * @sl: Pointer to seqlock_t
831 *
832 * Return: count, to be passed to read_seqretry()
833 */
834static inline unsigned read_seqbegin(const seqlock_t *sl)
835{
836 return read_seqcount_begin(&sl->seqcount);
837}
838
839/**
840 * read_seqretry() - end a seqlock_t read side section
841 * @sl: Pointer to seqlock_t
842 * @start: count, from read_seqbegin()
843 *
844 * read_seqretry closes the read side critical section of given seqlock_t.
845 * If the critical section was invalid, it must be ignored (and typically
846 * retried).
847 *
848 * Return: true if a read section retry is required, else false
849 */
850static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
851{
852 return read_seqcount_retry(&sl->seqcount, start);
853}
854
855/*
856 * For all seqlock_t write side functions, use the internal
857 * do_write_seqcount_begin() instead of generic write_seqcount_begin().
858 * This way, no redundant lockdep_assert_held() checks are added.
859 */
860
861/**
862 * write_seqlock() - start a seqlock_t write side critical section
863 * @sl: Pointer to seqlock_t
864 *
865 * write_seqlock opens a write side critical section for the given
866 * seqlock_t. It also implicitly acquires the spinlock_t embedded inside
867 * that sequential lock. All seqlock_t write side sections are thus
868 * automatically serialized and non-preemptible.
869 *
870 * Context: if the seqlock_t read section, or other write side critical
871 * sections, can be invoked from hardirq or softirq contexts, use the
872 * _irqsave or _bh variants of this function instead.
873 */
874static inline void write_seqlock(seqlock_t *sl)
875{
876 spin_lock(lock: &sl->lock);
877 do_write_seqcount_begin(s: &sl->seqcount.seqcount);
878}
879
880/**
881 * write_sequnlock() - end a seqlock_t write side critical section
882 * @sl: Pointer to seqlock_t
883 *
884 * write_sequnlock closes the (serialized and non-preemptible) write side
885 * critical section of given seqlock_t.
886 */
887static inline void write_sequnlock(seqlock_t *sl)
888{
889 do_write_seqcount_end(s: &sl->seqcount.seqcount);
890 spin_unlock(lock: &sl->lock);
891}
892
893/**
894 * write_seqlock_bh() - start a softirqs-disabled seqlock_t write section
895 * @sl: Pointer to seqlock_t
896 *
897 * _bh variant of write_seqlock(). Use only if the read side section, or
898 * other write side sections, can be invoked from softirq contexts.
899 */
900static inline void write_seqlock_bh(seqlock_t *sl)
901{
902 spin_lock_bh(lock: &sl->lock);
903 do_write_seqcount_begin(s: &sl->seqcount.seqcount);
904}
905
906/**
907 * write_sequnlock_bh() - end a softirqs-disabled seqlock_t write section
908 * @sl: Pointer to seqlock_t
909 *
910 * write_sequnlock_bh closes the serialized, non-preemptible, and
911 * softirqs-disabled, seqlock_t write side critical section opened with
912 * write_seqlock_bh().
913 */
914static inline void write_sequnlock_bh(seqlock_t *sl)
915{
916 do_write_seqcount_end(s: &sl->seqcount.seqcount);
917 spin_unlock_bh(lock: &sl->lock);
918}
919
920/**
921 * write_seqlock_irq() - start a non-interruptible seqlock_t write section
922 * @sl: Pointer to seqlock_t
923 *
924 * _irq variant of write_seqlock(). Use only if the read side section, or
925 * other write sections, can be invoked from hardirq contexts.
926 */
927static inline void write_seqlock_irq(seqlock_t *sl)
928{
929 spin_lock_irq(lock: &sl->lock);
930 do_write_seqcount_begin(s: &sl->seqcount.seqcount);
931}
932
933/**
934 * write_sequnlock_irq() - end a non-interruptible seqlock_t write section
935 * @sl: Pointer to seqlock_t
936 *
937 * write_sequnlock_irq closes the serialized and non-interruptible
938 * seqlock_t write side section opened with write_seqlock_irq().
939 */
940static inline void write_sequnlock_irq(seqlock_t *sl)
941{
942 do_write_seqcount_end(s: &sl->seqcount.seqcount);
943 spin_unlock_irq(lock: &sl->lock);
944}
945
946static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
947{
948 unsigned long flags;
949
950 spin_lock_irqsave(&sl->lock, flags);
951 do_write_seqcount_begin(s: &sl->seqcount.seqcount);
952 return flags;
953}
954
955/**
956 * write_seqlock_irqsave() - start a non-interruptible seqlock_t write
957 * section
958 * @lock: Pointer to seqlock_t
959 * @flags: Stack-allocated storage for saving caller's local interrupt
960 * state, to be passed to write_sequnlock_irqrestore().
961 *
962 * _irqsave variant of write_seqlock(). Use it only if the read side
963 * section, or other write sections, can be invoked from hardirq context.
964 */
965#define write_seqlock_irqsave(lock, flags) \
966 do { flags = __write_seqlock_irqsave(lock); } while (0)
967
968/**
969 * write_sequnlock_irqrestore() - end non-interruptible seqlock_t write
970 * section
971 * @sl: Pointer to seqlock_t
972 * @flags: Caller's saved interrupt state, from write_seqlock_irqsave()
973 *
974 * write_sequnlock_irqrestore closes the serialized and non-interruptible
975 * seqlock_t write section previously opened with write_seqlock_irqsave().
976 */
977static inline void
978write_sequnlock_irqrestore(seqlock_t *sl, unsigned long flags)
979{
980 do_write_seqcount_end(s: &sl->seqcount.seqcount);
981 spin_unlock_irqrestore(lock: &sl->lock, flags);
982}
983
984/**
985 * read_seqlock_excl() - begin a seqlock_t locking reader section
986 * @sl: Pointer to seqlock_t
987 *
988 * read_seqlock_excl opens a seqlock_t locking reader critical section. A
989 * locking reader exclusively locks out *both* other writers *and* other
990 * locking readers, but it does not update the embedded sequence number.
991 *
992 * Locking readers act like a normal spin_lock()/spin_unlock().
993 *
994 * Context: if the seqlock_t write section, *or other read sections*, can
995 * be invoked from hardirq or softirq contexts, use the _irqsave or _bh
996 * variant of this function instead.
997 *
998 * The opened read section must be closed with read_sequnlock_excl().
999 */
1000static inline void read_seqlock_excl(seqlock_t *sl)
1001{
1002 spin_lock(lock: &sl->lock);
1003}
1004
1005/**
1006 * read_sequnlock_excl() - end a seqlock_t locking reader critical section
1007 * @sl: Pointer to seqlock_t
1008 */
1009static inline void read_sequnlock_excl(seqlock_t *sl)
1010{
1011 spin_unlock(lock: &sl->lock);
1012}
1013
1014/**
1015 * read_seqlock_excl_bh() - start a seqlock_t locking reader section with
1016 * softirqs disabled
1017 * @sl: Pointer to seqlock_t
1018 *
1019 * _bh variant of read_seqlock_excl(). Use this variant only if the
1020 * seqlock_t write side section, *or other read sections*, can be invoked
1021 * from softirq contexts.
1022 */
1023static inline void read_seqlock_excl_bh(seqlock_t *sl)
1024{
1025 spin_lock_bh(lock: &sl->lock);
1026}
1027
1028/**
1029 * read_sequnlock_excl_bh() - stop a seqlock_t softirq-disabled locking
1030 * reader section
1031 * @sl: Pointer to seqlock_t
1032 */
1033static inline void read_sequnlock_excl_bh(seqlock_t *sl)
1034{
1035 spin_unlock_bh(lock: &sl->lock);
1036}
1037
1038/**
1039 * read_seqlock_excl_irq() - start a non-interruptible seqlock_t locking
1040 * reader section
1041 * @sl: Pointer to seqlock_t
1042 *
1043 * _irq variant of read_seqlock_excl(). Use this only if the seqlock_t
1044 * write side section, *or other read sections*, can be invoked from a
1045 * hardirq context.
1046 */
1047static inline void read_seqlock_excl_irq(seqlock_t *sl)
1048{
1049 spin_lock_irq(lock: &sl->lock);
1050}
1051
1052/**
1053 * read_sequnlock_excl_irq() - end an interrupts-disabled seqlock_t
1054 * locking reader section
1055 * @sl: Pointer to seqlock_t
1056 */
1057static inline void read_sequnlock_excl_irq(seqlock_t *sl)
1058{
1059 spin_unlock_irq(lock: &sl->lock);
1060}
1061
1062static inline unsigned long __read_seqlock_excl_irqsave(seqlock_t *sl)
1063{
1064 unsigned long flags;
1065
1066 spin_lock_irqsave(&sl->lock, flags);
1067 return flags;
1068}
1069
1070/**
1071 * read_seqlock_excl_irqsave() - start a non-interruptible seqlock_t
1072 * locking reader section
1073 * @lock: Pointer to seqlock_t
1074 * @flags: Stack-allocated storage for saving caller's local interrupt
1075 * state, to be passed to read_sequnlock_excl_irqrestore().
1076 *
1077 * _irqsave variant of read_seqlock_excl(). Use this only if the seqlock_t
1078 * write side section, *or other read sections*, can be invoked from a
1079 * hardirq context.
1080 */
1081#define read_seqlock_excl_irqsave(lock, flags) \
1082 do { flags = __read_seqlock_excl_irqsave(lock); } while (0)
1083
1084/**
1085 * read_sequnlock_excl_irqrestore() - end non-interruptible seqlock_t
1086 * locking reader section
1087 * @sl: Pointer to seqlock_t
1088 * @flags: Caller saved interrupt state, from read_seqlock_excl_irqsave()
1089 */
1090static inline void
1091read_sequnlock_excl_irqrestore(seqlock_t *sl, unsigned long flags)
1092{
1093 spin_unlock_irqrestore(lock: &sl->lock, flags);
1094}
1095
1096/**
1097 * read_seqbegin_or_lock() - begin a seqlock_t lockless or locking reader
1098 * @lock: Pointer to seqlock_t
1099 * @seq : Marker and return parameter. If the passed value is even, the
1100 * reader will become a *lockless* seqlock_t reader as in read_seqbegin().
1101 * If the passed value is odd, the reader will become a *locking* reader
1102 * as in read_seqlock_excl(). In the first call to this function, the
1103 * caller *must* initialize and pass an even value to @seq; this way, a
1104 * lockless read can be optimistically tried first.
1105 *
1106 * read_seqbegin_or_lock is an API designed to optimistically try a normal
1107 * lockless seqlock_t read section first. If an odd counter is found, the
1108 * lockless read trial has failed, and the next read iteration transforms
1109 * itself into a full seqlock_t locking reader.
1110 *
1111 * This is typically used to avoid seqlock_t lockless readers starvation
1112 * (too much retry loops) in the case of a sharp spike in write side
1113 * activity.
1114 *
1115 * Context: if the seqlock_t write section, *or other read sections*, can
1116 * be invoked from hardirq or softirq contexts, use the _irqsave or _bh
1117 * variant of this function instead.
1118 *
1119 * Check Documentation/locking/seqlock.rst for template example code.
1120 *
1121 * Return: the encountered sequence counter value, through the @seq
1122 * parameter, which is overloaded as a return parameter. This returned
1123 * value must be checked with need_seqretry(). If the read section need to
1124 * be retried, this returned value must also be passed as the @seq
1125 * parameter of the next read_seqbegin_or_lock() iteration.
1126 */
1127static inline void read_seqbegin_or_lock(seqlock_t *lock, int *seq)
1128{
1129 if (!(*seq & 1)) /* Even */
1130 *seq = read_seqbegin(sl: lock);
1131 else /* Odd */
1132 read_seqlock_excl(sl: lock);
1133}
1134
1135/**
1136 * need_seqretry() - validate seqlock_t "locking or lockless" read section
1137 * @lock: Pointer to seqlock_t
1138 * @seq: sequence count, from read_seqbegin_or_lock()
1139 *
1140 * Return: true if a read section retry is required, false otherwise
1141 */
1142static inline int need_seqretry(seqlock_t *lock, int seq)
1143{
1144 return !(seq & 1) && read_seqretry(sl: lock, start: seq);
1145}
1146
1147/**
1148 * done_seqretry() - end seqlock_t "locking or lockless" reader section
1149 * @lock: Pointer to seqlock_t
1150 * @seq: count, from read_seqbegin_or_lock()
1151 *
1152 * done_seqretry finishes the seqlock_t read side critical section started
1153 * with read_seqbegin_or_lock() and validated by need_seqretry().
1154 */
1155static inline void done_seqretry(seqlock_t *lock, int seq)
1156{
1157 if (seq & 1)
1158 read_sequnlock_excl(sl: lock);
1159}
1160
1161/**
1162 * read_seqbegin_or_lock_irqsave() - begin a seqlock_t lockless reader, or
1163 * a non-interruptible locking reader
1164 * @lock: Pointer to seqlock_t
1165 * @seq: Marker and return parameter. Check read_seqbegin_or_lock().
1166 *
1167 * This is the _irqsave variant of read_seqbegin_or_lock(). Use it only if
1168 * the seqlock_t write section, *or other read sections*, can be invoked
1169 * from hardirq context.
1170 *
1171 * Note: Interrupts will be disabled only for "locking reader" mode.
1172 *
1173 * Return:
1174 *
1175 * 1. The saved local interrupts state in case of a locking reader, to
1176 * be passed to done_seqretry_irqrestore().
1177 *
1178 * 2. The encountered sequence counter value, returned through @seq
1179 * overloaded as a return parameter. Check read_seqbegin_or_lock().
1180 */
1181static inline unsigned long
1182read_seqbegin_or_lock_irqsave(seqlock_t *lock, int *seq)
1183{
1184 unsigned long flags = 0;
1185
1186 if (!(*seq & 1)) /* Even */
1187 *seq = read_seqbegin(sl: lock);
1188 else /* Odd */
1189 read_seqlock_excl_irqsave(lock, flags);
1190
1191 return flags;
1192}
1193
1194/**
1195 * done_seqretry_irqrestore() - end a seqlock_t lockless reader, or a
1196 * non-interruptible locking reader section
1197 * @lock: Pointer to seqlock_t
1198 * @seq: Count, from read_seqbegin_or_lock_irqsave()
1199 * @flags: Caller's saved local interrupt state in case of a locking
1200 * reader, also from read_seqbegin_or_lock_irqsave()
1201 *
1202 * This is the _irqrestore variant of done_seqretry(). The read section
1203 * must've been opened with read_seqbegin_or_lock_irqsave(), and validated
1204 * by need_seqretry().
1205 */
1206static inline void
1207done_seqretry_irqrestore(seqlock_t *lock, int seq, unsigned long flags)
1208{
1209 if (seq & 1)
1210 read_sequnlock_excl_irqrestore(sl: lock, flags);
1211}
1212#endif /* __LINUX_SEQLOCK_H */
1213

source code of linux/include/linux/seqlock.h

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