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/spinlock.h>
22
23#include <asm/processor.h>
24
25/*
26 * The seqlock seqcount_t interface does not prescribe a precise sequence of
27 * read begin/retry/end. For readers, typically there is a call to
28 * read_seqcount_begin() and read_seqcount_retry(), however, there are more
29 * esoteric cases which do not follow this pattern.
30 *
31 * As a consequence, we take the following best-effort approach for raw usage
32 * via seqcount_t under KCSAN: upon beginning a seq-reader critical section,
33 * pessimistically mark the next KCSAN_SEQLOCK_REGION_MAX memory accesses as
34 * atomics; if there is a matching read_seqcount_retry() call, no following
35 * memory operations are considered atomic. Usage of the seqlock_t interface
36 * is not affected.
37 */
38#define KCSAN_SEQLOCK_REGION_MAX 1000
39
40/*
41 * Sequence counters (seqcount_t)
42 *
43 * This is the raw counting mechanism, without any writer protection.
44 *
45 * Write side critical sections must be serialized and non-preemptible.
46 *
47 * If readers can be invoked from hardirq or softirq contexts,
48 * interrupts or bottom halves must also be respectively disabled before
49 * entering the write section.
50 *
51 * This mechanism can't be used if the protected data contains pointers,
52 * as the writer can invalidate a pointer that a reader is following.
53 *
54 * If the write serialization mechanism is one of the common kernel
55 * locking primitives, use a sequence counter with associated lock
56 * (seqcount_LOCKNAME_t) instead.
57 *
58 * If it's desired to automatically handle the sequence counter writer
59 * serialization and non-preemptibility requirements, use a sequential
60 * lock (seqlock_t) instead.
61 *
62 * See Documentation/locking/seqlock.rst
63 */
64typedef struct seqcount {
65 unsigned sequence;
66#ifdef CONFIG_DEBUG_LOCK_ALLOC
67 struct lockdep_map dep_map;
68#endif
69} seqcount_t;
70
71static inline void __seqcount_init(seqcount_t *s, const char *name,
72 struct lock_class_key *key)
73{
74 /*
75 * Make sure we are not reinitializing a held lock:
76 */
77 lockdep_init_map(&s->dep_map, name, key, 0);
78 s->sequence = 0;
79}
80
81#ifdef CONFIG_DEBUG_LOCK_ALLOC
82
83# define SEQCOUNT_DEP_MAP_INIT(lockname) \
84 .dep_map = { .name = #lockname }
85
86/**
87 * seqcount_init() - runtime initializer for seqcount_t
88 * @s: Pointer to the seqcount_t instance
89 */
90# define seqcount_init(s) \
91 do { \
92 static struct lock_class_key __key; \
93 __seqcount_init((s), #s, &__key); \
94 } while (0)
95
96static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
97{
98 seqcount_t *l = (seqcount_t *)s;
99 unsigned long flags;
100
101 local_irq_save(flags);
102 seqcount_acquire_read(&l->dep_map, 0, 0, _RET_IP_);
103 seqcount_release(&l->dep_map, _RET_IP_);
104 local_irq_restore(flags);
105}
106
107#else
108# define SEQCOUNT_DEP_MAP_INIT(lockname)
109# define seqcount_init(s) __seqcount_init(s, NULL, NULL)
110# define seqcount_lockdep_reader_access(x)
111#endif
112
113/**
114 * SEQCNT_ZERO() - static initializer for seqcount_t
115 * @name: Name of the seqcount_t instance
116 */
117#define SEQCNT_ZERO(name) { .sequence = 0, SEQCOUNT_DEP_MAP_INIT(name) }
118
119/*
120 * Sequence counters with associated locks (seqcount_LOCKNAME_t)
121 *
122 * A sequence counter which associates the lock used for writer
123 * serialization at initialization time. This enables lockdep to validate
124 * that the write side critical section is properly serialized.
125 *
126 * For associated locks which do not implicitly disable preemption,
127 * preemption protection is enforced in the write side function.
128 *
129 * Lockdep is never used in any for the raw write variants.
130 *
131 * See Documentation/locking/seqlock.rst
132 */
133
134/*
135 * For PREEMPT_RT, seqcount_LOCKNAME_t write side critical sections cannot
136 * disable preemption. It can lead to higher latencies, and the write side
137 * sections will not be able to acquire locks which become sleeping locks
138 * (e.g. spinlock_t).
139 *
140 * To remain preemptible while avoiding a possible livelock caused by the
141 * reader preempting the writer, use a different technique: let the reader
142 * detect if a seqcount_LOCKNAME_t writer is in progress. If that is the
143 * case, acquire then release the associated LOCKNAME writer serialization
144 * lock. This will allow any possibly-preempted writer to make progress
145 * until the end of its writer serialization lock critical section.
146 *
147 * This lock-unlock technique must be implemented for all of PREEMPT_RT
148 * sleeping locks. See Documentation/locking/locktypes.rst
149 */
150#if defined(CONFIG_LOCKDEP) || defined(CONFIG_PREEMPT_RT)
151#define __SEQ_LOCK(expr) expr
152#else
153#define __SEQ_LOCK(expr)
154#endif
155
156/*
157 * typedef seqcount_LOCKNAME_t - sequence counter with LOCKNAME associated
158 * @seqcount: The real sequence counter
159 * @lock: Pointer to the associated lock
160 *
161 * A plain sequence counter with external writer synchronization by
162 * LOCKNAME @lock. The lock is associated to the sequence counter in the
163 * static initializer or init function. This enables lockdep to validate
164 * that the write side critical section is properly serialized.
165 *
166 * LOCKNAME: raw_spinlock, spinlock, rwlock or mutex
167 */
168
169/*
170 * seqcount_LOCKNAME_init() - runtime initializer for seqcount_LOCKNAME_t
171 * @s: Pointer to the seqcount_LOCKNAME_t instance
172 * @lock: Pointer to the associated lock
173 */
174
175#define seqcount_LOCKNAME_init(s, _lock, lockname) \
176 do { \
177 seqcount_##lockname##_t *____s = (s); \
178 seqcount_init(&____s->seqcount); \
179 __SEQ_LOCK(____s->lock = (_lock)); \
180 } while (0)
181
182#define seqcount_raw_spinlock_init(s, lock) seqcount_LOCKNAME_init(s, lock, raw_spinlock)
183#define seqcount_spinlock_init(s, lock) seqcount_LOCKNAME_init(s, lock, spinlock)
184#define seqcount_rwlock_init(s, lock) seqcount_LOCKNAME_init(s, lock, rwlock)
185#define seqcount_mutex_init(s, lock) seqcount_LOCKNAME_init(s, lock, mutex)
186
187/*
188 * SEQCOUNT_LOCKNAME() - Instantiate seqcount_LOCKNAME_t and helpers
189 * seqprop_LOCKNAME_*() - Property accessors for seqcount_LOCKNAME_t
190 *
191 * @lockname: "LOCKNAME" part of seqcount_LOCKNAME_t
192 * @locktype: LOCKNAME canonical C data type
193 * @preemptible: preemptibility of above locktype
194 * @lockmember: argument for lockdep_assert_held()
195 * @lockbase: associated lock release function (prefix only)
196 * @lock_acquire: associated lock acquisition function (full call)
197 */
198#define SEQCOUNT_LOCKNAME(lockname, locktype, preemptible, lockmember, lockbase, lock_acquire) \
199typedef struct seqcount_##lockname { \
200 seqcount_t seqcount; \
201 __SEQ_LOCK(locktype *lock); \
202} seqcount_##lockname##_t; \
203 \
204static __always_inline seqcount_t * \
205__seqprop_##lockname##_ptr(seqcount_##lockname##_t *s) \
206{ \
207 return &s->seqcount; \
208} \
209 \
210static __always_inline unsigned \
211__seqprop_##lockname##_sequence(const seqcount_##lockname##_t *s) \
212{ \
213 unsigned seq = READ_ONCE(s->seqcount.sequence); \
214 \
215 if (!IS_ENABLED(CONFIG_PREEMPT_RT)) \
216 return seq; \
217 \
218 if (preemptible && unlikely(seq & 1)) { \
219 __SEQ_LOCK(lock_acquire); \
220 __SEQ_LOCK(lockbase##_unlock(s->lock)); \
221 \
222 /* \
223 * Re-read the sequence counter since the (possibly \
224 * preempted) writer made progress. \
225 */ \
226 seq = READ_ONCE(s->seqcount.sequence); \
227 } \
228 \
229 return seq; \
230} \
231 \
232static __always_inline bool \
233__seqprop_##lockname##_preemptible(const seqcount_##lockname##_t *s) \
234{ \
235 if (!IS_ENABLED(CONFIG_PREEMPT_RT)) \
236 return preemptible; \
237 \
238 /* PREEMPT_RT relies on the above LOCK+UNLOCK */ \
239 return false; \
240} \
241 \
242static __always_inline void \
243__seqprop_##lockname##_assert(const seqcount_##lockname##_t *s) \
244{ \
245 __SEQ_LOCK(lockdep_assert_held(lockmember)); \
246}
247
248/*
249 * __seqprop() for seqcount_t
250 */
251
252static inline seqcount_t *__seqprop_ptr(seqcount_t *s)
253{
254 return s;
255}
256
257static inline unsigned __seqprop_sequence(const seqcount_t *s)
258{
259 return READ_ONCE(s->sequence);
260}
261
262static inline bool __seqprop_preemptible(const seqcount_t *s)
263{
264 return false;
265}
266
267static inline void __seqprop_assert(const seqcount_t *s)
268{
269 lockdep_assert_preemption_disabled();
270}
271
272#define __SEQ_RT IS_ENABLED(CONFIG_PREEMPT_RT)
273
274SEQCOUNT_LOCKNAME(raw_spinlock, raw_spinlock_t, false, s->lock, raw_spin, raw_spin_lock(s->lock))
275SEQCOUNT_LOCKNAME(spinlock, spinlock_t, __SEQ_RT, s->lock, spin, spin_lock(s->lock))
276SEQCOUNT_LOCKNAME(rwlock, rwlock_t, __SEQ_RT, s->lock, read, read_lock(s->lock))
277SEQCOUNT_LOCKNAME(mutex, struct mutex, true, s->lock, mutex, mutex_lock(s->lock))
278
279/*
280 * SEQCNT_LOCKNAME_ZERO - static initializer for seqcount_LOCKNAME_t
281 * @name: Name of the seqcount_LOCKNAME_t instance
282 * @lock: Pointer to the associated LOCKNAME
283 */
284
285#define SEQCOUNT_LOCKNAME_ZERO(seq_name, assoc_lock) { \
286 .seqcount = SEQCNT_ZERO(seq_name.seqcount), \
287 __SEQ_LOCK(.lock = (assoc_lock)) \
288}
289
290#define SEQCNT_RAW_SPINLOCK_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(name, lock)
291#define SEQCNT_SPINLOCK_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(name, lock)
292#define SEQCNT_RWLOCK_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(name, lock)
293#define SEQCNT_MUTEX_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(name, lock)
294#define SEQCNT_WW_MUTEX_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(name, lock)
295
296#define __seqprop_case(s, lockname, prop) \
297 seqcount_##lockname##_t: __seqprop_##lockname##_##prop((void *)(s))
298
299#define __seqprop(s, prop) _Generic(*(s), \
300 seqcount_t: __seqprop_##prop((void *)(s)), \
301 __seqprop_case((s), raw_spinlock, prop), \
302 __seqprop_case((s), spinlock, prop), \
303 __seqprop_case((s), rwlock, prop), \
304 __seqprop_case((s), mutex, prop))
305
306#define seqprop_ptr(s) __seqprop(s, ptr)
307#define seqprop_sequence(s) __seqprop(s, sequence)
308#define seqprop_preemptible(s) __seqprop(s, preemptible)
309#define seqprop_assert(s) __seqprop(s, assert)
310
311/**
312 * __read_seqcount_begin() - begin a seqcount_t read section w/o barrier
313 * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
314 *
315 * __read_seqcount_begin is like read_seqcount_begin, but has no smp_rmb()
316 * barrier. Callers should ensure that smp_rmb() or equivalent ordering is
317 * provided before actually loading any of the variables that are to be
318 * protected in this critical section.
319 *
320 * Use carefully, only in critical code, and comment how the barrier is
321 * provided.
322 *
323 * Return: count to be passed to read_seqcount_retry()
324 */
325#define __read_seqcount_begin(s) \
326({ \
327 unsigned __seq; \
328 \
329 while ((__seq = seqprop_sequence(s)) & 1) \
330 cpu_relax(); \
331 \
332 kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX); \
333 __seq; \
334})
335
336/**
337 * raw_read_seqcount_begin() - begin a seqcount_t read section w/o lockdep
338 * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
339 *
340 * Return: count to be passed to read_seqcount_retry()
341 */
342#define raw_read_seqcount_begin(s) \
343({ \
344 unsigned _seq = __read_seqcount_begin(s); \
345 \
346 smp_rmb(); \
347 _seq; \
348})
349
350/**
351 * read_seqcount_begin() - begin a seqcount_t read critical section
352 * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
353 *
354 * Return: count to be passed to read_seqcount_retry()
355 */
356#define read_seqcount_begin(s) \
357({ \
358 seqcount_lockdep_reader_access(seqprop_ptr(s)); \
359 raw_read_seqcount_begin(s); \
360})
361
362/**
363 * raw_read_seqcount() - read the raw seqcount_t counter value
364 * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
365 *
366 * raw_read_seqcount opens a read critical section of the given
367 * seqcount_t, without any lockdep checking, and without checking or
368 * masking the sequence counter LSB. Calling code is responsible for
369 * handling that.
370 *
371 * Return: count to be passed to read_seqcount_retry()
372 */
373#define raw_read_seqcount(s) \
374({ \
375 unsigned __seq = seqprop_sequence(s); \
376 \
377 smp_rmb(); \
378 kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX); \
379 __seq; \
380})
381
382/**
383 * raw_seqcount_begin() - begin a seqcount_t read critical section w/o
384 * lockdep and w/o counter stabilization
385 * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
386 *
387 * raw_seqcount_begin opens a read critical section of the given
388 * seqcount_t. Unlike read_seqcount_begin(), this function will not wait
389 * for the count to stabilize. If a writer is active when it begins, it
390 * will fail the read_seqcount_retry() at the end of the read critical
391 * section instead of stabilizing at the beginning of it.
392 *
393 * Use this only in special kernel hot paths where the read section is
394 * small and has a high probability of success through other external
395 * means. It will save a single branching instruction.
396 *
397 * Return: count to be passed to read_seqcount_retry()
398 */
399#define raw_seqcount_begin(s) \
400({ \
401 /* \
402 * If the counter is odd, let read_seqcount_retry() fail \
403 * by decrementing the counter. \
404 */ \
405 raw_read_seqcount(s) & ~1; \
406})
407
408/**
409 * __read_seqcount_retry() - end a seqcount_t read section w/o barrier
410 * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
411 * @start: count, from read_seqcount_begin()
412 *
413 * __read_seqcount_retry is like read_seqcount_retry, but has no smp_rmb()
414 * barrier. Callers should ensure that smp_rmb() or equivalent ordering is
415 * provided before actually loading any of the variables that are to be
416 * protected in this critical section.
417 *
418 * Use carefully, only in critical code, and comment how the barrier is
419 * provided.
420 *
421 * Return: true if a read section retry is required, else false
422 */
423#define __read_seqcount_retry(s, start) \
424 do___read_seqcount_retry(seqprop_ptr(s), start)
425
426static inline int do___read_seqcount_retry(const seqcount_t *s, unsigned start)
427{
428 kcsan_atomic_next(0);
429 return unlikely(READ_ONCE(s->sequence) != start);
430}
431
432/**
433 * read_seqcount_retry() - end a seqcount_t read critical section
434 * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
435 * @start: count, from read_seqcount_begin()
436 *
437 * read_seqcount_retry closes the read critical section of given
438 * seqcount_t. If the critical section was invalid, it must be ignored
439 * (and typically retried).
440 *
441 * Return: true if a read section retry is required, else false
442 */
443#define read_seqcount_retry(s, start) \
444 do_read_seqcount_retry(seqprop_ptr(s), start)
445
446static inline int do_read_seqcount_retry(const seqcount_t *s, unsigned start)
447{
448 smp_rmb();
449 return do___read_seqcount_retry(s, start);
450}
451
452/**
453 * raw_write_seqcount_begin() - start a seqcount_t write section w/o lockdep
454 * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
455 *
456 * Context: check write_seqcount_begin()
457 */
458#define raw_write_seqcount_begin(s) \
459do { \
460 if (seqprop_preemptible(s)) \
461 preempt_disable(); \
462 \
463 do_raw_write_seqcount_begin(seqprop_ptr(s)); \
464} while (0)
465
466static inline void do_raw_write_seqcount_begin(seqcount_t *s)
467{
468 kcsan_nestable_atomic_begin();
469 s->sequence++;
470 smp_wmb();
471}
472
473/**
474 * raw_write_seqcount_end() - end a seqcount_t write section w/o lockdep
475 * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
476 *
477 * Context: check write_seqcount_end()
478 */
479#define raw_write_seqcount_end(s) \
480do { \
481 do_raw_write_seqcount_end(seqprop_ptr(s)); \
482 \
483 if (seqprop_preemptible(s)) \
484 preempt_enable(); \
485} while (0)
486
487static inline void do_raw_write_seqcount_end(seqcount_t *s)
488{
489 smp_wmb();
490 s->sequence++;
491 kcsan_nestable_atomic_end();
492}
493
494/**
495 * write_seqcount_begin_nested() - start a seqcount_t write section with
496 * custom lockdep nesting level
497 * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
498 * @subclass: lockdep nesting level
499 *
500 * See Documentation/locking/lockdep-design.rst
501 * Context: check write_seqcount_begin()
502 */
503#define write_seqcount_begin_nested(s, subclass) \
504do { \
505 seqprop_assert(s); \
506 \
507 if (seqprop_preemptible(s)) \
508 preempt_disable(); \
509 \
510 do_write_seqcount_begin_nested(seqprop_ptr(s), subclass); \
511} while (0)
512
513static inline void do_write_seqcount_begin_nested(seqcount_t *s, int subclass)
514{
515 do_raw_write_seqcount_begin(s);
516 seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
517}
518
519/**
520 * write_seqcount_begin() - start a seqcount_t write side critical section
521 * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
522 *
523 * Context: sequence counter write side sections must be serialized and
524 * non-preemptible. Preemption will be automatically disabled if and
525 * only if the seqcount write serialization lock is associated, and
526 * preemptible. If readers can be invoked from hardirq or softirq
527 * context, interrupts or bottom halves must be respectively disabled.
528 */
529#define write_seqcount_begin(s) \
530do { \
531 seqprop_assert(s); \
532 \
533 if (seqprop_preemptible(s)) \
534 preempt_disable(); \
535 \
536 do_write_seqcount_begin(seqprop_ptr(s)); \
537} while (0)
538
539static inline void do_write_seqcount_begin(seqcount_t *s)
540{
541 do_write_seqcount_begin_nested(s, 0);
542}
543
544/**
545 * write_seqcount_end() - end a seqcount_t write side critical section
546 * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
547 *
548 * Context: Preemption will be automatically re-enabled if and only if
549 * the seqcount write serialization lock is associated, and preemptible.
550 */
551#define write_seqcount_end(s) \
552do { \
553 do_write_seqcount_end(seqprop_ptr(s)); \
554 \
555 if (seqprop_preemptible(s)) \
556 preempt_enable(); \
557} while (0)
558
559static inline void do_write_seqcount_end(seqcount_t *s)
560{
561 seqcount_release(&s->dep_map, _RET_IP_);
562 do_raw_write_seqcount_end(s);
563}
564
565/**
566 * raw_write_seqcount_barrier() - do a seqcount_t write barrier
567 * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
568 *
569 * This can be used to provide an ordering guarantee instead of the usual
570 * consistency guarantee. It is one wmb cheaper, because it can collapse
571 * the two back-to-back wmb()s.
572 *
573 * Note that writes surrounding the barrier should be declared atomic (e.g.
574 * via WRITE_ONCE): a) to ensure the writes become visible to other threads
575 * atomically, avoiding compiler optimizations; b) to document which writes are
576 * meant to propagate to the reader critical section. This is necessary because
577 * neither writes before and after the barrier are enclosed in a seq-writer
578 * critical section that would ensure readers are aware of ongoing writes::
579 *
580 * seqcount_t seq;
581 * bool X = true, Y = false;
582 *
583 * void read(void)
584 * {
585 * bool x, y;
586 *
587 * do {
588 * int s = read_seqcount_begin(&seq);
589 *
590 * x = X; y = Y;
591 *
592 * } while (read_seqcount_retry(&seq, s));
593 *
594 * BUG_ON(!x && !y);
595 * }
596 *
597 * void write(void)
598 * {
599 * WRITE_ONCE(Y, true);
600 *
601 * raw_write_seqcount_barrier(seq);
602 *
603 * WRITE_ONCE(X, false);
604 * }
605 */
606#define raw_write_seqcount_barrier(s) \
607 do_raw_write_seqcount_barrier(seqprop_ptr(s))
608
609static inline void do_raw_write_seqcount_barrier(seqcount_t *s)
610{
611 kcsan_nestable_atomic_begin();
612 s->sequence++;
613 smp_wmb();
614 s->sequence++;
615 kcsan_nestable_atomic_end();
616}
617
618/**
619 * write_seqcount_invalidate() - invalidate in-progress seqcount_t read
620 * side operations
621 * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
622 *
623 * After write_seqcount_invalidate, no seqcount_t read side operations
624 * will complete successfully and see data older than this.
625 */
626#define write_seqcount_invalidate(s) \
627 do_write_seqcount_invalidate(seqprop_ptr(s))
628
629static inline void do_write_seqcount_invalidate(seqcount_t *s)
630{
631 smp_wmb();
632 kcsan_nestable_atomic_begin();
633 s->sequence+=2;
634 kcsan_nestable_atomic_end();
635}
636
637/*
638 * Latch sequence counters (seqcount_latch_t)
639 *
640 * A sequence counter variant where the counter even/odd value is used to
641 * switch between two copies of protected data. This allows the read path,
642 * typically NMIs, to safely interrupt the write side critical section.
643 *
644 * As the write sections are fully preemptible, no special handling for
645 * PREEMPT_RT is needed.
646 */
647typedef struct {
648 seqcount_t seqcount;
649} seqcount_latch_t;
650
651/**
652 * SEQCNT_LATCH_ZERO() - static initializer for seqcount_latch_t
653 * @seq_name: Name of the seqcount_latch_t instance
654 */
655#define SEQCNT_LATCH_ZERO(seq_name) { \
656 .seqcount = SEQCNT_ZERO(seq_name.seqcount), \
657}
658
659/**
660 * seqcount_latch_init() - runtime initializer for seqcount_latch_t
661 * @s: Pointer to the seqcount_latch_t instance
662 */
663#define seqcount_latch_init(s) seqcount_init(&(s)->seqcount)
664
665/**
666 * raw_read_seqcount_latch() - pick even/odd latch data copy
667 * @s: Pointer to seqcount_latch_t
668 *
669 * See raw_write_seqcount_latch() for details and a full reader/writer
670 * usage example.
671 *
672 * Return: sequence counter raw value. Use the lowest bit as an index for
673 * picking which data copy to read. The full counter must then be checked
674 * with read_seqcount_latch_retry().
675 */
676static inline unsigned raw_read_seqcount_latch(const seqcount_latch_t *s)
677{
678 /*
679 * Pairs with the first smp_wmb() in raw_write_seqcount_latch().
680 * Due to the dependent load, a full smp_rmb() is not needed.
681 */
682 return READ_ONCE(s->seqcount.sequence);
683}
684
685/**
686 * read_seqcount_latch_retry() - end a seqcount_latch_t read section
687 * @s: Pointer to seqcount_latch_t
688 * @start: count, from raw_read_seqcount_latch()
689 *
690 * Return: true if a read section retry is required, else false
691 */
692static inline int
693read_seqcount_latch_retry(const seqcount_latch_t *s, unsigned start)
694{
695 return read_seqcount_retry(&s->seqcount, start);
696}
697
698/**
699 * raw_write_seqcount_latch() - redirect latch readers to even/odd copy
700 * @s: Pointer to seqcount_latch_t
701 *
702 * The latch technique is a multiversion concurrency control method that allows
703 * queries during non-atomic modifications. If you can guarantee queries never
704 * interrupt the modification -- e.g. the concurrency is strictly between CPUs
705 * -- you most likely do not need this.
706 *
707 * Where the traditional RCU/lockless data structures rely on atomic
708 * modifications to ensure queries observe either the old or the new state the
709 * latch allows the same for non-atomic updates. The trade-off is doubling the
710 * cost of storage; we have to maintain two copies of the entire data
711 * structure.
712 *
713 * Very simply put: we first modify one copy and then the other. This ensures
714 * there is always one copy in a stable state, ready to give us an answer.
715 *
716 * The basic form is a data structure like::
717 *
718 * struct latch_struct {
719 * seqcount_latch_t seq;
720 * struct data_struct data[2];
721 * };
722 *
723 * Where a modification, which is assumed to be externally serialized, does the
724 * following::
725 *
726 * void latch_modify(struct latch_struct *latch, ...)
727 * {
728 * smp_wmb(); // Ensure that the last data[1] update is visible
729 * latch->seq.sequence++;
730 * smp_wmb(); // Ensure that the seqcount update is visible
731 *
732 * modify(latch->data[0], ...);
733 *
734 * smp_wmb(); // Ensure that the data[0] update is visible
735 * latch->seq.sequence++;
736 * smp_wmb(); // Ensure that the seqcount update is visible
737 *
738 * modify(latch->data[1], ...);
739 * }
740 *
741 * The query will have a form like::
742 *
743 * struct entry *latch_query(struct latch_struct *latch, ...)
744 * {
745 * struct entry *entry;
746 * unsigned seq, idx;
747 *
748 * do {
749 * seq = raw_read_seqcount_latch(&latch->seq);
750 *
751 * idx = seq & 0x01;
752 * entry = data_query(latch->data[idx], ...);
753 *
754 * // This includes needed smp_rmb()
755 * } while (read_seqcount_latch_retry(&latch->seq, seq));
756 *
757 * return entry;
758 * }
759 *
760 * So during the modification, queries are first redirected to data[1]. Then we
761 * modify data[0]. When that is complete, we redirect queries back to data[0]
762 * and we can modify data[1].
763 *
764 * NOTE:
765 *
766 * The non-requirement for atomic modifications does _NOT_ include
767 * the publishing of new entries in the case where data is a dynamic
768 * data structure.
769 *
770 * An iteration might start in data[0] and get suspended long enough
771 * to miss an entire modification sequence, once it resumes it might
772 * observe the new entry.
773 *
774 * NOTE2:
775 *
776 * When data is a dynamic data structure; one should use regular RCU
777 * patterns to manage the lifetimes of the objects within.
778 */
779static inline void raw_write_seqcount_latch(seqcount_latch_t *s)
780{
781 smp_wmb(); /* prior stores before incrementing "sequence" */
782 s->seqcount.sequence++;
783 smp_wmb(); /* increment "sequence" before following stores */
784}
785
786/*
787 * Sequential locks (seqlock_t)
788 *
789 * Sequence counters with an embedded spinlock for writer serialization
790 * and non-preemptibility.
791 *
792 * For more info, see:
793 * - Comments on top of seqcount_t
794 * - Documentation/locking/seqlock.rst
795 */
796typedef struct {
797 /*
798 * Make sure that readers don't starve writers on PREEMPT_RT: use
799 * seqcount_spinlock_t instead of seqcount_t. Check __SEQ_LOCK().
800 */
801 seqcount_spinlock_t seqcount;
802 spinlock_t lock;
803} seqlock_t;
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 unsigned ret = read_seqcount_begin(&sl->seqcount);
837
838 kcsan_atomic_next(0); /* non-raw usage, assume closing read_seqretry() */
839 kcsan_flat_atomic_begin();
840 return ret;
841}
842
843/**
844 * read_seqretry() - end a seqlock_t read side section
845 * @sl: Pointer to seqlock_t
846 * @start: count, from read_seqbegin()
847 *
848 * read_seqretry closes the read side critical section of given seqlock_t.
849 * If the critical section was invalid, it must be ignored (and typically
850 * retried).
851 *
852 * Return: true if a read section retry is required, else false
853 */
854static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
855{
856 /*
857 * Assume not nested: read_seqretry() may be called multiple times when
858 * completing read critical section.
859 */
860 kcsan_flat_atomic_end();
861
862 return read_seqcount_retry(&sl->seqcount, start);
863}
864
865/*
866 * For all seqlock_t write side functions, use the the internal
867 * do_write_seqcount_begin() instead of generic write_seqcount_begin().
868 * This way, no redundant lockdep_assert_held() checks are added.
869 */
870
871/**
872 * write_seqlock() - start a seqlock_t write side critical section
873 * @sl: Pointer to seqlock_t
874 *
875 * write_seqlock opens a write side critical section for the given
876 * seqlock_t. It also implicitly acquires the spinlock_t embedded inside
877 * that sequential lock. All seqlock_t write side sections are thus
878 * automatically serialized and non-preemptible.
879 *
880 * Context: if the seqlock_t read section, or other write side critical
881 * sections, can be invoked from hardirq or softirq contexts, use the
882 * _irqsave or _bh variants of this function instead.
883 */
884static inline void write_seqlock(seqlock_t *sl)
885{
886 spin_lock(&sl->lock);
887 do_write_seqcount_begin(&sl->seqcount.seqcount);
888}
889
890/**
891 * write_sequnlock() - end a seqlock_t write side critical section
892 * @sl: Pointer to seqlock_t
893 *
894 * write_sequnlock closes the (serialized and non-preemptible) write side
895 * critical section of given seqlock_t.
896 */
897static inline void write_sequnlock(seqlock_t *sl)
898{
899 do_write_seqcount_end(&sl->seqcount.seqcount);
900 spin_unlock(&sl->lock);
901}
902
903/**
904 * write_seqlock_bh() - start a softirqs-disabled seqlock_t write section
905 * @sl: Pointer to seqlock_t
906 *
907 * _bh variant of write_seqlock(). Use only if the read side section, or
908 * other write side sections, can be invoked from softirq contexts.
909 */
910static inline void write_seqlock_bh(seqlock_t *sl)
911{
912 spin_lock_bh(&sl->lock);
913 do_write_seqcount_begin(&sl->seqcount.seqcount);
914}
915
916/**
917 * write_sequnlock_bh() - end a softirqs-disabled seqlock_t write section
918 * @sl: Pointer to seqlock_t
919 *
920 * write_sequnlock_bh closes the serialized, non-preemptible, and
921 * softirqs-disabled, seqlock_t write side critical section opened with
922 * write_seqlock_bh().
923 */
924static inline void write_sequnlock_bh(seqlock_t *sl)
925{
926 do_write_seqcount_end(&sl->seqcount.seqcount);
927 spin_unlock_bh(&sl->lock);
928}
929
930/**
931 * write_seqlock_irq() - start a non-interruptible seqlock_t write section
932 * @sl: Pointer to seqlock_t
933 *
934 * _irq variant of write_seqlock(). Use only if the read side section, or
935 * other write sections, can be invoked from hardirq contexts.
936 */
937static inline void write_seqlock_irq(seqlock_t *sl)
938{
939 spin_lock_irq(&sl->lock);
940 do_write_seqcount_begin(&sl->seqcount.seqcount);
941}
942
943/**
944 * write_sequnlock_irq() - end a non-interruptible seqlock_t write section
945 * @sl: Pointer to seqlock_t
946 *
947 * write_sequnlock_irq closes the serialized and non-interruptible
948 * seqlock_t write side section opened with write_seqlock_irq().
949 */
950static inline void write_sequnlock_irq(seqlock_t *sl)
951{
952 do_write_seqcount_end(&sl->seqcount.seqcount);
953 spin_unlock_irq(&sl->lock);
954}
955
956static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
957{
958 unsigned long flags;
959
960 spin_lock_irqsave(&sl->lock, flags);
961 do_write_seqcount_begin(&sl->seqcount.seqcount);
962 return flags;
963}
964
965/**
966 * write_seqlock_irqsave() - start a non-interruptible seqlock_t write
967 * section
968 * @lock: Pointer to seqlock_t
969 * @flags: Stack-allocated storage for saving caller's local interrupt
970 * state, to be passed to write_sequnlock_irqrestore().
971 *
972 * _irqsave variant of write_seqlock(). Use it only if the read side
973 * section, or other write sections, can be invoked from hardirq context.
974 */
975#define write_seqlock_irqsave(lock, flags) \
976 do { flags = __write_seqlock_irqsave(lock); } while (0)
977
978/**
979 * write_sequnlock_irqrestore() - end non-interruptible seqlock_t write
980 * section
981 * @sl: Pointer to seqlock_t
982 * @flags: Caller's saved interrupt state, from write_seqlock_irqsave()
983 *
984 * write_sequnlock_irqrestore closes the serialized and non-interruptible
985 * seqlock_t write section previously opened with write_seqlock_irqsave().
986 */
987static inline void
988write_sequnlock_irqrestore(seqlock_t *sl, unsigned long flags)
989{
990 do_write_seqcount_end(&sl->seqcount.seqcount);
991 spin_unlock_irqrestore(&sl->lock, flags);
992}
993
994/**
995 * read_seqlock_excl() - begin a seqlock_t locking reader section
996 * @sl: Pointer to seqlock_t
997 *
998 * read_seqlock_excl opens a seqlock_t locking reader critical section. A
999 * locking reader exclusively locks out *both* other writers *and* other
1000 * locking readers, but it does not update the embedded sequence number.
1001 *
1002 * Locking readers act like a normal spin_lock()/spin_unlock().
1003 *
1004 * Context: if the seqlock_t write section, *or other read sections*, can
1005 * be invoked from hardirq or softirq contexts, use the _irqsave or _bh
1006 * variant of this function instead.
1007 *
1008 * The opened read section must be closed with read_sequnlock_excl().
1009 */
1010static inline void read_seqlock_excl(seqlock_t *sl)
1011{
1012 spin_lock(&sl->lock);
1013}
1014
1015/**
1016 * read_sequnlock_excl() - end a seqlock_t locking reader critical section
1017 * @sl: Pointer to seqlock_t
1018 */
1019static inline void read_sequnlock_excl(seqlock_t *sl)
1020{
1021 spin_unlock(&sl->lock);
1022}
1023
1024/**
1025 * read_seqlock_excl_bh() - start a seqlock_t locking reader section with
1026 * softirqs disabled
1027 * @sl: Pointer to seqlock_t
1028 *
1029 * _bh variant of read_seqlock_excl(). Use this variant only if the
1030 * seqlock_t write side section, *or other read sections*, can be invoked
1031 * from softirq contexts.
1032 */
1033static inline void read_seqlock_excl_bh(seqlock_t *sl)
1034{
1035 spin_lock_bh(&sl->lock);
1036}
1037
1038/**
1039 * read_sequnlock_excl_bh() - stop a seqlock_t softirq-disabled locking
1040 * reader section
1041 * @sl: Pointer to seqlock_t
1042 */
1043static inline void read_sequnlock_excl_bh(seqlock_t *sl)
1044{
1045 spin_unlock_bh(&sl->lock);
1046}
1047
1048/**
1049 * read_seqlock_excl_irq() - start a non-interruptible seqlock_t locking
1050 * reader section
1051 * @sl: Pointer to seqlock_t
1052 *
1053 * _irq variant of read_seqlock_excl(). Use this only if the seqlock_t
1054 * write side section, *or other read sections*, can be invoked from a
1055 * hardirq context.
1056 */
1057static inline void read_seqlock_excl_irq(seqlock_t *sl)
1058{
1059 spin_lock_irq(&sl->lock);
1060}
1061
1062/**
1063 * read_sequnlock_excl_irq() - end an interrupts-disabled seqlock_t
1064 * locking reader section
1065 * @sl: Pointer to seqlock_t
1066 */
1067static inline void read_sequnlock_excl_irq(seqlock_t *sl)
1068{
1069 spin_unlock_irq(&sl->lock);
1070}
1071
1072static inline unsigned long __read_seqlock_excl_irqsave(seqlock_t *sl)
1073{
1074 unsigned long flags;
1075
1076 spin_lock_irqsave(&sl->lock, flags);
1077 return flags;
1078}
1079
1080/**
1081 * read_seqlock_excl_irqsave() - start a non-interruptible seqlock_t
1082 * locking reader section
1083 * @lock: Pointer to seqlock_t
1084 * @flags: Stack-allocated storage for saving caller's local interrupt
1085 * state, to be passed to read_sequnlock_excl_irqrestore().
1086 *
1087 * _irqsave variant of read_seqlock_excl(). Use this only if the seqlock_t
1088 * write side section, *or other read sections*, can be invoked from a
1089 * hardirq context.
1090 */
1091#define read_seqlock_excl_irqsave(lock, flags) \
1092 do { flags = __read_seqlock_excl_irqsave(lock); } while (0)
1093
1094/**
1095 * read_sequnlock_excl_irqrestore() - end non-interruptible seqlock_t
1096 * locking reader section
1097 * @sl: Pointer to seqlock_t
1098 * @flags: Caller saved interrupt state, from read_seqlock_excl_irqsave()
1099 */
1100static inline void
1101read_sequnlock_excl_irqrestore(seqlock_t *sl, unsigned long flags)
1102{
1103 spin_unlock_irqrestore(&sl->lock, flags);
1104}
1105
1106/**
1107 * read_seqbegin_or_lock() - begin a seqlock_t lockless or locking reader
1108 * @lock: Pointer to seqlock_t
1109 * @seq : Marker and return parameter. If the passed value is even, the
1110 * reader will become a *lockless* seqlock_t reader as in read_seqbegin().
1111 * If the passed value is odd, the reader will become a *locking* reader
1112 * as in read_seqlock_excl(). In the first call to this function, the
1113 * caller *must* initialize and pass an even value to @seq; this way, a
1114 * lockless read can be optimistically tried first.
1115 *
1116 * read_seqbegin_or_lock is an API designed to optimistically try a normal
1117 * lockless seqlock_t read section first. If an odd counter is found, the
1118 * lockless read trial has failed, and the next read iteration transforms
1119 * itself into a full seqlock_t locking reader.
1120 *
1121 * This is typically used to avoid seqlock_t lockless readers starvation
1122 * (too much retry loops) in the case of a sharp spike in write side
1123 * activity.
1124 *
1125 * Context: if the seqlock_t write section, *or other read sections*, can
1126 * be invoked from hardirq or softirq contexts, use the _irqsave or _bh
1127 * variant of this function instead.
1128 *
1129 * Check Documentation/locking/seqlock.rst for template example code.
1130 *
1131 * Return: the encountered sequence counter value, through the @seq
1132 * parameter, which is overloaded as a return parameter. This returned
1133 * value must be checked with need_seqretry(). If the read section need to
1134 * be retried, this returned value must also be passed as the @seq
1135 * parameter of the next read_seqbegin_or_lock() iteration.
1136 */
1137static inline void read_seqbegin_or_lock(seqlock_t *lock, int *seq)
1138{
1139 if (!(*seq & 1)) /* Even */
1140 *seq = read_seqbegin(lock);
1141 else /* Odd */
1142 read_seqlock_excl(lock);
1143}
1144
1145/**
1146 * need_seqretry() - validate seqlock_t "locking or lockless" read section
1147 * @lock: Pointer to seqlock_t
1148 * @seq: sequence count, from read_seqbegin_or_lock()
1149 *
1150 * Return: true if a read section retry is required, false otherwise
1151 */
1152static inline int need_seqretry(seqlock_t *lock, int seq)
1153{
1154 return !(seq & 1) && read_seqretry(lock, seq);
1155}
1156
1157/**
1158 * done_seqretry() - end seqlock_t "locking or lockless" reader section
1159 * @lock: Pointer to seqlock_t
1160 * @seq: count, from read_seqbegin_or_lock()
1161 *
1162 * done_seqretry finishes the seqlock_t read side critical section started
1163 * with read_seqbegin_or_lock() and validated by need_seqretry().
1164 */
1165static inline void done_seqretry(seqlock_t *lock, int seq)
1166{
1167 if (seq & 1)
1168 read_sequnlock_excl(lock);
1169}
1170
1171/**
1172 * read_seqbegin_or_lock_irqsave() - begin a seqlock_t lockless reader, or
1173 * a non-interruptible locking reader
1174 * @lock: Pointer to seqlock_t
1175 * @seq: Marker and return parameter. Check read_seqbegin_or_lock().
1176 *
1177 * This is the _irqsave variant of read_seqbegin_or_lock(). Use it only if
1178 * the seqlock_t write section, *or other read sections*, can be invoked
1179 * from hardirq context.
1180 *
1181 * Note: Interrupts will be disabled only for "locking reader" mode.
1182 *
1183 * Return:
1184 *
1185 * 1. The saved local interrupts state in case of a locking reader, to
1186 * be passed to done_seqretry_irqrestore().
1187 *
1188 * 2. The encountered sequence counter value, returned through @seq
1189 * overloaded as a return parameter. Check read_seqbegin_or_lock().
1190 */
1191static inline unsigned long
1192read_seqbegin_or_lock_irqsave(seqlock_t *lock, int *seq)
1193{
1194 unsigned long flags = 0;
1195
1196 if (!(*seq & 1)) /* Even */
1197 *seq = read_seqbegin(lock);
1198 else /* Odd */
1199 read_seqlock_excl_irqsave(lock, flags);
1200
1201 return flags;
1202}
1203
1204/**
1205 * done_seqretry_irqrestore() - end a seqlock_t lockless reader, or a
1206 * non-interruptible locking reader section
1207 * @lock: Pointer to seqlock_t
1208 * @seq: Count, from read_seqbegin_or_lock_irqsave()
1209 * @flags: Caller's saved local interrupt state in case of a locking
1210 * reader, also from read_seqbegin_or_lock_irqsave()
1211 *
1212 * This is the _irqrestore variant of done_seqretry(). The read section
1213 * must've been opened with read_seqbegin_or_lock_irqsave(), and validated
1214 * by need_seqretry().
1215 */
1216static inline void
1217done_seqretry_irqrestore(seqlock_t *lock, int seq, unsigned long flags)
1218{
1219 if (seq & 1)
1220 read_sequnlock_excl_irqrestore(lock, flags);
1221}
1222#endif /* __LINUX_SEQLOCK_H */
1223

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