Line data Source code
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 : */
64 : typedef struct seqcount {
65 : unsigned sequence;
66 : #ifdef CONFIG_DEBUG_LOCK_ALLOC
67 : struct lockdep_map dep_map;
68 : #endif
69 : } seqcount_t;
70 :
71 : static 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 1743427214 : lockdep_init_map(&s->dep_map, name, key, 0);
78 1743427214 : 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 :
96 : static 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) \
199 : typedef struct seqcount_##lockname { \
200 : seqcount_t seqcount; \
201 : __SEQ_LOCK(locktype *lock); \
202 : } seqcount_##lockname##_t; \
203 : \
204 : static __always_inline seqcount_t * \
205 : __seqprop_##lockname##_ptr(seqcount_##lockname##_t *s) \
206 : { \
207 : return &s->seqcount; \
208 : } \
209 : \
210 : static __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 : \
232 : static __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 : \
242 : static __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 :
252 : static inline seqcount_t *__seqprop_ptr(seqcount_t *s)
253 : {
254 : return s;
255 : }
256 :
257 : static inline unsigned __seqprop_sequence(const seqcount_t *s)
258 : {
259 : return READ_ONCE(s->sequence);
260 : }
261 :
262 : static inline bool __seqprop_preemptible(const seqcount_t *s)
263 : {
264 : return false;
265 : }
266 :
267 : static 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 :
274 : SEQCOUNT_LOCKNAME(raw_spinlock, raw_spinlock_t, false, s->lock, raw_spin, raw_spin_lock(s->lock))
275 >11198*10^7 : SEQCOUNT_LOCKNAME(spinlock, spinlock_t, __SEQ_RT, s->lock, spin, spin_lock(s->lock))
276 : SEQCOUNT_LOCKNAME(rwlock, rwlock_t, __SEQ_RT, s->lock, read, read_lock(s->lock))
277 : SEQCOUNT_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 :
426 : static inline int do___read_seqcount_retry(const seqcount_t *s, unsigned start)
427 : {
428 >17535*10^7 : kcsan_atomic_next(0);
429 24243696185 : 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 :
446 0 : static inline int do_read_seqcount_retry(const seqcount_t *s, unsigned start)
447 : {
448 >18529*10^7 : smp_rmb();
449 >16735*10^7 : 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) \
459 : do { \
460 : if (seqprop_preemptible(s)) \
461 : preempt_disable(); \
462 : \
463 : do_raw_write_seqcount_begin(seqprop_ptr(s)); \
464 : } while (0)
465 :
466 : static inline void do_raw_write_seqcount_begin(seqcount_t *s)
467 : {
468 2660532016 : kcsan_nestable_atomic_begin();
469 2660532016 : s->sequence++;
470 3463122770 : 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) \
480 : do { \
481 : do_raw_write_seqcount_end(seqprop_ptr(s)); \
482 : \
483 : if (seqprop_preemptible(s)) \
484 : preempt_enable(); \
485 : } while (0)
486 :
487 : static inline void do_raw_write_seqcount_end(seqcount_t *s)
488 : {
489 3460942543 : smp_wmb();
490 2657561504 : s->sequence++;
491 2547848544 : 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) \
504 : do { \
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 :
513 : static inline void do_write_seqcount_begin_nested(seqcount_t *s, int subclass)
514 : {
515 1021701900 : do_raw_write_seqcount_begin(s);
516 1072979810 : 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) \
530 : do { \
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 :
539 : static inline void do_write_seqcount_begin(seqcount_t *s)
540 : {
541 1021701900 : 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) \
552 : do { \
553 : do_write_seqcount_end(seqprop_ptr(s)); \
554 : \
555 : if (seqprop_preemptible(s)) \
556 : preempt_enable(); \
557 : } while (0)
558 :
559 : static inline void do_write_seqcount_end(seqcount_t *s)
560 : {
561 1131351018 : seqcount_release(&s->dep_map, _RET_IP_);
562 1021638058 : 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 :
609 : static 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 :
629 : static inline void do_write_seqcount_invalidate(seqcount_t *s)
630 : {
631 1001267942 : smp_wmb();
632 1001267641 : kcsan_nestable_atomic_begin();
633 1001267641 : s->sequence+=2;
634 1001267641 : kcsan_nestable_atomic_end();
635 1001267641 : }
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 : */
647 : typedef 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 raw_read_seqcount_latch_retry().
675 : */
676 : static __always_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 : * raw_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 : */
692 : static __always_inline int
693 : raw_read_seqcount_latch_retry(const seqcount_latch_t *s, unsigned start)
694 : {
695 : smp_rmb();
696 : return unlikely(READ_ONCE(s->seqcount.sequence) != start);
697 : }
698 :
699 : /**
700 : * raw_write_seqcount_latch() - redirect latch readers to even/odd copy
701 : * @s: Pointer to seqcount_latch_t
702 : *
703 : * The latch technique is a multiversion concurrency control method that allows
704 : * queries during non-atomic modifications. If you can guarantee queries never
705 : * interrupt the modification -- e.g. the concurrency is strictly between CPUs
706 : * -- you most likely do not need this.
707 : *
708 : * Where the traditional RCU/lockless data structures rely on atomic
709 : * modifications to ensure queries observe either the old or the new state the
710 : * latch allows the same for non-atomic updates. The trade-off is doubling the
711 : * cost of storage; we have to maintain two copies of the entire data
712 : * structure.
713 : *
714 : * Very simply put: we first modify one copy and then the other. This ensures
715 : * there is always one copy in a stable state, ready to give us an answer.
716 : *
717 : * The basic form is a data structure like::
718 : *
719 : * struct latch_struct {
720 : * seqcount_latch_t seq;
721 : * struct data_struct data[2];
722 : * };
723 : *
724 : * Where a modification, which is assumed to be externally serialized, does the
725 : * following::
726 : *
727 : * void latch_modify(struct latch_struct *latch, ...)
728 : * {
729 : * smp_wmb(); // Ensure that the last data[1] update is visible
730 : * latch->seq.sequence++;
731 : * smp_wmb(); // Ensure that the seqcount update is visible
732 : *
733 : * modify(latch->data[0], ...);
734 : *
735 : * smp_wmb(); // Ensure that the data[0] update is visible
736 : * latch->seq.sequence++;
737 : * smp_wmb(); // Ensure that the seqcount update is visible
738 : *
739 : * modify(latch->data[1], ...);
740 : * }
741 : *
742 : * The query will have a form like::
743 : *
744 : * struct entry *latch_query(struct latch_struct *latch, ...)
745 : * {
746 : * struct entry *entry;
747 : * unsigned seq, idx;
748 : *
749 : * do {
750 : * seq = raw_read_seqcount_latch(&latch->seq);
751 : *
752 : * idx = seq & 0x01;
753 : * entry = data_query(latch->data[idx], ...);
754 : *
755 : * // This includes needed smp_rmb()
756 : * } while (raw_read_seqcount_latch_retry(&latch->seq, seq));
757 : *
758 : * return entry;
759 : * }
760 : *
761 : * So during the modification, queries are first redirected to data[1]. Then we
762 : * modify data[0]. When that is complete, we redirect queries back to data[0]
763 : * and we can modify data[1].
764 : *
765 : * NOTE:
766 : *
767 : * The non-requirement for atomic modifications does _NOT_ include
768 : * the publishing of new entries in the case where data is a dynamic
769 : * data structure.
770 : *
771 : * An iteration might start in data[0] and get suspended long enough
772 : * to miss an entire modification sequence, once it resumes it might
773 : * observe the new entry.
774 : *
775 : * NOTE2:
776 : *
777 : * When data is a dynamic data structure; one should use regular RCU
778 : * patterns to manage the lifetimes of the objects within.
779 : */
780 : static inline void raw_write_seqcount_latch(seqcount_latch_t *s)
781 : {
782 : smp_wmb(); /* prior stores before incrementing "sequence" */
783 : s->seqcount.sequence++;
784 : smp_wmb(); /* increment "sequence" before following stores */
785 : }
786 :
787 : /*
788 : * Sequential locks (seqlock_t)
789 : *
790 : * Sequence counters with an embedded spinlock for writer serialization
791 : * and non-preemptibility.
792 : *
793 : * For more info, see:
794 : * - Comments on top of seqcount_t
795 : * - Documentation/locking/seqlock.rst
796 : */
797 : typedef struct {
798 : /*
799 : * Make sure that readers don't starve writers on PREEMPT_RT: use
800 : * seqcount_spinlock_t instead of seqcount_t. Check __SEQ_LOCK().
801 : */
802 : seqcount_spinlock_t seqcount;
803 : spinlock_t lock;
804 : } seqlock_t;
805 :
806 : #define __SEQLOCK_UNLOCKED(lockname) \
807 : { \
808 : .seqcount = SEQCNT_SPINLOCK_ZERO(lockname, &(lockname).lock), \
809 : .lock = __SPIN_LOCK_UNLOCKED(lockname) \
810 : }
811 :
812 : /**
813 : * seqlock_init() - dynamic initializer for seqlock_t
814 : * @sl: Pointer to the seqlock_t instance
815 : */
816 : #define seqlock_init(sl) \
817 : do { \
818 : spin_lock_init(&(sl)->lock); \
819 : seqcount_spinlock_init(&(sl)->seqcount, &(sl)->lock); \
820 : } while (0)
821 :
822 : /**
823 : * DEFINE_SEQLOCK(sl) - Define a statically allocated seqlock_t
824 : * @sl: Name of the seqlock_t instance
825 : */
826 : #define DEFINE_SEQLOCK(sl) \
827 : seqlock_t sl = __SEQLOCK_UNLOCKED(sl)
828 :
829 : /**
830 : * read_seqbegin() - start a seqlock_t read side critical section
831 : * @sl: Pointer to seqlock_t
832 : *
833 : * Return: count, to be passed to read_seqretry()
834 : */
835 : static inline unsigned read_seqbegin(const seqlock_t *sl)
836 : {
837 5463218139 : unsigned ret = read_seqcount_begin(&sl->seqcount);
838 :
839 5417812429 : kcsan_atomic_next(0); /* non-raw usage, assume closing read_seqretry() */
840 5417812429 : kcsan_flat_atomic_begin();
841 5380201362 : return ret;
842 : }
843 :
844 : /**
845 : * read_seqretry() - end a seqlock_t read side section
846 : * @sl: Pointer to seqlock_t
847 : * @start: count, from read_seqbegin()
848 : *
849 : * read_seqretry closes the read side critical section of given seqlock_t.
850 : * If the critical section was invalid, it must be ignored (and typically
851 : * retried).
852 : *
853 : * Return: true if a read section retry is required, else false
854 : */
855 : static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
856 : {
857 : /*
858 : * Assume not nested: read_seqretry() may be called multiple times when
859 : * completing read critical section.
860 : */
861 25981944003 : kcsan_flat_atomic_end();
862 :
863 25928165974 : return read_seqcount_retry(&sl->seqcount, start);
864 : }
865 :
866 : /*
867 : * For all seqlock_t write side functions, use the the internal
868 : * do_write_seqcount_begin() instead of generic write_seqcount_begin().
869 : * This way, no redundant lockdep_assert_held() checks are added.
870 : */
871 :
872 : /**
873 : * write_seqlock() - start a seqlock_t write side critical section
874 : * @sl: Pointer to seqlock_t
875 : *
876 : * write_seqlock opens a write side critical section for the given
877 : * seqlock_t. It also implicitly acquires the spinlock_t embedded inside
878 : * that sequential lock. All seqlock_t write side sections are thus
879 : * automatically serialized and non-preemptible.
880 : *
881 : * Context: if the seqlock_t read section, or other write side critical
882 : * sections, can be invoked from hardirq or softirq contexts, use the
883 : * _irqsave or _bh variants of this function instead.
884 : */
885 : static inline void write_seqlock(seqlock_t *sl)
886 : {
887 65264114 : spin_lock(&sl->lock);
888 65267768 : do_write_seqcount_begin(&sl->seqcount.seqcount);
889 3704036 : }
890 :
891 : /**
892 : * write_sequnlock() - end a seqlock_t write side critical section
893 : * @sl: Pointer to seqlock_t
894 : *
895 : * write_sequnlock closes the (serialized and non-preemptible) write side
896 : * critical section of given seqlock_t.
897 : */
898 : static inline void write_sequnlock(seqlock_t *sl)
899 : {
900 65267768 : do_write_seqcount_end(&sl->seqcount.seqcount);
901 65267768 : spin_unlock(&sl->lock);
902 3704036 : }
903 :
904 : /**
905 : * write_seqlock_bh() - start a softirqs-disabled seqlock_t write section
906 : * @sl: Pointer to seqlock_t
907 : *
908 : * _bh variant of write_seqlock(). Use only if the read side section, or
909 : * other write side sections, can be invoked from softirq contexts.
910 : */
911 : static inline void write_seqlock_bh(seqlock_t *sl)
912 : {
913 : spin_lock_bh(&sl->lock);
914 : do_write_seqcount_begin(&sl->seqcount.seqcount);
915 : }
916 :
917 : /**
918 : * write_sequnlock_bh() - end a softirqs-disabled seqlock_t write section
919 : * @sl: Pointer to seqlock_t
920 : *
921 : * write_sequnlock_bh closes the serialized, non-preemptible, and
922 : * softirqs-disabled, seqlock_t write side critical section opened with
923 : * write_seqlock_bh().
924 : */
925 : static inline void write_sequnlock_bh(seqlock_t *sl)
926 : {
927 : do_write_seqcount_end(&sl->seqcount.seqcount);
928 : spin_unlock_bh(&sl->lock);
929 : }
930 :
931 : /**
932 : * write_seqlock_irq() - start a non-interruptible seqlock_t write section
933 : * @sl: Pointer to seqlock_t
934 : *
935 : * _irq variant of write_seqlock(). Use only if the read side section, or
936 : * other write sections, can be invoked from hardirq contexts.
937 : */
938 : static inline void write_seqlock_irq(seqlock_t *sl)
939 : {
940 : spin_lock_irq(&sl->lock);
941 : do_write_seqcount_begin(&sl->seqcount.seqcount);
942 : }
943 :
944 : /**
945 : * write_sequnlock_irq() - end a non-interruptible seqlock_t write section
946 : * @sl: Pointer to seqlock_t
947 : *
948 : * write_sequnlock_irq closes the serialized and non-interruptible
949 : * seqlock_t write side section opened with write_seqlock_irq().
950 : */
951 : static inline void write_sequnlock_irq(seqlock_t *sl)
952 : {
953 : do_write_seqcount_end(&sl->seqcount.seqcount);
954 : spin_unlock_irq(&sl->lock);
955 : }
956 :
957 : static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
958 : {
959 : unsigned long flags;
960 :
961 : spin_lock_irqsave(&sl->lock, flags);
962 : do_write_seqcount_begin(&sl->seqcount.seqcount);
963 : return flags;
964 : }
965 :
966 : /**
967 : * write_seqlock_irqsave() - start a non-interruptible seqlock_t write
968 : * section
969 : * @lock: Pointer to seqlock_t
970 : * @flags: Stack-allocated storage for saving caller's local interrupt
971 : * state, to be passed to write_sequnlock_irqrestore().
972 : *
973 : * _irqsave variant of write_seqlock(). Use it only if the read side
974 : * section, or other write sections, can be invoked from hardirq context.
975 : */
976 : #define write_seqlock_irqsave(lock, flags) \
977 : do { flags = __write_seqlock_irqsave(lock); } while (0)
978 :
979 : /**
980 : * write_sequnlock_irqrestore() - end non-interruptible seqlock_t write
981 : * section
982 : * @sl: Pointer to seqlock_t
983 : * @flags: Caller's saved interrupt state, from write_seqlock_irqsave()
984 : *
985 : * write_sequnlock_irqrestore closes the serialized and non-interruptible
986 : * seqlock_t write section previously opened with write_seqlock_irqsave().
987 : */
988 : static inline void
989 : write_sequnlock_irqrestore(seqlock_t *sl, unsigned long flags)
990 : {
991 : do_write_seqcount_end(&sl->seqcount.seqcount);
992 : spin_unlock_irqrestore(&sl->lock, flags);
993 : }
994 :
995 : /**
996 : * read_seqlock_excl() - begin a seqlock_t locking reader section
997 : * @sl: Pointer to seqlock_t
998 : *
999 : * read_seqlock_excl opens a seqlock_t locking reader critical section. A
1000 : * locking reader exclusively locks out *both* other writers *and* other
1001 : * locking readers, but it does not update the embedded sequence number.
1002 : *
1003 : * Locking readers act like a normal spin_lock()/spin_unlock().
1004 : *
1005 : * Context: if the seqlock_t write section, *or other read sections*, can
1006 : * be invoked from hardirq or softirq contexts, use the _irqsave or _bh
1007 : * variant of this function instead.
1008 : *
1009 : * The opened read section must be closed with read_sequnlock_excl().
1010 : */
1011 : static inline void read_seqlock_excl(seqlock_t *sl)
1012 : {
1013 3885536 : spin_lock(&sl->lock);
1014 225399 : }
1015 :
1016 : /**
1017 : * read_sequnlock_excl() - end a seqlock_t locking reader critical section
1018 : * @sl: Pointer to seqlock_t
1019 : */
1020 : static inline void read_sequnlock_excl(seqlock_t *sl)
1021 : {
1022 3886208 : spin_unlock(&sl->lock);
1023 225399 : }
1024 :
1025 : /**
1026 : * read_seqlock_excl_bh() - start a seqlock_t locking reader section with
1027 : * softirqs disabled
1028 : * @sl: Pointer to seqlock_t
1029 : *
1030 : * _bh variant of read_seqlock_excl(). Use this variant only if the
1031 : * seqlock_t write side section, *or other read sections*, can be invoked
1032 : * from softirq contexts.
1033 : */
1034 : static inline void read_seqlock_excl_bh(seqlock_t *sl)
1035 : {
1036 : spin_lock_bh(&sl->lock);
1037 : }
1038 :
1039 : /**
1040 : * read_sequnlock_excl_bh() - stop a seqlock_t softirq-disabled locking
1041 : * reader section
1042 : * @sl: Pointer to seqlock_t
1043 : */
1044 : static inline void read_sequnlock_excl_bh(seqlock_t *sl)
1045 : {
1046 : spin_unlock_bh(&sl->lock);
1047 : }
1048 :
1049 : /**
1050 : * read_seqlock_excl_irq() - start a non-interruptible seqlock_t locking
1051 : * reader section
1052 : * @sl: Pointer to seqlock_t
1053 : *
1054 : * _irq variant of read_seqlock_excl(). Use this only if the seqlock_t
1055 : * write side section, *or other read sections*, can be invoked from a
1056 : * hardirq context.
1057 : */
1058 : static inline void read_seqlock_excl_irq(seqlock_t *sl)
1059 : {
1060 : spin_lock_irq(&sl->lock);
1061 : }
1062 :
1063 : /**
1064 : * read_sequnlock_excl_irq() - end an interrupts-disabled seqlock_t
1065 : * locking reader section
1066 : * @sl: Pointer to seqlock_t
1067 : */
1068 : static inline void read_sequnlock_excl_irq(seqlock_t *sl)
1069 : {
1070 : spin_unlock_irq(&sl->lock);
1071 : }
1072 :
1073 : static inline unsigned long __read_seqlock_excl_irqsave(seqlock_t *sl)
1074 : {
1075 : unsigned long flags;
1076 :
1077 : spin_lock_irqsave(&sl->lock, flags);
1078 : return flags;
1079 : }
1080 :
1081 : /**
1082 : * read_seqlock_excl_irqsave() - start a non-interruptible seqlock_t
1083 : * locking reader section
1084 : * @lock: Pointer to seqlock_t
1085 : * @flags: Stack-allocated storage for saving caller's local interrupt
1086 : * state, to be passed to read_sequnlock_excl_irqrestore().
1087 : *
1088 : * _irqsave variant of read_seqlock_excl(). Use this only if the seqlock_t
1089 : * write side section, *or other read sections*, can be invoked from a
1090 : * hardirq context.
1091 : */
1092 : #define read_seqlock_excl_irqsave(lock, flags) \
1093 : do { flags = __read_seqlock_excl_irqsave(lock); } while (0)
1094 :
1095 : /**
1096 : * read_sequnlock_excl_irqrestore() - end non-interruptible seqlock_t
1097 : * locking reader section
1098 : * @sl: Pointer to seqlock_t
1099 : * @flags: Caller saved interrupt state, from read_seqlock_excl_irqsave()
1100 : */
1101 : static inline void
1102 : read_sequnlock_excl_irqrestore(seqlock_t *sl, unsigned long flags)
1103 : {
1104 : spin_unlock_irqrestore(&sl->lock, flags);
1105 : }
1106 :
1107 : /**
1108 : * read_seqbegin_or_lock() - begin a seqlock_t lockless or locking reader
1109 : * @lock: Pointer to seqlock_t
1110 : * @seq : Marker and return parameter. If the passed value is even, the
1111 : * reader will become a *lockless* seqlock_t reader as in read_seqbegin().
1112 : * If the passed value is odd, the reader will become a *locking* reader
1113 : * as in read_seqlock_excl(). In the first call to this function, the
1114 : * caller *must* initialize and pass an even value to @seq; this way, a
1115 : * lockless read can be optimistically tried first.
1116 : *
1117 : * read_seqbegin_or_lock is an API designed to optimistically try a normal
1118 : * lockless seqlock_t read section first. If an odd counter is found, the
1119 : * lockless read trial has failed, and the next read iteration transforms
1120 : * itself into a full seqlock_t locking reader.
1121 : *
1122 : * This is typically used to avoid seqlock_t lockless readers starvation
1123 : * (too much retry loops) in the case of a sharp spike in write side
1124 : * activity.
1125 : *
1126 : * Context: if the seqlock_t write section, *or other read sections*, can
1127 : * be invoked from hardirq or softirq contexts, use the _irqsave or _bh
1128 : * variant of this function instead.
1129 : *
1130 : * Check Documentation/locking/seqlock.rst for template example code.
1131 : *
1132 : * Return: the encountered sequence counter value, through the @seq
1133 : * parameter, which is overloaded as a return parameter. This returned
1134 : * value must be checked with need_seqretry(). If the read section need to
1135 : * be retried, this returned value must also be passed as the @seq
1136 : * parameter of the next read_seqbegin_or_lock() iteration.
1137 : */
1138 3342566712 : static inline void read_seqbegin_or_lock(seqlock_t *lock, int *seq)
1139 : {
1140 3342566712 : if (!(*seq & 1)) /* Even */
1141 3342551295 : *seq = read_seqbegin(lock);
1142 : else /* Odd */
1143 224727 : read_seqlock_excl(lock);
1144 3342776694 : }
1145 :
1146 : /**
1147 : * need_seqretry() - validate seqlock_t "locking or lockless" read section
1148 : * @lock: Pointer to seqlock_t
1149 : * @seq: sequence count, from read_seqbegin_or_lock()
1150 : *
1151 : * Return: true if a read section retry is required, false otherwise
1152 : */
1153 : static inline int need_seqretry(seqlock_t *lock, int seq)
1154 : {
1155 6646605613 : return !(seq & 1) && read_seqretry(lock, seq);
1156 : }
1157 :
1158 : /**
1159 : * done_seqretry() - end seqlock_t "locking or lockless" reader section
1160 : * @lock: Pointer to seqlock_t
1161 : * @seq: count, from read_seqbegin_or_lock()
1162 : *
1163 : * done_seqretry finishes the seqlock_t read side critical section started
1164 : * with read_seqbegin_or_lock() and validated by need_seqretry().
1165 : */
1166 : static inline void done_seqretry(seqlock_t *lock, int seq)
1167 : {
1168 3340226139 : if (seq & 1)
1169 225399 : read_sequnlock_excl(lock);
1170 : }
1171 :
1172 : /**
1173 : * read_seqbegin_or_lock_irqsave() - begin a seqlock_t lockless reader, or
1174 : * a non-interruptible locking reader
1175 : * @lock: Pointer to seqlock_t
1176 : * @seq: Marker and return parameter. Check read_seqbegin_or_lock().
1177 : *
1178 : * This is the _irqsave variant of read_seqbegin_or_lock(). Use it only if
1179 : * the seqlock_t write section, *or other read sections*, can be invoked
1180 : * from hardirq context.
1181 : *
1182 : * Note: Interrupts will be disabled only for "locking reader" mode.
1183 : *
1184 : * Return:
1185 : *
1186 : * 1. The saved local interrupts state in case of a locking reader, to
1187 : * be passed to done_seqretry_irqrestore().
1188 : *
1189 : * 2. The encountered sequence counter value, returned through @seq
1190 : * overloaded as a return parameter. Check read_seqbegin_or_lock().
1191 : */
1192 : static inline unsigned long
1193 : read_seqbegin_or_lock_irqsave(seqlock_t *lock, int *seq)
1194 : {
1195 : unsigned long flags = 0;
1196 :
1197 : if (!(*seq & 1)) /* Even */
1198 : *seq = read_seqbegin(lock);
1199 : else /* Odd */
1200 : read_seqlock_excl_irqsave(lock, flags);
1201 :
1202 : return flags;
1203 : }
1204 :
1205 : /**
1206 : * done_seqretry_irqrestore() - end a seqlock_t lockless reader, or a
1207 : * non-interruptible locking reader section
1208 : * @lock: Pointer to seqlock_t
1209 : * @seq: Count, from read_seqbegin_or_lock_irqsave()
1210 : * @flags: Caller's saved local interrupt state in case of a locking
1211 : * reader, also from read_seqbegin_or_lock_irqsave()
1212 : *
1213 : * This is the _irqrestore variant of done_seqretry(). The read section
1214 : * must've been opened with read_seqbegin_or_lock_irqsave(), and validated
1215 : * by need_seqretry().
1216 : */
1217 : static inline void
1218 : done_seqretry_irqrestore(seqlock_t *lock, int seq, unsigned long flags)
1219 : {
1220 : if (seq & 1)
1221 : read_sequnlock_excl_irqrestore(lock, flags);
1222 : }
1223 : #endif /* __LINUX_SEQLOCK_H */
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