1 /* SPDX-License-Identifier: GPL-2.0 */
6  * seqcount_t / seqlock_t - a reader-writer consistency mechanism with
7  * lockless readers (read-only retry loops), and no writer starvation.
12  * - Based on x86_64 vsyscall gettimeofday: Keith Owens, Andrea Arcangeli
13  * - Sequence counters with associated locks, (C) 2020 Linutronix GmbH
17 #include <linux/kcsan-checks.h>
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,
46  * Write side critical sections must be serialized and non-preemptible.
50  * entering the write section.
60  * serialization and non-preemptibility requirements, use a sequential
78 	lockdep_init_map(&s->dep_map, name, key, 0);  in __seqcount_init()
79 	s->sequence = 0;  in __seqcount_init()
88  * seqcount_init() - runtime initializer for seqcount_t
103 	seqcount_acquire_read(&l->dep_map, 0, 0, _RET_IP_);  in seqcount_lockdep_reader_access()
104 	seqcount_release(&l->dep_map, _RET_IP_);  in seqcount_lockdep_reader_access()
115  * SEQCNT_ZERO() - static initializer for seqcount_t
125  * that the write side critical section is properly serialized.
145  * lock. This will allow any possibly-preempted writer to make progress
146  * until the end of its writer serialization lock critical section.
148  * This lock-unlock technique must be implemented for all of PREEMPT_RT
158  * typedef seqcount_LOCKNAME_t - sequence counter with LOCKNAME associated
165  * that the write side critical section is properly serialized.
171  * seqcount_LOCKNAME_init() - runtime initializer for seqcount_LOCKNAME_t
179 		seqcount_init(&____s->seqcount);			\
180 		__SEQ_LOCK(____s->lock = (_lock));			\
190  * SEQCOUNT_LOCKNAME()	- Instantiate seqcount_LOCKNAME_t and helpers
191  * seqprop_LOCKNAME_*()	- Property accessors for seqcount_LOCKNAME_t
209 	return &s->seqcount;						\
215 	unsigned seq = READ_ONCE(s->seqcount.sequence);			\
222 		__SEQ_LOCK(lockbase##_unlock(s->lock));			\
225 		 * Re-read the sequence counter since the (possibly	\
228 		seq = READ_ONCE(s->seqcount.sequence);			\
261 	return READ_ONCE(s->sequence);  in __seqprop_sequence()
276 …OUNT_LOCKNAME(raw_spinlock, raw_spinlock_t,  false,    s->lock,        raw_spin, raw_spin_lock(s->…
277 SEQCOUNT_LOCKNAME(spinlock,     spinlock_t,      __SEQ_RT, s->lock,        spin,     spin_lock(s->l…
278 SEQCOUNT_LOCKNAME(rwlock,       rwlock_t,        __SEQ_RT, s->lock,        read,     read_lock(s->l…
279 SEQCOUNT_LOCKNAME(mutex,        struct mutex,    true,     s->lock,        mutex,    mutex_lock(s->…
280 …UNT_LOCKNAME(ww_mutex,     struct ww_mutex, true,     &s->lock->base, ww_mutex, ww_mutex_lock(s->l…
283  * SEQCNT_LOCKNAME_ZERO - static initializer for seqcount_LOCKNAME_t
316  * __read_seqcount_begin() - begin a seqcount_t read section w/o barrier
322  * protected in this critical section.
341  * raw_read_seqcount_begin() - begin a seqcount_t read section w/o lockdep
355  * read_seqcount_begin() - begin a seqcount_t read critical section
367  * raw_read_seqcount() - read the raw seqcount_t counter value
370  * raw_read_seqcount opens a read critical section of the given
387  * raw_seqcount_begin() - begin a seqcount_t read critical section w/o
391  * raw_seqcount_begin opens a read critical section of the given
395  * section instead of stabilizing at the beginning of it.
397  * Use this only in special kernel hot paths where the read section is
413  * __read_seqcount_retry() - end a seqcount_t read section w/o barrier
420  * protected in this critical section.
425  * Return: true if a read section retry is required, else false
433 	return unlikely(READ_ONCE(s->sequence) != start);  in do___read_seqcount_retry()
437  * read_seqcount_retry() - end a seqcount_t read critical section
441  * read_seqcount_retry closes the read critical section of given
442  * seqcount_t.  If the critical section was invalid, it must be ignored
445  * Return: true if a read section retry is required, else false
457  * raw_write_seqcount_begin() - start a seqcount_t write section w/o lockdep
473 	s->sequence++;  in do_raw_write_seqcount_begin()
478  * raw_write_seqcount_end() - end a seqcount_t write section w/o lockdep
494 	s->sequence++;  in do_raw_write_seqcount_end()
499  * write_seqcount_begin_nested() - start a seqcount_t write section with
504  * See Documentation/locking/lockdep-design.rst
520 	seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);  in do_write_seqcount_begin_nested()
524  * write_seqcount_begin() - start a seqcount_t write side critical section
528  * non-preemptible. Preemption will be automatically disabled if and
549  * write_seqcount_end() - end a seqcount_t write side critical section
552  * Context: Preemption will be automatically re-enabled if and only if
565 	seqcount_release(&s->dep_map, _RET_IP_);  in do_write_seqcount_end()
570  * raw_write_seqcount_barrier() - do a seqcount_t write barrier
575  * the two back-to-back wmb()s.
580  * meant to propagate to the reader critical section. This is necessary because
581  * neither writes before and after the barrier are enclosed in a seq-writer
582  * critical section that would ensure readers are aware of ongoing writes::
616 	s->sequence++;  in do_raw_write_seqcount_barrier()
618 	s->sequence++;  in do_raw_write_seqcount_barrier()
623  * write_seqcount_invalidate() - invalidate in-progress seqcount_t read
637 	s->sequence+=2;  in do_write_seqcount_invalidate()
646  * typically NMIs, to safely interrupt the write side critical section.
656  * SEQCNT_LATCH_ZERO() - static initializer for seqcount_latch_t
664  * seqcount_latch_init() - runtime initializer for seqcount_latch_t
667 #define seqcount_latch_init(s) seqcount_init(&(s)->seqcount)
670  * raw_read_seqcount_latch() - pick even/odd latch data copy
686 	return READ_ONCE(s->seqcount.sequence);  in raw_read_seqcount_latch()
690  * read_seqcount_latch_retry() - end a seqcount_latch_t read section
694  * Return: true if a read section retry is required, else false
699 	return read_seqcount_retry(&s->seqcount, start);  in read_seqcount_latch_retry()
703  * raw_write_seqcount_latch() - redirect latch readers to even/odd copy
707  * queries during non-atomic modifications. If you can guarantee queries never
708  * interrupt the modification -- e.g. the concurrency is strictly between CPUs
709  * -- you most likely do not need this.
713  * latch allows the same for non-atomic updates. The trade-off is doubling the
733  *		latch->seq.sequence++;
736  *		modify(latch->data[0], ...);
739  *		latch->seq.sequence++;
742  *		modify(latch->data[1], ...);
753  *			seq = raw_read_seqcount_latch(&latch->seq);
756  *			entry = data_query(latch->data[idx], ...);
759  *		} while (read_seqcount_latch_retry(&latch->seq, seq));
770  *	The non-requirement for atomic modifications does _NOT_ include
786 	s->seqcount.sequence++;  in raw_write_seqcount_latch()
794  * and non-preemptibility.
797  *    - Comments on top of seqcount_t
798  *    - Documentation/locking/seqlock.rst
816  * seqlock_init() - dynamic initializer for seqlock_t
821 		spin_lock_init(&(sl)->lock);				\
822 		seqcount_spinlock_init(&(sl)->seqcount, &(sl)->lock);	\
826  * DEFINE_SEQLOCK(sl) - Define a statically allocated seqlock_t
833  * read_seqbegin() - start a seqlock_t read side critical section
840 	unsigned ret = read_seqcount_begin(&sl->seqcount);  in read_seqbegin()
842 	kcsan_atomic_next(0);  /* non-raw usage, assume closing read_seqretry() */  in read_seqbegin()
848  * read_seqretry() - end a seqlock_t read side section
852  * read_seqretry closes the read side critical section of given seqlock_t.
853  * If the critical section was invalid, it must be ignored (and typically
856  * Return: true if a read section retry is required, else false
862 	 * completing read critical section.  in read_seqretry()
866 	return read_seqcount_retry(&sl->seqcount, start);  in read_seqretry()
876  * write_seqlock() - start a seqlock_t write side critical section
879  * write_seqlock opens a write side critical section for the given
882  * automatically serialized and non-preemptible.
884  * Context: if the seqlock_t read section, or other write side critical
890 	spin_lock(&sl->lock);  in write_seqlock()
891 	do_write_seqcount_begin(&sl->seqcount.seqcount);  in write_seqlock()
895  * write_sequnlock() - end a seqlock_t write side critical section
898  * write_sequnlock closes the (serialized and non-preemptible) write side
899  * critical section of given seqlock_t.
903 	do_write_seqcount_end(&sl->seqcount.seqcount);  in write_sequnlock()
904 	spin_unlock(&sl->lock);  in write_sequnlock()
908  * write_seqlock_bh() - start a softirqs-disabled seqlock_t write section
911  * _bh variant of write_seqlock(). Use only if the read side section, or
916 	spin_lock_bh(&sl->lock);  in write_seqlock_bh()
917 	do_write_seqcount_begin(&sl->seqcount.seqcount);  in write_seqlock_bh()
921  * write_sequnlock_bh() - end a softirqs-disabled seqlock_t write section
924  * write_sequnlock_bh closes the serialized, non-preemptible, and
925  * softirqs-disabled, seqlock_t write side critical section opened with
930 	do_write_seqcount_end(&sl->seqcount.seqcount);  in write_sequnlock_bh()
931 	spin_unlock_bh(&sl->lock);  in write_sequnlock_bh()
935  * write_seqlock_irq() - start a non-interruptible seqlock_t write section
938  * _irq variant of write_seqlock(). Use only if the read side section, or
943 	spin_lock_irq(&sl->lock);  in write_seqlock_irq()
944 	do_write_seqcount_begin(&sl->seqcount.seqcount);  in write_seqlock_irq()
948  * write_sequnlock_irq() - end a non-interruptible seqlock_t write section
951  * write_sequnlock_irq closes the serialized and non-interruptible
952  * seqlock_t write side section opened with write_seqlock_irq().
956 	do_write_seqcount_end(&sl->seqcount.seqcount);  in write_sequnlock_irq()
957 	spin_unlock_irq(&sl->lock);  in write_sequnlock_irq()
964 	spin_lock_irqsave(&sl->lock, flags);  in __write_seqlock_irqsave()
965 	do_write_seqcount_begin(&sl->seqcount.seqcount);  in __write_seqlock_irqsave()
970  * write_seqlock_irqsave() - start a non-interruptible seqlock_t write
971  *                           section
973  * @flags: Stack-allocated storage for saving caller's local interrupt
977  * section, or other write sections, can be invoked from hardirq context.
983  * write_sequnlock_irqrestore() - end non-interruptible seqlock_t write
984  *                                section
988  * write_sequnlock_irqrestore closes the serialized and non-interruptible
989  * seqlock_t write section previously opened with write_seqlock_irqsave().
994 	do_write_seqcount_end(&sl->seqcount.seqcount);  in write_sequnlock_irqrestore()
995 	spin_unlock_irqrestore(&sl->lock, flags);  in write_sequnlock_irqrestore()
999  * read_seqlock_excl() - begin a seqlock_t locking reader section
1002  * read_seqlock_excl opens a seqlock_t locking reader critical section.  A
1008  * Context: if the seqlock_t write section, *or other read sections*, can
1012  * The opened read section must be closed with read_sequnlock_excl().
1016 	spin_lock(&sl->lock);  in read_seqlock_excl()
1020  * read_sequnlock_excl() - end a seqlock_t locking reader critical section
1025 	spin_unlock(&sl->lock);  in read_sequnlock_excl()
1029  * read_seqlock_excl_bh() - start a seqlock_t locking reader section with
1034  * seqlock_t write side section, *or other read sections*, can be invoked
1039 	spin_lock_bh(&sl->lock);  in read_seqlock_excl_bh()
1043  * read_sequnlock_excl_bh() - stop a seqlock_t softirq-disabled locking
1044  *			      reader section
1049 	spin_unlock_bh(&sl->lock);  in read_sequnlock_excl_bh()
1053  * read_seqlock_excl_irq() - start a non-interruptible seqlock_t locking
1054  *			     reader section
1058  * write side section, *or other read sections*, can be invoked from a
1063 	spin_lock_irq(&sl->lock);  in read_seqlock_excl_irq()
1067  * read_sequnlock_excl_irq() - end an interrupts-disabled seqlock_t
1068  *                             locking reader section
1073 	spin_unlock_irq(&sl->lock);  in read_sequnlock_excl_irq()
1080 	spin_lock_irqsave(&sl->lock, flags);  in __read_seqlock_excl_irqsave()
1085  * read_seqlock_excl_irqsave() - start a non-interruptible seqlock_t
1086  *				 locking reader section
1088  * @flags: Stack-allocated storage for saving caller's local interrupt
1092  * write side section, *or other read sections*, can be invoked from a
1099  * read_sequnlock_excl_irqrestore() - end non-interruptible seqlock_t
1100  *				      locking reader section
1107 	spin_unlock_irqrestore(&sl->lock, flags);  in read_sequnlock_excl_irqrestore()
1111  * read_seqbegin_or_lock() - begin a seqlock_t lockless or locking reader
1121  * lockless seqlock_t read section first.  If an odd counter is found, the
1129  * Context: if the seqlock_t write section, *or other read sections*, can
1137  * value must be checked with need_seqretry(). If the read section need to
1150  * need_seqretry() - validate seqlock_t "locking or lockless" read section
1154  * Return: true if a read section retry is required, false otherwise
1162  * done_seqretry() - end seqlock_t "locking or lockless" reader section
1166  * done_seqretry finishes the seqlock_t read side critical section started
1176  * read_seqbegin_or_lock_irqsave() - begin a seqlock_t lockless reader, or
1177  *                                   a non-interruptible locking reader
1182  * the seqlock_t write section, *or other read sections*, can be invoked
1209  * done_seqretry_irqrestore() - end a seqlock_t lockless reader, or a
1210  *				non-interruptible locking reader section
1216  * This is the _irqrestore variant of done_seqretry(). The read section