seqlock.h

   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(&s->dep_map, name, key, 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(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, 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(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(&sl->lock);
 877	do_write_seqcount_begin(&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(&sl->seqcount.seqcount);
 890	spin_unlock(&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(&sl->lock);
 903	do_write_seqcount_begin(&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(&sl->seqcount.seqcount);
 917	spin_unlock_bh(&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(&sl->lock);
 930	do_write_seqcount_begin(&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(&sl->seqcount.seqcount);
 943	spin_unlock_irq(&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(&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(&sl->seqcount.seqcount);
 981	spin_unlock_irqrestore(&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(&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(&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(&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(&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(&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(&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(&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(lock);
1131	else			/* Odd */
1132		read_seqlock_excl(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(lock, 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(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(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(lock, flags);
1211}
1212
1213enum ss_state {
1214	ss_done = 0,
1215	ss_lock,
1216	ss_lock_irqsave,
1217	ss_lockless,
1218};
1219
1220struct ss_tmp {
1221	enum ss_state	state;
1222	unsigned long	data;
1223	spinlock_t	*lock;
1224	spinlock_t	*lock_irqsave;
1225};
1226
1227static __always_inline void __scoped_seqlock_cleanup(struct ss_tmp *sst)
1228{
1229	if (sst->lock)
1230		spin_unlock(sst->lock);
1231	if (sst->lock_irqsave)
1232		spin_unlock_irqrestore(sst->lock_irqsave, sst->data);
1233}
1234
1235extern void __scoped_seqlock_invalid_target(void);
1236
1237#if (defined(CONFIG_CC_IS_GCC) && CONFIG_GCC_VERSION < 90000) || defined(CONFIG_KASAN)
1238/*
1239 * For some reason some GCC-8 architectures (nios2, alpha) have trouble
1240 * determining that the ss_done state is impossible in __scoped_seqlock_next()
1241 * below.
1242 *
1243 * Similarly KASAN is known to confuse compilers enough to break this. But we
1244 * don't care about code quality for KASAN builds anyway.
1245 */
1246static inline void __scoped_seqlock_bug(void) { }
1247#else
1248/*
1249 * Canary for compiler optimization -- if the compiler doesn't realize this is
1250 * an impossible state, it very likely generates sub-optimal code here.
1251 */
1252extern void __scoped_seqlock_bug(void);
1253#endif
1254
1255static __always_inline void
1256__scoped_seqlock_next(struct ss_tmp *sst, seqlock_t *lock, enum ss_state target)
1257{
1258	switch (sst->state) {
1259	case ss_done:
1260		__scoped_seqlock_bug();
1261		return;
1262
1263	case ss_lock:
1264	case ss_lock_irqsave:
1265		sst->state = ss_done;
1266		return;
1267
1268	case ss_lockless:
1269		if (!read_seqretry(lock, sst->data)) {
1270			sst->state = ss_done;
1271			return;
1272		}
1273		break;
1274	}
1275
1276	switch (target) {
1277	case ss_done:
1278		__scoped_seqlock_invalid_target();
1279		return;
1280
1281	case ss_lock:
1282		sst->lock = &lock->lock;
1283		spin_lock(sst->lock);
1284		sst->state = ss_lock;
1285		return;
1286
1287	case ss_lock_irqsave:
1288		sst->lock_irqsave = &lock->lock;
1289		spin_lock_irqsave(sst->lock_irqsave, sst->data);
1290		sst->state = ss_lock_irqsave;
1291		return;
1292
1293	case ss_lockless:
1294		sst->data = read_seqbegin(lock);
1295		return;
1296	}
1297}
1298
1299#define __scoped_seqlock_read(_seqlock, _target, _s)			\
1300	for (struct ss_tmp _s __cleanup(__scoped_seqlock_cleanup) =	\
1301	     { .state = ss_lockless, .data = read_seqbegin(_seqlock) };	\
1302	     _s.state != ss_done;					\
1303	     __scoped_seqlock_next(&_s, _seqlock, _target))
1304
1305/**
1306 * scoped_seqlock_read() - execute the read-side critical section
1307 *                         without manual sequence counter handling
1308 *                         or calls to other helpers
1309 * @_seqlock: pointer to seqlock_t protecting the data
1310 * @_target: an enum ss_state: one of {ss_lock, ss_lock_irqsave, ss_lockless}
1311 *           indicating the type of critical read section
1312 *
1313 * Example::
1314 *
1315 *     scoped_seqlock_read (&lock, ss_lock) {
1316 *         // read-side critical section
1317 *     }
1318 *
1319 * Starts with a lockess pass first. If it fails, restarts the critical
1320 * section with the lock held.
1321 */
1322#define scoped_seqlock_read(_seqlock, _target)				\
1323	__scoped_seqlock_read(_seqlock, _target, __UNIQUE_ID(seqlock))
1324
1325#endif /* __LINUX_SEQLOCK_H */