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>
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
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,
35  * atomics; if there is a matching read_seqcount_retry() call, no following
45 	 * Make sure we are not reinitializing a held lock:  in __seqcount_init()
47 	lockdep_init_map(&s->dep_map, name, key, 0);  in __seqcount_init()
48 	s->sequence = 0;  in __seqcount_init()
57  * seqcount_init() - runtime initializer for seqcount_t
72 	seqcount_acquire_read(&l->dep_map, 0, 0, _RET_IP_);  in seqcount_lockdep_reader_access()
73 	seqcount_release(&l->dep_map, _RET_IP_);  in seqcount_lockdep_reader_access()
84  * SEQCNT_ZERO() - static initializer for seqcount_t
92  * A sequence counter which associates the lock used for writer
94  * that the write side critical section is properly serialized.
97  * preemption protection is enforced in the write side function.
105  * typedef seqcount_LOCKNAME_t - sequence counter with LOCKNAME associated
109  * A plain sequence counter with external writer synchronization by
112  * that the write side critical section is properly serialized.
118  * seqcount_LOCKNAME_init() - runtime initializer for seqcount_LOCKNAME_t
126 		seqcount_init(&____s->seqcount);			\
127 		__SEQ_LOCK(____s->lock = (_lock));			\
136  * SEQCOUNT_LOCKNAME()	- Instantiate seqcount_LOCKNAME_t and helpers
137  * seqprop_LOCKNAME_*()	- Property accessors for seqcount_LOCKNAME_t
148 	return &s->seqcount;						\
154 	return &s->seqcount;						\
160 	unsigned seq = smp_load_acquire(&s->seqcount.sequence);		\
166 		__SEQ_LOCK(lockbase##_lock(s->lock));			\
167 		__SEQ_LOCK(lockbase##_unlock(s->lock));			\
170 		 * Re-read the sequence counter since the (possibly	\
173 		seq = smp_load_acquire(&s->seqcount.sequence);		\
192 	__SEQ_LOCK(lockdep_assert_held(s->lock));			\
211 	return smp_load_acquire(&s->sequence);  in __seqprop_sequence()
233  * SEQCNT_LOCKNAME_ZERO - static initializer for seqcount_LOCKNAME_t  in SEQCOUNT_LOCKNAME()
266  * __read_seqcount_begin() - begin a seqcount_t read section
283  * raw_read_seqcount_begin() - begin a seqcount_t read section w/o lockdep
291  * read_seqcount_begin() - begin a seqcount_t read critical section
303  * raw_read_seqcount() - read the raw seqcount_t counter value
306  * raw_read_seqcount opens a read critical section of the given
322  * raw_seqcount_begin() - begin a seqcount_t read critical section w/o
326  * raw_seqcount_begin opens a read critical section of the given
328  * for the count to stabilize. If a writer is active when it begins, it
333  * small and has a high probability of success through other external
334  * means. It will save a single branching instruction.
348  * __read_seqcount_retry() - end a seqcount_t read section w/o barrier
360  * Return: true if a read section retry is required, else false
368 	return unlikely(READ_ONCE(s->sequence) != start);
372  * read_seqcount_retry() - end a seqcount_t read critical section
380  * Return: true if a read section retry is required, else false
392  * raw_write_seqcount_begin() - start a seqcount_t write section w/o lockdep
408 	s->sequence++;  in do_raw_write_seqcount_begin()
413  * raw_write_seqcount_end() - end a seqcount_t write section w/o lockdep
429 	s->sequence++;  in do_raw_write_seqcount_end()
434  * write_seqcount_begin_nested() - start a seqcount_t write section with
439  * See Documentation/locking/lockdep-design.rst
454 	seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);  in do_write_seqcount_begin_nested()
459  * write_seqcount_begin() - start a seqcount_t write side critical section
462  * Context: sequence counter write side sections must be serialized and
463  * non-preemptible. Preemption will be automatically disabled if and
484  * write_seqcount_end() - end a seqcount_t write side critical section
487  * Context: Preemption will be automatically re-enabled if and only if
500 	seqcount_release(&s->dep_map, _RET_IP_);  in do_write_seqcount_end()
505  * raw_write_seqcount_barrier() - do a seqcount_t write barrier
510  * the two back-to-back wmb()s.
513  * via WRITE_ONCE): a) to ensure the writes become visible to other threads
516  * neither writes before nor after the barrier are enclosed in a seq-writer
551 	s->sequence++;  in do_raw_write_seqcount_barrier()
553 	s->sequence++;  in do_raw_write_seqcount_barrier()
558  * write_seqcount_invalidate() - invalidate in-progress seqcount_t read
559  *                               side operations
562  * After write_seqcount_invalidate, no seqcount_t read side operations
572 	s->sequence+=2;  in do_write_seqcount_invalidate()
579  * A sequence counter variant where the counter even/odd value is used to
581  * typically NMIs, to safely interrupt the write side critical section.
591  * SEQCNT_LATCH_ZERO() - static initializer for seqcount_latch_t
599  * seqcount_latch_init() - runtime initializer for seqcount_latch_t
602 #define seqcount_latch_init(s) seqcount_init(&(s)->seqcount)
605  * raw_read_seqcount_latch() - pick even/odd latch data copy
608  * See raw_write_seqcount_latch() for details and a full reader/writer
619 	 * Due to the dependent load, a full smp_rmb() is not needed.  in raw_read_seqcount_latch()
621 	return READ_ONCE(s->seqcount.sequence);  in raw_read_seqcount_latch()
625  * read_seqcount_latch() - pick even/odd latch data copy
628  * See write_seqcount_latch() for details and a full reader/writer usage
642  * raw_read_seqcount_latch_retry() - end a seqcount_latch_t read section
646  * Return: true if a read section retry is required, else false
652 	return unlikely(READ_ONCE(s->seqcount.sequence) != start);  in raw_read_seqcount_latch_retry()
656  * read_seqcount_latch_retry() - end a seqcount_latch_t read section
660  * Return: true if a read section retry is required, else false
670  * raw_write_seqcount_latch() - redirect latch readers to even/odd copy
676 	s->seqcount.sequence++;  in raw_write_seqcount_latch()
681  * write_seqcount_latch_begin() - redirect latch readers to odd copy
684  * The latch technique is a multiversion concurrency control method that allows
685  * queries during non-atomic modifications. If you can guarantee queries never
686  * interrupt the modification -- e.g. the concurrency is strictly between CPUs
687  * -- you most likely do not need this.
691  * latch allows the same for non-atomic updates. The trade-off is doubling the
696  * there is always one copy in a stable state, ready to give us an answer.
698  * The basic form is a data structure like::
705  * Where a modification, which is assumed to be externally serialized, does the
710  *		write_seqcount_latch_begin(&latch->seq);
711  *		modify(latch->data[0], ...);
712  *		write_seqcount_latch(&latch->seq);
713  *		modify(latch->data[1], ...);
714  *		write_seqcount_latch_end(&latch->seq);
717  * The query will have a form like::
725  *			seq = read_seqcount_latch(&latch->seq);
728  *			entry = data_query(latch->data[idx], ...);
731  *		} while (read_seqcount_latch_retry(&latch->seq, seq));
742  *	The non-requirement for atomic modifications does _NOT_ include
743  *	the publishing of new entries in the case where data is a dynamic
752  *	When data is a dynamic data structure; one should use regular RCU
762  * write_seqcount_latch() - redirect latch readers to even copy
771  * write_seqcount_latch_end() - end a seqcount_latch_t write section
774  * Marks the end of a seqcount_latch_t writer section, after all copies of the
775  * latch-protected data have been updated.
789  * seqlock_init() - dynamic initializer for seqlock_t
794 		spin_lock_init(&(sl)->lock);				\
795 		seqcount_spinlock_init(&(sl)->seqcount, &(sl)->lock);	\
799  * DEFINE_SEQLOCK(sl) - Define a statically allocated seqlock_t
806  * read_seqbegin() - start a seqlock_t read side critical section
813 	return read_seqcount_begin(&sl->seqcount);  in read_seqbegin()
817  * read_seqretry() - end a seqlock_t read side section
821  * read_seqretry closes the read side critical section of given seqlock_t.
825  * Return: true if a read section retry is required, else false
829 	return read_seqcount_retry(&sl->seqcount, start);  in read_seqretry()
833  * For all seqlock_t write side functions, use the internal
839  * write_seqlock() - start a seqlock_t write side critical section
842  * write_seqlock opens a write side critical section for the given
844  * that sequential lock. All seqlock_t write side sections are thus
845  * automatically serialized and non-preemptible.
847  * Context: if the seqlock_t read section, or other write side critical
853 	spin_lock(&sl->lock);  in write_seqlock()
854 	do_write_seqcount_begin(&sl->seqcount.seqcount);  in write_seqlock()
858  * write_sequnlock() - end a seqlock_t write side critical section
861  * write_sequnlock closes the (serialized and non-preemptible) write side
866 	do_write_seqcount_end(&sl->seqcount.seqcount);  in write_sequnlock()
867 	spin_unlock(&sl->lock);  in write_sequnlock()
871  * write_seqlock_bh() - start a softirqs-disabled seqlock_t write section
874  * _bh variant of write_seqlock(). Use only if the read side section, or
875  * other write side sections, can be invoked from softirq contexts.
879 	spin_lock_bh(&sl->lock);  in write_seqlock_bh()
880 	do_write_seqcount_begin(&sl->seqcount.seqcount);  in write_seqlock_bh()
884  * write_sequnlock_bh() - end a softirqs-disabled seqlock_t write section
887  * write_sequnlock_bh closes the serialized, non-preemptible, and
888  * softirqs-disabled, seqlock_t write side critical section opened with
893 	do_write_seqcount_end(&sl->seqcount.seqcount);  in write_sequnlock_bh()
894 	spin_unlock_bh(&sl->lock);  in write_sequnlock_bh()
898  * write_seqlock_irq() - start a non-interruptible seqlock_t write section
901  * _irq variant of write_seqlock(). Use only if the read side section, or
906 	spin_lock_irq(&sl->lock);  in write_seqlock_irq()
907 	do_write_seqcount_begin(&sl->seqcount.seqcount);  in write_seqlock_irq()
911  * write_sequnlock_irq() - end a non-interruptible seqlock_t write section
914  * write_sequnlock_irq closes the serialized and non-interruptible
915  * seqlock_t write side section opened with write_seqlock_irq().
919 	do_write_seqcount_end(&sl->seqcount.seqcount);  in write_sequnlock_irq()
920 	spin_unlock_irq(&sl->lock);  in write_sequnlock_irq()
927 	spin_lock_irqsave(&sl->lock, flags);  in __write_seqlock_irqsave()
928 	do_write_seqcount_begin(&sl->seqcount.seqcount);  in __write_seqlock_irqsave()
933  * write_seqlock_irqsave() - start a non-interruptible seqlock_t write
936  * @flags: Stack-allocated storage for saving caller's local interrupt
939  * _irqsave variant of write_seqlock(). Use it only if the read side
946  * write_sequnlock_irqrestore() - end non-interruptible seqlock_t write
951  * write_sequnlock_irqrestore closes the serialized and non-interruptible
957 	do_write_seqcount_end(&sl->seqcount.seqcount);  in write_sequnlock_irqrestore()
958 	spin_unlock_irqrestore(&sl->lock, flags);  in write_sequnlock_irqrestore()
962  * read_seqlock_excl() - begin a seqlock_t locking reader section
965  * read_seqlock_excl opens a seqlock_t locking reader critical section.  A
969  * Locking readers act like a normal spin_lock()/spin_unlock().
979 	spin_lock(&sl->lock);  in read_seqlock_excl()
983  * read_sequnlock_excl() - end a seqlock_t locking reader critical section
988 	spin_unlock(&sl->lock);  in read_sequnlock_excl()
992  * read_seqlock_excl_bh() - start a seqlock_t locking reader section with
997  * seqlock_t write side section, *or other read sections*, can be invoked
1002 	spin_lock_bh(&sl->lock);  in read_seqlock_excl_bh()
1006  * read_sequnlock_excl_bh() - stop a seqlock_t softirq-disabled locking
1012 	spin_unlock_bh(&sl->lock);  in read_sequnlock_excl_bh()
1016  * read_seqlock_excl_irq() - start a non-interruptible seqlock_t locking
1021  * write side section, *or other read sections*, can be invoked from a
1026 	spin_lock_irq(&sl->lock);  in read_seqlock_excl_irq()
1030  * read_sequnlock_excl_irq() - end an interrupts-disabled seqlock_t
1036 	spin_unlock_irq(&sl->lock);  in read_sequnlock_excl_irq()
1043 	spin_lock_irqsave(&sl->lock, flags);  in __read_seqlock_excl_irqsave()
1048  * read_seqlock_excl_irqsave() - start a non-interruptible seqlock_t
1051  * @flags: Stack-allocated storage for saving caller's local interrupt
1055  * write side section, *or other read sections*, can be invoked from a
1062  * read_sequnlock_excl_irqrestore() - end non-interruptible seqlock_t
1070 	spin_unlock_irqrestore(&sl->lock, flags);  in read_sequnlock_excl_irqrestore()
1074  * read_seqbegin_or_lock() - begin a seqlock_t lockless or locking reader
1077  * reader will become a *lockless* seqlock_t reader as in read_seqbegin().
1078  * If the passed value is odd, the reader will become a *locking* reader
1080  * caller *must* initialize and pass an even value to @seq; this way, a
1083  * read_seqbegin_or_lock is an API designed to optimistically try a normal
1086  * itself into a full seqlock_t locking reader.
1089  * (too much retry loops) in the case of a sharp spike in write side
1099  * parameter, which is overloaded as a return parameter. This returned
1113  * need_seqretry() - validate seqlock_t "locking or lockless" read section
1117  * Return: true if a read section retry is required, false otherwise
1125  * done_seqretry() - end seqlock_t "locking or lockless" reader section
1129  * done_seqretry finishes the seqlock_t read side critical section started
1139  * read_seqbegin_or_lock_irqsave() - begin a seqlock_t lockless reader, or
1140  *                                   a non-interruptible locking reader
1152  *   1. The saved local interrupts state in case of a locking reader, to
1156  *      overloaded as a return parameter. Check read_seqbegin_or_lock().
1172  * done_seqretry_irqrestore() - end a seqlock_t lockless reader, or a
1173  *				non-interruptible locking reader section
1176  * @flags: Caller's saved local interrupt state in case of a locking