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Diffstat (limited to 'include/linux/seqlock.h')
| -rw-r--r-- | include/linux/seqlock.h | 1222 |
1 files changed, 1222 insertions, 0 deletions
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h new file mode 100644 index 000000000..d778af83c --- /dev/null +++ b/include/linux/seqlock.h @@ -0,0 +1,1222 @@ +/* SPDX-License-Identifier: GPL-2.0 */ +#ifndef __LINUX_SEQLOCK_H +#define __LINUX_SEQLOCK_H + +/* + * seqcount_t / seqlock_t - a reader-writer consistency mechanism with + * lockless readers (read-only retry loops), and no writer starvation. + * + * See Documentation/locking/seqlock.rst + * + * Copyrights: + * - Based on x86_64 vsyscall gettimeofday: Keith Owens, Andrea Arcangeli + * - Sequence counters with associated locks, (C) 2020 Linutronix GmbH + */ + +#include <linux/compiler.h> +#include <linux/kcsan-checks.h> +#include <linux/lockdep.h> +#include <linux/mutex.h> +#include <linux/preempt.h> +#include <linux/spinlock.h> + +#include <asm/processor.h> + +/* + * The seqlock seqcount_t interface does not prescribe a precise sequence of + * read begin/retry/end. For readers, typically there is a call to + * read_seqcount_begin() and read_seqcount_retry(), however, there are more + * esoteric cases which do not follow this pattern. + * + * As a consequence, we take the following best-effort approach for raw usage + * via seqcount_t under KCSAN: upon beginning a seq-reader critical section, + * pessimistically mark the next KCSAN_SEQLOCK_REGION_MAX memory accesses as + * atomics; if there is a matching read_seqcount_retry() call, no following + * memory operations are considered atomic. Usage of the seqlock_t interface + * is not affected. + */ +#define KCSAN_SEQLOCK_REGION_MAX 1000 + +/* + * Sequence counters (seqcount_t) + * + * This is the raw counting mechanism, without any writer protection. + * + * Write side critical sections must be serialized and non-preemptible. + * + * If readers can be invoked from hardirq or softirq contexts, + * interrupts or bottom halves must also be respectively disabled before + * entering the write section. + * + * This mechanism can't be used if the protected data contains pointers, + * as the writer can invalidate a pointer that a reader is following. + * + * If the write serialization mechanism is one of the common kernel + * locking primitives, use a sequence counter with associated lock + * (seqcount_LOCKNAME_t) instead. + * + * If it's desired to automatically handle the sequence counter writer + * serialization and non-preemptibility requirements, use a sequential + * lock (seqlock_t) instead. + * + * See Documentation/locking/seqlock.rst + */ +typedef struct seqcount { + unsigned sequence; +#ifdef CONFIG_DEBUG_LOCK_ALLOC + struct lockdep_map dep_map; +#endif +} seqcount_t; + +static inline void __seqcount_init(seqcount_t *s, const char *name, + struct lock_class_key *key) +{ + /* + * Make sure we are not reinitializing a held lock: + */ + lockdep_init_map(&s->dep_map, name, key, 0); + s->sequence = 0; +} + +#ifdef CONFIG_DEBUG_LOCK_ALLOC + +# define SEQCOUNT_DEP_MAP_INIT(lockname) \ + .dep_map = { .name = #lockname } + +/** + * seqcount_init() - runtime initializer for seqcount_t + * @s: Pointer to the seqcount_t instance + */ +# define seqcount_init(s) \ + do { \ + static struct lock_class_key __key; \ + __seqcount_init((s), #s, &__key); \ + } while (0) + +static inline void seqcount_lockdep_reader_access(const seqcount_t *s) +{ + seqcount_t *l = (seqcount_t *)s; + unsigned long flags; + + local_irq_save(flags); + seqcount_acquire_read(&l->dep_map, 0, 0, _RET_IP_); + seqcount_release(&l->dep_map, _RET_IP_); + local_irq_restore(flags); +} + +#else +# define SEQCOUNT_DEP_MAP_INIT(lockname) +# define seqcount_init(s) __seqcount_init(s, NULL, NULL) +# define seqcount_lockdep_reader_access(x) +#endif + +/** + * SEQCNT_ZERO() - static initializer for seqcount_t + * @name: Name of the seqcount_t instance + */ +#define SEQCNT_ZERO(name) { .sequence = 0, SEQCOUNT_DEP_MAP_INIT(name) } + +/* + * Sequence counters with associated locks (seqcount_LOCKNAME_t) + * + * A sequence counter which associates the lock used for writer + * serialization at initialization time. This enables lockdep to validate + * that the write side critical section is properly serialized. + * + * For associated locks which do not implicitly disable preemption, + * preemption protection is enforced in the write side function. + * + * Lockdep is never used in any for the raw write variants. + * + * See Documentation/locking/seqlock.rst + */ + +/* + * For PREEMPT_RT, seqcount_LOCKNAME_t write side critical sections cannot + * disable preemption. It can lead to higher latencies, and the write side + * sections will not be able to acquire locks which become sleeping locks + * (e.g. spinlock_t). + * + * To remain preemptible while avoiding a possible livelock caused by the + * reader preempting the writer, use a different technique: let the reader + * detect if a seqcount_LOCKNAME_t writer is in progress. If that is the + * case, acquire then release the associated LOCKNAME writer serialization + * lock. This will allow any possibly-preempted writer to make progress + * until the end of its writer serialization lock critical section. + * + * This lock-unlock technique must be implemented for all of PREEMPT_RT + * sleeping locks. See Documentation/locking/locktypes.rst + */ +#if defined(CONFIG_LOCKDEP) || defined(CONFIG_PREEMPT_RT) +#define __SEQ_LOCK(expr) expr +#else +#define __SEQ_LOCK(expr) +#endif + +/* + * typedef seqcount_LOCKNAME_t - sequence counter with LOCKNAME associated + * @seqcount: The real sequence counter + * @lock: Pointer to the associated lock + * + * A plain sequence counter with external writer synchronization by + * LOCKNAME @lock. The lock is associated to the sequence counter in the + * static initializer or init function. This enables lockdep to validate + * that the write side critical section is properly serialized. + * + * LOCKNAME: raw_spinlock, spinlock, rwlock or mutex + */ + +/* + * seqcount_LOCKNAME_init() - runtime initializer for seqcount_LOCKNAME_t + * @s: Pointer to the seqcount_LOCKNAME_t instance + * @lock: Pointer to the associated lock + */ + +#define seqcount_LOCKNAME_init(s, _lock, lockname) \ + do { \ + seqcount_##lockname##_t *____s = (s); \ + seqcount_init(&____s->seqcount); \ + __SEQ_LOCK(____s->lock = (_lock)); \ + } while (0) + +#define seqcount_raw_spinlock_init(s, lock) seqcount_LOCKNAME_init(s, lock, raw_spinlock) +#define seqcount_spinlock_init(s, lock) seqcount_LOCKNAME_init(s, lock, spinlock) +#define seqcount_rwlock_init(s, lock) seqcount_LOCKNAME_init(s, lock, rwlock) +#define seqcount_mutex_init(s, lock) seqcount_LOCKNAME_init(s, lock, mutex) + +/* + * SEQCOUNT_LOCKNAME() - Instantiate seqcount_LOCKNAME_t and helpers + * seqprop_LOCKNAME_*() - Property accessors for seqcount_LOCKNAME_t + * + * @lockname: "LOCKNAME" part of seqcount_LOCKNAME_t + * @locktype: LOCKNAME canonical C data type + * @preemptible: preemptibility of above locktype + * @lockmember: argument for lockdep_assert_held() + * @lockbase: associated lock release function (prefix only) + * @lock_acquire: associated lock acquisition function (full call) + */ +#define SEQCOUNT_LOCKNAME(lockname, locktype, preemptible, lockmember, lockbase, lock_acquire) \ +typedef struct seqcount_##lockname { \ + seqcount_t seqcount; \ + __SEQ_LOCK(locktype *lock); \ +} seqcount_##lockname##_t; \ + \ +static __always_inline seqcount_t * \ +__seqprop_##lockname##_ptr(seqcount_##lockname##_t *s) \ +{ \ + return &s->seqcount; \ +} \ + \ +static __always_inline unsigned \ +__seqprop_##lockname##_sequence(const seqcount_##lockname##_t *s) \ +{ \ + unsigned seq = READ_ONCE(s->seqcount.sequence); \ + \ + if (!IS_ENABLED(CONFIG_PREEMPT_RT)) \ + return seq; \ + \ + if (preemptible && unlikely(seq & 1)) { \ + __SEQ_LOCK(lock_acquire); \ + __SEQ_LOCK(lockbase##_unlock(s->lock)); \ + \ + /* \ + * Re-read the sequence counter since the (possibly \ + * preempted) writer made progress. \ + */ \ + seq = READ_ONCE(s->seqcount.sequence); \ + } \ + \ + return seq; \ +} \ + \ +static __always_inline bool \ +__seqprop_##lockname##_preemptible(const seqcount_##lockname##_t *s) \ +{ \ + if (!IS_ENABLED(CONFIG_PREEMPT_RT)) \ + return preemptible; \ + \ + /* PREEMPT_RT relies on the above LOCK+UNLOCK */ \ + return false; \ +} \ + \ +static __always_inline void \ +__seqprop_##lockname##_assert(const seqcount_##lockname##_t *s) \ +{ \ + __SEQ_LOCK(lockdep_assert_held(lockmember)); \ +} + +/* + * __seqprop() for seqcount_t + */ + +static inline seqcount_t *__seqprop_ptr(seqcount_t *s) +{ + return s; +} + +static inline unsigned __seqprop_sequence(const seqcount_t *s) +{ + return READ_ONCE(s->sequence); +} + +static inline bool __seqprop_preemptible(const seqcount_t *s) +{ + return false; +} + +static inline void __seqprop_assert(const seqcount_t *s) +{ + lockdep_assert_preemption_disabled(); +} + +#define __SEQ_RT IS_ENABLED(CONFIG_PREEMPT_RT) + +SEQCOUNT_LOCKNAME(raw_spinlock, raw_spinlock_t, false, s->lock, raw_spin, raw_spin_lock(s->lock)) +SEQCOUNT_LOCKNAME(spinlock, spinlock_t, __SEQ_RT, s->lock, spin, spin_lock(s->lock)) +SEQCOUNT_LOCKNAME(rwlock, rwlock_t, __SEQ_RT, s->lock, read, read_lock(s->lock)) +SEQCOUNT_LOCKNAME(mutex, struct mutex, true, s->lock, mutex, mutex_lock(s->lock)) + +/* + * SEQCNT_LOCKNAME_ZERO - static initializer for seqcount_LOCKNAME_t + * @name: Name of the seqcount_LOCKNAME_t instance + * @lock: Pointer to the associated LOCKNAME + */ + +#define SEQCOUNT_LOCKNAME_ZERO(seq_name, assoc_lock) { \ + .seqcount = SEQCNT_ZERO(seq_name.seqcount), \ + __SEQ_LOCK(.lock = (assoc_lock)) \ +} + +#define SEQCNT_RAW_SPINLOCK_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(name, lock) +#define SEQCNT_SPINLOCK_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(name, lock) +#define SEQCNT_RWLOCK_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(name, lock) +#define SEQCNT_MUTEX_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(name, lock) +#define SEQCNT_WW_MUTEX_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(name, lock) + +#define __seqprop_case(s, lockname, prop) \ + seqcount_##lockname##_t: __seqprop_##lockname##_##prop((void *)(s)) + +#define __seqprop(s, prop) _Generic(*(s), \ + seqcount_t: __seqprop_##prop((void *)(s)), \ + __seqprop_case((s), raw_spinlock, prop), \ + __seqprop_case((s), spinlock, prop), \ + __seqprop_case((s), rwlock, prop), \ + __seqprop_case((s), mutex, prop)) + +#define seqprop_ptr(s) __seqprop(s, ptr) +#define seqprop_sequence(s) __seqprop(s, sequence) +#define seqprop_preemptible(s) __seqprop(s, preemptible) +#define seqprop_assert(s) __seqprop(s, assert) + +/** + * __read_seqcount_begin() - begin a seqcount_t read section w/o barrier + * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants + * + * __read_seqcount_begin is like read_seqcount_begin, but has no smp_rmb() + * barrier. Callers should ensure that smp_rmb() or equivalent ordering is + * provided before actually loading any of the variables that are to be + * protected in this critical section. + * + * Use carefully, only in critical code, and comment how the barrier is + * provided. + * + * Return: count to be passed to read_seqcount_retry() + */ +#define __read_seqcount_begin(s) \ +({ \ + unsigned __seq; \ + \ + while ((__seq = seqprop_sequence(s)) & 1) \ + cpu_relax(); \ + \ + kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX); \ + __seq; \ +}) + +/** + * raw_read_seqcount_begin() - begin a seqcount_t read section w/o lockdep + * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants + * + * Return: count to be passed to read_seqcount_retry() + */ +#define raw_read_seqcount_begin(s) \ +({ \ + unsigned _seq = __read_seqcount_begin(s); \ + \ + smp_rmb(); \ + _seq; \ +}) + +/** + * read_seqcount_begin() - begin a seqcount_t read critical section + * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants + * + * Return: count to be passed to read_seqcount_retry() + */ +#define read_seqcount_begin(s) \ +({ \ + seqcount_lockdep_reader_access(seqprop_ptr(s)); \ + raw_read_seqcount_begin(s); \ +}) + +/** + * raw_read_seqcount() - read the raw seqcount_t counter value + * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants + * + * raw_read_seqcount opens a read critical section of the given + * seqcount_t, without any lockdep checking, and without checking or + * masking the sequence counter LSB. Calling code is responsible for + * handling that. + * + * Return: count to be passed to read_seqcount_retry() + */ +#define raw_read_seqcount(s) \ +({ \ + unsigned __seq = seqprop_sequence(s); \ + \ + smp_rmb(); \ + kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX); \ + __seq; \ +}) + +/** + * raw_seqcount_begin() - begin a seqcount_t read critical section w/o + * lockdep and w/o counter stabilization + * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants + * + * raw_seqcount_begin opens a read critical section of the given + * seqcount_t. Unlike read_seqcount_begin(), this function will not wait + * for the count to stabilize. If a writer is active when it begins, it + * will fail the read_seqcount_retry() at the end of the read critical + * section instead of stabilizing at the beginning of it. + * + * Use this only in special kernel hot paths where the read section is + * small and has a high probability of success through other external + * means. It will save a single branching instruction. + * + * Return: count to be passed to read_seqcount_retry() + */ +#define raw_seqcount_begin(s) \ +({ \ + /* \ + * If the counter is odd, let read_seqcount_retry() fail \ + * by decrementing the counter. \ + */ \ + raw_read_seqcount(s) & ~1; \ +}) + +/** + * __read_seqcount_retry() - end a seqcount_t read section w/o barrier + * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants + * @start: count, from read_seqcount_begin() + * + * __read_seqcount_retry is like read_seqcount_retry, but has no smp_rmb() + * barrier. Callers should ensure that smp_rmb() or equivalent ordering is + * provided before actually loading any of the variables that are to be + * protected in this critical section. + * + * Use carefully, only in critical code, and comment how the barrier is + * provided. + * + * Return: true if a read section retry is required, else false + */ +#define __read_seqcount_retry(s, start) \ + do___read_seqcount_retry(seqprop_ptr(s), start) + +static inline int do___read_seqcount_retry(const seqcount_t *s, unsigned start) +{ + kcsan_atomic_next(0); + return unlikely(READ_ONCE(s->sequence) != start); +} + +/** + * read_seqcount_retry() - end a seqcount_t read critical section + * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants + * @start: count, from read_seqcount_begin() + * + * read_seqcount_retry closes the read critical section of given + * seqcount_t. If the critical section was invalid, it must be ignored + * (and typically retried). + * + * Return: true if a read section retry is required, else false + */ +#define read_seqcount_retry(s, start) \ + do_read_seqcount_retry(seqprop_ptr(s), start) + +static inline int do_read_seqcount_retry(const seqcount_t *s, unsigned start) +{ + smp_rmb(); + return do___read_seqcount_retry(s, start); +} + +/** + * raw_write_seqcount_begin() - start a seqcount_t write section w/o lockdep + * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants + * + * Context: check write_seqcount_begin() + */ +#define raw_write_seqcount_begin(s) \ +do { \ + if (seqprop_preemptible(s)) \ + preempt_disable(); \ + \ + do_raw_write_seqcount_begin(seqprop_ptr(s)); \ +} while (0) + +static inline void do_raw_write_seqcount_begin(seqcount_t *s) +{ + kcsan_nestable_atomic_begin(); + s->sequence++; + smp_wmb(); +} + +/** + * raw_write_seqcount_end() - end a seqcount_t write section w/o lockdep + * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants + * + * Context: check write_seqcount_end() + */ +#define raw_write_seqcount_end(s) \ +do { \ + do_raw_write_seqcount_end(seqprop_ptr(s)); \ + \ + if (seqprop_preemptible(s)) \ + preempt_enable(); \ +} while (0) + +static inline void do_raw_write_seqcount_end(seqcount_t *s) +{ + smp_wmb(); + s->sequence++; + kcsan_nestable_atomic_end(); +} + +/** + * write_seqcount_begin_nested() - start a seqcount_t write section with + * custom lockdep nesting level + * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants + * @subclass: lockdep nesting level + * + * See Documentation/locking/lockdep-design.rst + * Context: check write_seqcount_begin() + */ +#define write_seqcount_begin_nested(s, subclass) \ +do { \ + seqprop_assert(s); \ + \ + if (seqprop_preemptible(s)) \ + preempt_disable(); \ + \ + do_write_seqcount_begin_nested(seqprop_ptr(s), subclass); \ +} while (0) + +static inline void do_write_seqcount_begin_nested(seqcount_t *s, int subclass) +{ + seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_); + do_raw_write_seqcount_begin(s); +} + +/** + * write_seqcount_begin() - start a seqcount_t write side critical section + * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants + * + * Context: sequence counter write side sections must be serialized and + * non-preemptible. Preemption will be automatically disabled if and + * only if the seqcount write serialization lock is associated, and + * preemptible. If readers can be invoked from hardirq or softirq + * context, interrupts or bottom halves must be respectively disabled. + */ +#define write_seqcount_begin(s) \ +do { \ + seqprop_assert(s); \ + \ + if (seqprop_preemptible(s)) \ + preempt_disable(); \ + \ + do_write_seqcount_begin(seqprop_ptr(s)); \ +} while (0) + +static inline void do_write_seqcount_begin(seqcount_t *s) +{ + do_write_seqcount_begin_nested(s, 0); +} + +/** + * write_seqcount_end() - end a seqcount_t write side critical section + * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants + * + * Context: Preemption will be automatically re-enabled if and only if + * the seqcount write serialization lock is associated, and preemptible. + */ +#define write_seqcount_end(s) \ +do { \ + do_write_seqcount_end(seqprop_ptr(s)); \ + \ + if (seqprop_preemptible(s)) \ + preempt_enable(); \ +} while (0) + +static inline void do_write_seqcount_end(seqcount_t *s) +{ + seqcount_release(&s->dep_map, _RET_IP_); + do_raw_write_seqcount_end(s); +} + +/** + * raw_write_seqcount_barrier() - do a seqcount_t write barrier + * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants + * + * This can be used to provide an ordering guarantee instead of the usual + * consistency guarantee. It is one wmb cheaper, because it can collapse + * the two back-to-back wmb()s. + * + * Note that writes surrounding the barrier should be declared atomic (e.g. + * via WRITE_ONCE): a) to ensure the writes become visible to other threads + * atomically, avoiding compiler optimizations; b) to document which writes are + * meant to propagate to the reader critical section. This is necessary because + * neither writes before and after the barrier are enclosed in a seq-writer + * critical section that would ensure readers are aware of ongoing writes:: + * + * seqcount_t seq; + * bool X = true, Y = false; + * + * void read(void) + * { + * bool x, y; + * + * do { + * int s = read_seqcount_begin(&seq); + * + * x = X; y = Y; + * + * } while (read_seqcount_retry(&seq, s)); + * + * BUG_ON(!x && !y); + * } + * + * void write(void) + * { + * WRITE_ONCE(Y, true); + * + * raw_write_seqcount_barrier(seq); + * + * WRITE_ONCE(X, false); + * } + */ +#define raw_write_seqcount_barrier(s) \ + do_raw_write_seqcount_barrier(seqprop_ptr(s)) + +static inline void do_raw_write_seqcount_barrier(seqcount_t *s) +{ + kcsan_nestable_atomic_begin(); + s->sequence++; + smp_wmb(); + s->sequence++; + kcsan_nestable_atomic_end(); +} + +/** + * write_seqcount_invalidate() - invalidate in-progress seqcount_t read + * side operations + * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants + * + * After write_seqcount_invalidate, no seqcount_t read side operations + * will complete successfully and see data older than this. + */ +#define write_seqcount_invalidate(s) \ + do_write_seqcount_invalidate(seqprop_ptr(s)) + +static inline void do_write_seqcount_invalidate(seqcount_t *s) +{ + smp_wmb(); + kcsan_nestable_atomic_begin(); + s->sequence+=2; + kcsan_nestable_atomic_end(); +} + +/* + * Latch sequence counters (seqcount_latch_t) + * + * A sequence counter variant where the counter even/odd value is used to + * switch between two copies of protected data. This allows the read path, + * typically NMIs, to safely interrupt the write side critical section. + * + * As the write sections are fully preemptible, no special handling for + * PREEMPT_RT is needed. + */ +typedef struct { + seqcount_t seqcount; +} seqcount_latch_t; + +/** + * SEQCNT_LATCH_ZERO() - static initializer for seqcount_latch_t + * @seq_name: Name of the seqcount_latch_t instance + */ +#define SEQCNT_LATCH_ZERO(seq_name) { \ + .seqcount = SEQCNT_ZERO(seq_name.seqcount), \ +} + +/** + * seqcount_latch_init() - runtime initializer for seqcount_latch_t + * @s: Pointer to the seqcount_latch_t instance + */ +#define seqcount_latch_init(s) seqcount_init(&(s)->seqcount) + +/** + * raw_read_seqcount_latch() - pick even/odd latch data copy + * @s: Pointer to seqcount_latch_t + * + * See raw_write_seqcount_latch() for details and a full reader/writer + * usage example. + * + * Return: sequence counter raw value. Use the lowest bit as an index for + * picking which data copy to read. The full counter must then be checked + * with read_seqcount_latch_retry(). + */ +static inline unsigned raw_read_seqcount_latch(const seqcount_latch_t *s) +{ + /* + * Pairs with the first smp_wmb() in raw_write_seqcount_latch(). + * Due to the dependent load, a full smp_rmb() is not needed. + */ + return READ_ONCE(s->seqcount.sequence); +} + +/** + * read_seqcount_latch_retry() - end a seqcount_latch_t read section + * @s: Pointer to seqcount_latch_t + * @start: count, from raw_read_seqcount_latch() + * + * Return: true if a read section retry is required, else false + */ +static inline int +read_seqcount_latch_retry(const seqcount_latch_t *s, unsigned start) +{ + return read_seqcount_retry(&s->seqcount, start); +} + +/** + * raw_write_seqcount_latch() - redirect latch readers to even/odd copy + * @s: Pointer to seqcount_latch_t + * + * The latch technique is a multiversion concurrency control method that allows + * queries during non-atomic modifications. If you can guarantee queries never + * interrupt the modification -- e.g. the concurrency is strictly between CPUs + * -- you most likely do not need this. + * + * Where the traditional RCU/lockless data structures rely on atomic + * modifications to ensure queries observe either the old or the new state the + * latch allows the same for non-atomic updates. The trade-off is doubling the + * cost of storage; we have to maintain two copies of the entire data + * structure. + * + * Very simply put: we first modify one copy and then the other. This ensures + * there is always one copy in a stable state, ready to give us an answer. + * + * The basic form is a data structure like:: + * + * struct latch_struct { + * seqcount_latch_t seq; + * struct data_struct data[2]; + * }; + * + * Where a modification, which is assumed to be externally serialized, does the + * following:: + * + * void latch_modify(struct latch_struct *latch, ...) + * { + * smp_wmb(); // Ensure that the last data[1] update is visible + * latch->seq.sequence++; + * smp_wmb(); // Ensure that the seqcount update is visible + * + * modify(latch->data[0], ...); + * + * smp_wmb(); // Ensure that the data[0] update is visible + * latch->seq.sequence++; + * smp_wmb(); // Ensure that the seqcount update is visible + * + * modify(latch->data[1], ...); + * } + * + * The query will have a form like:: + * + * struct entry *latch_query(struct latch_struct *latch, ...) + * { + * struct entry *entry; + * unsigned seq, idx; + * + * do { + * seq = raw_read_seqcount_latch(&latch->seq); + * + * idx = seq & 0x01; + * entry = data_query(latch->data[idx], ...); + * + * // This includes needed smp_rmb() + * } while (read_seqcount_latch_retry(&latch->seq, seq)); + * + * return entry; + * } + * + * So during the modification, queries are first redirected to data[1]. Then we + * modify data[0]. When that is complete, we redirect queries back to data[0] + * and we can modify data[1]. + * + * NOTE: + * + * The non-requirement for atomic modifications does _NOT_ include + * the publishing of new entries in the case where data is a dynamic + * data structure. + * + * An iteration might start in data[0] and get suspended long enough + * to miss an entire modification sequence, once it resumes it might + * observe the new entry. + * + * NOTE2: + * + * When data is a dynamic data structure; one should use regular RCU + * patterns to manage the lifetimes of the objects within. + */ +static inline void raw_write_seqcount_latch(seqcount_latch_t *s) +{ + smp_wmb(); /* prior stores before incrementing "sequence" */ + s->seqcount.sequence++; + smp_wmb(); /* increment "sequence" before following stores */ +} + +/* + * Sequential locks (seqlock_t) + * + * Sequence counters with an embedded spinlock for writer serialization + * and non-preemptibility. + * + * For more info, see: + * - Comments on top of seqcount_t + * - Documentation/locking/seqlock.rst + */ +typedef struct { + /* + * Make sure that readers don't starve writers on PREEMPT_RT: use + * seqcount_spinlock_t instead of seqcount_t. Check __SEQ_LOCK(). + */ + seqcount_spinlock_t seqcount; + spinlock_t lock; +} seqlock_t; + +#define __SEQLOCK_UNLOCKED(lockname) \ + { \ + .seqcount = SEQCNT_SPINLOCK_ZERO(lockname, &(lockname).lock), \ + .lock = __SPIN_LOCK_UNLOCKED(lockname) \ + } + +/** + * seqlock_init() - dynamic initializer for seqlock_t + * @sl: Pointer to the seqlock_t instance + */ +#define seqlock_init(sl) \ + do { \ + spin_lock_init(&(sl)->lock); \ + seqcount_spinlock_init(&(sl)->seqcount, &(sl)->lock); \ + } while (0) + +/** + * DEFINE_SEQLOCK(sl) - Define a statically allocated seqlock_t + * @sl: Name of the seqlock_t instance + */ +#define DEFINE_SEQLOCK(sl) \ + seqlock_t sl = __SEQLOCK_UNLOCKED(sl) + +/** + * read_seqbegin() - start a seqlock_t read side critical section + * @sl: Pointer to seqlock_t + * + * Return: count, to be passed to read_seqretry() + */ +static inline unsigned read_seqbegin(const seqlock_t *sl) +{ + unsigned ret = read_seqcount_begin(&sl->seqcount); + + kcsan_atomic_next(0); /* non-raw usage, assume closing read_seqretry() */ + kcsan_flat_atomic_begin(); + return ret; +} + +/** + * read_seqretry() - end a seqlock_t read side section + * @sl: Pointer to seqlock_t + * @start: count, from read_seqbegin() + * + * read_seqretry closes the read side critical section of given seqlock_t. + * If the critical section was invalid, it must be ignored (and typically + * retried). + * + * Return: true if a read section retry is required, else false + */ +static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start) +{ + /* + * Assume not nested: read_seqretry() may be called multiple times when + * completing read critical section. + */ + kcsan_flat_atomic_end(); + + return read_seqcount_retry(&sl->seqcount, start); +} + +/* + * For all seqlock_t write side functions, use the the internal + * do_write_seqcount_begin() instead of generic write_seqcount_begin(). + * This way, no redundant lockdep_assert_held() checks are added. + */ + +/** + * write_seqlock() - start a seqlock_t write side critical section + * @sl: Pointer to seqlock_t + * + * write_seqlock opens a write side critical section for the given + * seqlock_t. It also implicitly acquires the spinlock_t embedded inside + * that sequential lock. All seqlock_t write side sections are thus + * automatically serialized and non-preemptible. + * + * Context: if the seqlock_t read section, or other write side critical + * sections, can be invoked from hardirq or softirq contexts, use the + * _irqsave or _bh variants of this function instead. + */ +static inline void write_seqlock(seqlock_t *sl) +{ + spin_lock(&sl->lock); + do_write_seqcount_begin(&sl->seqcount.seqcount); +} + +/** + * write_sequnlock() - end a seqlock_t write side critical section + * @sl: Pointer to seqlock_t + * + * write_sequnlock closes the (serialized and non-preemptible) write side + * critical section of given seqlock_t. + */ +static inline void write_sequnlock(seqlock_t *sl) +{ + do_write_seqcount_end(&sl->seqcount.seqcount); + spin_unlock(&sl->lock); +} + +/** + * write_seqlock_bh() - start a softirqs-disabled seqlock_t write section + * @sl: Pointer to seqlock_t + * + * _bh variant of write_seqlock(). Use only if the read side section, or + * other write side sections, can be invoked from softirq contexts. + */ +static inline void write_seqlock_bh(seqlock_t *sl) +{ + spin_lock_bh(&sl->lock); + do_write_seqcount_begin(&sl->seqcount.seqcount); +} + +/** + * write_sequnlock_bh() - end a softirqs-disabled seqlock_t write section + * @sl: Pointer to seqlock_t + * + * write_sequnlock_bh closes the serialized, non-preemptible, and + * softirqs-disabled, seqlock_t write side critical section opened with + * write_seqlock_bh(). + */ +static inline void write_sequnlock_bh(seqlock_t *sl) +{ + do_write_seqcount_end(&sl->seqcount.seqcount); + spin_unlock_bh(&sl->lock); +} + +/** + * write_seqlock_irq() - start a non-interruptible seqlock_t write section + * @sl: Pointer to seqlock_t + * + * _irq variant of write_seqlock(). Use only if the read side section, or + * other write sections, can be invoked from hardirq contexts. + */ +static inline void write_seqlock_irq(seqlock_t *sl) +{ + spin_lock_irq(&sl->lock); + do_write_seqcount_begin(&sl->seqcount.seqcount); +} + +/** + * write_sequnlock_irq() - end a non-interruptible seqlock_t write section + * @sl: Pointer to seqlock_t + * + * write_sequnlock_irq closes the serialized and non-interruptible + * seqlock_t write side section opened with write_seqlock_irq(). + */ +static inline void write_sequnlock_irq(seqlock_t *sl) +{ + do_write_seqcount_end(&sl->seqcount.seqcount); + spin_unlock_irq(&sl->lock); +} + +static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl) +{ + unsigned long flags; + + spin_lock_irqsave(&sl->lock, flags); + do_write_seqcount_begin(&sl->seqcount.seqcount); + return flags; +} + +/** + * write_seqlock_irqsave() - start a non-interruptible seqlock_t write + * section + * @lock: Pointer to seqlock_t + * @flags: Stack-allocated storage for saving caller's local interrupt + * state, to be passed to write_sequnlock_irqrestore(). + * + * _irqsave variant of write_seqlock(). Use it only if the read side + * section, or other write sections, can be invoked from hardirq context. + */ +#define write_seqlock_irqsave(lock, flags) \ + do { flags = __write_seqlock_irqsave(lock); } while (0) + +/** + * write_sequnlock_irqrestore() - end non-interruptible seqlock_t write + * section + * @sl: Pointer to seqlock_t + * @flags: Caller's saved interrupt state, from write_seqlock_irqsave() + * + * write_sequnlock_irqrestore closes the serialized and non-interruptible + * seqlock_t write section previously opened with write_seqlock_irqsave(). + */ +static inline void +write_sequnlock_irqrestore(seqlock_t *sl, unsigned long flags) +{ + do_write_seqcount_end(&sl->seqcount.seqcount); + spin_unlock_irqrestore(&sl->lock, flags); +} + +/** + * read_seqlock_excl() - begin a seqlock_t locking reader section + * @sl: Pointer to seqlock_t + * + * read_seqlock_excl opens a seqlock_t locking reader critical section. A + * locking reader exclusively locks out *both* other writers *and* other + * locking readers, but it does not update the embedded sequence number. + * + * Locking readers act like a normal spin_lock()/spin_unlock(). + * + * Context: if the seqlock_t write section, *or other read sections*, can + * be invoked from hardirq or softirq contexts, use the _irqsave or _bh + * variant of this function instead. + * + * The opened read section must be closed with read_sequnlock_excl(). + */ +static inline void read_seqlock_excl(seqlock_t *sl) +{ + spin_lock(&sl->lock); +} + +/** + * read_sequnlock_excl() - end a seqlock_t locking reader critical section + * @sl: Pointer to seqlock_t + */ +static inline void read_sequnlock_excl(seqlock_t *sl) +{ + spin_unlock(&sl->lock); +} + +/** + * read_seqlock_excl_bh() - start a seqlock_t locking reader section with + * softirqs disabled + * @sl: Pointer to seqlock_t + * + * _bh variant of read_seqlock_excl(). Use this variant only if the + * seqlock_t write side section, *or other read sections*, can be invoked + * from softirq contexts. + */ +static inline void read_seqlock_excl_bh(seqlock_t *sl) +{ + spin_lock_bh(&sl->lock); +} + +/** + * read_sequnlock_excl_bh() - stop a seqlock_t softirq-disabled locking + * reader section + * @sl: Pointer to seqlock_t + */ +static inline void read_sequnlock_excl_bh(seqlock_t *sl) +{ + spin_unlock_bh(&sl->lock); +} + +/** + * read_seqlock_excl_irq() - start a non-interruptible seqlock_t locking + * reader section + * @sl: Pointer to seqlock_t + * + * _irq variant of read_seqlock_excl(). Use this only if the seqlock_t + * write side section, *or other read sections*, can be invoked from a + * hardirq context. + */ +static inline void read_seqlock_excl_irq(seqlock_t *sl) +{ + spin_lock_irq(&sl->lock); +} + +/** + * read_sequnlock_excl_irq() - end an interrupts-disabled seqlock_t + * locking reader section + * @sl: Pointer to seqlock_t + */ +static inline void read_sequnlock_excl_irq(seqlock_t *sl) +{ + spin_unlock_irq(&sl->lock); +} + +static inline unsigned long __read_seqlock_excl_irqsave(seqlock_t *sl) +{ + unsigned long flags; + + spin_lock_irqsave(&sl->lock, flags); + return flags; +} + +/** + * read_seqlock_excl_irqsave() - start a non-interruptible seqlock_t + * locking reader section + * @lock: Pointer to seqlock_t + * @flags: Stack-allocated storage for saving caller's local interrupt + * state, to be passed to read_sequnlock_excl_irqrestore(). + * + * _irqsave variant of read_seqlock_excl(). Use this only if the seqlock_t + * write side section, *or other read sections*, can be invoked from a + * hardirq context. + */ +#define read_seqlock_excl_irqsave(lock, flags) \ + do { flags = __read_seqlock_excl_irqsave(lock); } while (0) + +/** + * read_sequnlock_excl_irqrestore() - end non-interruptible seqlock_t + * locking reader section + * @sl: Pointer to seqlock_t + * @flags: Caller saved interrupt state, from read_seqlock_excl_irqsave() + */ +static inline void +read_sequnlock_excl_irqrestore(seqlock_t *sl, unsigned long flags) +{ + spin_unlock_irqrestore(&sl->lock, flags); +} + +/** + * read_seqbegin_or_lock() - begin a seqlock_t lockless or locking reader + * @lock: Pointer to seqlock_t + * @seq : Marker and return parameter. If the passed value is even, the + * reader will become a *lockless* seqlock_t reader as in read_seqbegin(). + * If the passed value is odd, the reader will become a *locking* reader + * as in read_seqlock_excl(). In the first call to this function, the + * caller *must* initialize and pass an even value to @seq; this way, a + * lockless read can be optimistically tried first. + * + * read_seqbegin_or_lock is an API designed to optimistically try a normal + * lockless seqlock_t read section first. If an odd counter is found, the + * lockless read trial has failed, and the next read iteration transforms + * itself into a full seqlock_t locking reader. + * + * This is typically used to avoid seqlock_t lockless readers starvation + * (too much retry loops) in the case of a sharp spike in write side + * activity. + * + * Context: if the seqlock_t write section, *or other read sections*, can + * be invoked from hardirq or softirq contexts, use the _irqsave or _bh + * variant of this function instead. + * + * Check Documentation/locking/seqlock.rst for template example code. + * + * Return: the encountered sequence counter value, through the @seq + * parameter, which is overloaded as a return parameter. This returned + * value must be checked with need_seqretry(). If the read section need to + * be retried, this returned value must also be passed as the @seq + * parameter of the next read_seqbegin_or_lock() iteration. + */ +static inline void read_seqbegin_or_lock(seqlock_t *lock, int *seq) +{ + if (!(*seq & 1)) /* Even */ + *seq = read_seqbegin(lock); + else /* Odd */ + read_seqlock_excl(lock); +} + +/** + * need_seqretry() - validate seqlock_t "locking or lockless" read section + * @lock: Pointer to seqlock_t + * @seq: sequence count, from read_seqbegin_or_lock() + * + * Return: true if a read section retry is required, false otherwise + */ +static inline int need_seqretry(seqlock_t *lock, int seq) +{ + return !(seq & 1) && read_seqretry(lock, seq); +} + +/** + * done_seqretry() - end seqlock_t "locking or lockless" reader section + * @lock: Pointer to seqlock_t + * @seq: count, from read_seqbegin_or_lock() + * + * done_seqretry finishes the seqlock_t read side critical section started + * with read_seqbegin_or_lock() and validated by need_seqretry(). + */ +static inline void done_seqretry(seqlock_t *lock, int seq) +{ + if (seq & 1) + read_sequnlock_excl(lock); +} + +/** + * read_seqbegin_or_lock_irqsave() - begin a seqlock_t lockless reader, or + * a non-interruptible locking reader + * @lock: Pointer to seqlock_t + * @seq: Marker and return parameter. Check read_seqbegin_or_lock(). + * + * This is the _irqsave variant of read_seqbegin_or_lock(). Use it only if + * the seqlock_t write section, *or other read sections*, can be invoked + * from hardirq context. + * + * Note: Interrupts will be disabled only for "locking reader" mode. + * + * Return: + * + * 1. The saved local interrupts state in case of a locking reader, to + * be passed to done_seqretry_irqrestore(). + * + * 2. The encountered sequence counter value, returned through @seq + * overloaded as a return parameter. Check read_seqbegin_or_lock(). + */ +static inline unsigned long +read_seqbegin_or_lock_irqsave(seqlock_t *lock, int *seq) +{ + unsigned long flags = 0; + + if (!(*seq & 1)) /* Even */ + *seq = read_seqbegin(lock); + else /* Odd */ + read_seqlock_excl_irqsave(lock, flags); + + return flags; +} + +/** + * done_seqretry_irqrestore() - end a seqlock_t lockless reader, or a + * non-interruptible locking reader section + * @lock: Pointer to seqlock_t + * @seq: Count, from read_seqbegin_or_lock_irqsave() + * @flags: Caller's saved local interrupt state in case of a locking + * reader, also from read_seqbegin_or_lock_irqsave() + * + * This is the _irqrestore variant of done_seqretry(). The read section + * must've been opened with read_seqbegin_or_lock_irqsave(), and validated + * by need_seqretry(). + */ +static inline void +done_seqretry_irqrestore(seqlock_t *lock, int seq, unsigned long flags) +{ + if (seq & 1) + read_sequnlock_excl_irqrestore(lock, flags); +} +#endif /* __LINUX_SEQLOCK_H */ |