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4817a52b30
seqcount_init() must be a macro in order to preserve the static
variable that is used for the lockdep key. Don't then wrap it in an
inline function, which destroys that.
Luckily there aren't many users of this function, but fix it before it
becomes a problem.
Fixes: 80793c3471
("seqlock: Introduce seqcount_latch_t")
Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/YEeFEbNUVkZaXDp4@hirez.programming.kicks-ass.net
1226 lines
38 KiB
C
1226 lines
38 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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#ifndef __LINUX_SEQLOCK_H
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#define __LINUX_SEQLOCK_H
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/*
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* seqcount_t / seqlock_t - a reader-writer consistency mechanism with
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* lockless readers (read-only retry loops), and no writer starvation.
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*
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* See Documentation/locking/seqlock.rst
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*
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* Copyrights:
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* - Based on x86_64 vsyscall gettimeofday: Keith Owens, Andrea Arcangeli
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* - Sequence counters with associated locks, (C) 2020 Linutronix GmbH
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*/
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#include <linux/compiler.h>
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#include <linux/kcsan-checks.h>
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#include <linux/lockdep.h>
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#include <linux/mutex.h>
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#include <linux/ww_mutex.h>
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#include <linux/preempt.h>
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#include <linux/spinlock.h>
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#include <asm/processor.h>
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/*
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* The seqlock seqcount_t interface does not prescribe a precise sequence of
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* read begin/retry/end. For readers, typically there is a call to
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* read_seqcount_begin() and read_seqcount_retry(), however, there are more
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* esoteric cases which do not follow this pattern.
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*
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* As a consequence, we take the following best-effort approach for raw usage
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* via seqcount_t under KCSAN: upon beginning a seq-reader critical section,
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* pessimistically mark the next KCSAN_SEQLOCK_REGION_MAX memory accesses as
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* atomics; if there is a matching read_seqcount_retry() call, no following
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* memory operations are considered atomic. Usage of the seqlock_t interface
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* is not affected.
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*/
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#define KCSAN_SEQLOCK_REGION_MAX 1000
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/*
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* Sequence counters (seqcount_t)
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*
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* This is the raw counting mechanism, without any writer protection.
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*
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* Write side critical sections must be serialized and non-preemptible.
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*
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* If readers can be invoked from hardirq or softirq contexts,
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* interrupts or bottom halves must also be respectively disabled before
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* entering the write section.
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*
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* This mechanism can't be used if the protected data contains pointers,
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* as the writer can invalidate a pointer that a reader is following.
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*
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* If the write serialization mechanism is one of the common kernel
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* locking primitives, use a sequence counter with associated lock
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* (seqcount_LOCKNAME_t) instead.
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*
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* If it's desired to automatically handle the sequence counter writer
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* serialization and non-preemptibility requirements, use a sequential
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* lock (seqlock_t) instead.
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*
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* See Documentation/locking/seqlock.rst
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*/
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typedef struct seqcount {
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unsigned sequence;
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#ifdef CONFIG_DEBUG_LOCK_ALLOC
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struct lockdep_map dep_map;
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#endif
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} seqcount_t;
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static inline void __seqcount_init(seqcount_t *s, const char *name,
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struct lock_class_key *key)
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{
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/*
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* Make sure we are not reinitializing a held lock:
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*/
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lockdep_init_map(&s->dep_map, name, key, 0);
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s->sequence = 0;
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}
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#ifdef CONFIG_DEBUG_LOCK_ALLOC
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# define SEQCOUNT_DEP_MAP_INIT(lockname) \
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.dep_map = { .name = #lockname }
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/**
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* seqcount_init() - runtime initializer for seqcount_t
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* @s: Pointer to the seqcount_t instance
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*/
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# define seqcount_init(s) \
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do { \
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static struct lock_class_key __key; \
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__seqcount_init((s), #s, &__key); \
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} while (0)
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static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
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{
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seqcount_t *l = (seqcount_t *)s;
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unsigned long flags;
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local_irq_save(flags);
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seqcount_acquire_read(&l->dep_map, 0, 0, _RET_IP_);
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seqcount_release(&l->dep_map, _RET_IP_);
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local_irq_restore(flags);
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}
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#else
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# define SEQCOUNT_DEP_MAP_INIT(lockname)
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# define seqcount_init(s) __seqcount_init(s, NULL, NULL)
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# define seqcount_lockdep_reader_access(x)
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#endif
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/**
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* SEQCNT_ZERO() - static initializer for seqcount_t
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* @name: Name of the seqcount_t instance
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*/
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#define SEQCNT_ZERO(name) { .sequence = 0, SEQCOUNT_DEP_MAP_INIT(name) }
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/*
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* Sequence counters with associated locks (seqcount_LOCKNAME_t)
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*
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* A sequence counter which associates the lock used for writer
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* serialization at initialization time. This enables lockdep to validate
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* that the write side critical section is properly serialized.
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*
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* For associated locks which do not implicitly disable preemption,
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* preemption protection is enforced in the write side function.
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*
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* Lockdep is never used in any for the raw write variants.
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*
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* See Documentation/locking/seqlock.rst
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*/
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/*
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* For PREEMPT_RT, seqcount_LOCKNAME_t write side critical sections cannot
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* disable preemption. It can lead to higher latencies, and the write side
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* sections will not be able to acquire locks which become sleeping locks
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* (e.g. spinlock_t).
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*
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* To remain preemptible while avoiding a possible livelock caused by the
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* reader preempting the writer, use a different technique: let the reader
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* detect if a seqcount_LOCKNAME_t writer is in progress. If that is the
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* case, acquire then release the associated LOCKNAME writer serialization
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* lock. This will allow any possibly-preempted writer to make progress
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* until the end of its writer serialization lock critical section.
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*
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* This lock-unlock technique must be implemented for all of PREEMPT_RT
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* sleeping locks. See Documentation/locking/locktypes.rst
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*/
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#if defined(CONFIG_LOCKDEP) || defined(CONFIG_PREEMPT_RT)
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#define __SEQ_LOCK(expr) expr
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#else
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#define __SEQ_LOCK(expr)
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#endif
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/*
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* typedef seqcount_LOCKNAME_t - sequence counter with LOCKNAME associated
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* @seqcount: The real sequence counter
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* @lock: Pointer to the associated lock
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*
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* A plain sequence counter with external writer synchronization by
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* LOCKNAME @lock. The lock is associated to the sequence counter in the
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* static initializer or init function. This enables lockdep to validate
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* that the write side critical section is properly serialized.
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*
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* LOCKNAME: raw_spinlock, spinlock, rwlock, mutex, or ww_mutex.
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*/
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/*
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* seqcount_LOCKNAME_init() - runtime initializer for seqcount_LOCKNAME_t
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* @s: Pointer to the seqcount_LOCKNAME_t instance
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* @lock: Pointer to the associated lock
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*/
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#define seqcount_LOCKNAME_init(s, _lock, lockname) \
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do { \
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seqcount_##lockname##_t *____s = (s); \
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seqcount_init(&____s->seqcount); \
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__SEQ_LOCK(____s->lock = (_lock)); \
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} while (0)
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#define seqcount_raw_spinlock_init(s, lock) seqcount_LOCKNAME_init(s, lock, raw_spinlock)
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#define seqcount_spinlock_init(s, lock) seqcount_LOCKNAME_init(s, lock, spinlock)
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#define seqcount_rwlock_init(s, lock) seqcount_LOCKNAME_init(s, lock, rwlock);
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#define seqcount_mutex_init(s, lock) seqcount_LOCKNAME_init(s, lock, mutex);
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#define seqcount_ww_mutex_init(s, lock) seqcount_LOCKNAME_init(s, lock, ww_mutex);
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/*
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* SEQCOUNT_LOCKNAME() - Instantiate seqcount_LOCKNAME_t and helpers
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* seqprop_LOCKNAME_*() - Property accessors for seqcount_LOCKNAME_t
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*
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* @lockname: "LOCKNAME" part of seqcount_LOCKNAME_t
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* @locktype: LOCKNAME canonical C data type
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* @preemptible: preemptibility of above locktype
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* @lockmember: argument for lockdep_assert_held()
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* @lockbase: associated lock release function (prefix only)
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* @lock_acquire: associated lock acquisition function (full call)
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*/
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#define SEQCOUNT_LOCKNAME(lockname, locktype, preemptible, lockmember, lockbase, lock_acquire) \
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typedef struct seqcount_##lockname { \
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seqcount_t seqcount; \
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__SEQ_LOCK(locktype *lock); \
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} seqcount_##lockname##_t; \
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\
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static __always_inline seqcount_t * \
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__seqprop_##lockname##_ptr(seqcount_##lockname##_t *s) \
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{ \
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return &s->seqcount; \
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} \
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\
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static __always_inline unsigned \
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__seqprop_##lockname##_sequence(const seqcount_##lockname##_t *s) \
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{ \
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unsigned seq = READ_ONCE(s->seqcount.sequence); \
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\
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if (!IS_ENABLED(CONFIG_PREEMPT_RT)) \
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return seq; \
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\
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if (preemptible && unlikely(seq & 1)) { \
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__SEQ_LOCK(lock_acquire); \
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__SEQ_LOCK(lockbase##_unlock(s->lock)); \
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\
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/* \
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* Re-read the sequence counter since the (possibly \
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* preempted) writer made progress. \
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*/ \
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seq = READ_ONCE(s->seqcount.sequence); \
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} \
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\
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return seq; \
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} \
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\
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static __always_inline bool \
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__seqprop_##lockname##_preemptible(const seqcount_##lockname##_t *s) \
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{ \
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if (!IS_ENABLED(CONFIG_PREEMPT_RT)) \
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return preemptible; \
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\
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/* PREEMPT_RT relies on the above LOCK+UNLOCK */ \
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return false; \
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} \
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\
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static __always_inline void \
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__seqprop_##lockname##_assert(const seqcount_##lockname##_t *s) \
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{ \
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__SEQ_LOCK(lockdep_assert_held(lockmember)); \
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}
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/*
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* __seqprop() for seqcount_t
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*/
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static inline seqcount_t *__seqprop_ptr(seqcount_t *s)
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{
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return s;
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}
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static inline unsigned __seqprop_sequence(const seqcount_t *s)
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{
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return READ_ONCE(s->sequence);
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}
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static inline bool __seqprop_preemptible(const seqcount_t *s)
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{
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return false;
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}
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static inline void __seqprop_assert(const seqcount_t *s)
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{
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lockdep_assert_preemption_disabled();
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}
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#define __SEQ_RT IS_ENABLED(CONFIG_PREEMPT_RT)
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SEQCOUNT_LOCKNAME(raw_spinlock, raw_spinlock_t, false, s->lock, raw_spin, raw_spin_lock(s->lock))
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SEQCOUNT_LOCKNAME(spinlock, spinlock_t, __SEQ_RT, s->lock, spin, spin_lock(s->lock))
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SEQCOUNT_LOCKNAME(rwlock, rwlock_t, __SEQ_RT, s->lock, read, read_lock(s->lock))
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SEQCOUNT_LOCKNAME(mutex, struct mutex, true, s->lock, mutex, mutex_lock(s->lock))
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SEQCOUNT_LOCKNAME(ww_mutex, struct ww_mutex, true, &s->lock->base, ww_mutex, ww_mutex_lock(s->lock, NULL))
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/*
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* SEQCNT_LOCKNAME_ZERO - static initializer for seqcount_LOCKNAME_t
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* @name: Name of the seqcount_LOCKNAME_t instance
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* @lock: Pointer to the associated LOCKNAME
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*/
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#define SEQCOUNT_LOCKNAME_ZERO(seq_name, assoc_lock) { \
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.seqcount = SEQCNT_ZERO(seq_name.seqcount), \
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__SEQ_LOCK(.lock = (assoc_lock)) \
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}
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#define SEQCNT_RAW_SPINLOCK_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(name, lock)
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#define SEQCNT_SPINLOCK_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(name, lock)
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#define SEQCNT_RWLOCK_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(name, lock)
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#define SEQCNT_MUTEX_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(name, lock)
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#define SEQCNT_WW_MUTEX_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(name, lock)
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#define __seqprop_case(s, lockname, prop) \
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seqcount_##lockname##_t: __seqprop_##lockname##_##prop((void *)(s))
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#define __seqprop(s, prop) _Generic(*(s), \
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seqcount_t: __seqprop_##prop((void *)(s)), \
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__seqprop_case((s), raw_spinlock, prop), \
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__seqprop_case((s), spinlock, prop), \
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__seqprop_case((s), rwlock, prop), \
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__seqprop_case((s), mutex, prop), \
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__seqprop_case((s), ww_mutex, prop))
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#define seqprop_ptr(s) __seqprop(s, ptr)
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#define seqprop_sequence(s) __seqprop(s, sequence)
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#define seqprop_preemptible(s) __seqprop(s, preemptible)
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#define seqprop_assert(s) __seqprop(s, assert)
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/**
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* __read_seqcount_begin() - begin a seqcount_t read section w/o barrier
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* @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
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*
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* __read_seqcount_begin is like read_seqcount_begin, but has no smp_rmb()
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* barrier. Callers should ensure that smp_rmb() or equivalent ordering is
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* provided before actually loading any of the variables that are to be
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* protected in this critical section.
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*
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* Use carefully, only in critical code, and comment how the barrier is
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* provided.
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*
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* Return: count to be passed to read_seqcount_retry()
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*/
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#define __read_seqcount_begin(s) \
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({ \
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unsigned __seq; \
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\
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while ((__seq = seqprop_sequence(s)) & 1) \
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cpu_relax(); \
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\
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kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX); \
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__seq; \
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})
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/**
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* raw_read_seqcount_begin() - begin a seqcount_t read section w/o lockdep
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* @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
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*
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* Return: count to be passed to read_seqcount_retry()
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*/
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#define raw_read_seqcount_begin(s) \
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({ \
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unsigned _seq = __read_seqcount_begin(s); \
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\
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smp_rmb(); \
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_seq; \
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})
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/**
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* read_seqcount_begin() - begin a seqcount_t read critical section
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* @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
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*
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* Return: count to be passed to read_seqcount_retry()
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*/
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#define read_seqcount_begin(s) \
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({ \
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seqcount_lockdep_reader_access(seqprop_ptr(s)); \
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raw_read_seqcount_begin(s); \
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})
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/**
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* raw_read_seqcount() - read the raw seqcount_t counter value
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* @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
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*
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* raw_read_seqcount opens a read critical section of the given
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* seqcount_t, without any lockdep checking, and without checking or
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* masking the sequence counter LSB. Calling code is responsible for
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* handling that.
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*
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* Return: count to be passed to read_seqcount_retry()
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*/
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#define raw_read_seqcount(s) \
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({ \
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unsigned __seq = seqprop_sequence(s); \
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\
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smp_rmb(); \
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kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX); \
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__seq; \
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})
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/**
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* raw_seqcount_begin() - begin a seqcount_t read critical section w/o
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* lockdep and w/o counter stabilization
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* @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
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*
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* raw_seqcount_begin opens a read critical section of the given
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* seqcount_t. Unlike read_seqcount_begin(), this function will not wait
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* for the count to stabilize. If a writer is active when it begins, it
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* will fail the read_seqcount_retry() at the end of the read critical
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* section instead of stabilizing at the beginning of it.
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*
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* Use this only in special kernel hot paths where the read section is
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* small and has a high probability of success through other external
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* means. It will save a single branching instruction.
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*
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* Return: count to be passed to read_seqcount_retry()
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*/
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#define raw_seqcount_begin(s) \
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({ \
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/* \
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* If the counter is odd, let read_seqcount_retry() fail \
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* by decrementing the counter. \
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*/ \
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raw_read_seqcount(s) & ~1; \
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})
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/**
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* __read_seqcount_retry() - end a seqcount_t read section w/o barrier
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* @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
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* @start: count, from read_seqcount_begin()
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*
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* __read_seqcount_retry is like read_seqcount_retry, but has no smp_rmb()
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* barrier. Callers should ensure that smp_rmb() or equivalent ordering is
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* provided before actually loading any of the variables that are to be
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* protected in this critical section.
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*
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* Use carefully, only in critical code, and comment how the barrier is
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* provided.
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*
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* Return: true if a read section retry is required, else false
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*/
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#define __read_seqcount_retry(s, start) \
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do___read_seqcount_retry(seqprop_ptr(s), start)
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static inline int do___read_seqcount_retry(const seqcount_t *s, unsigned start)
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{
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kcsan_atomic_next(0);
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return unlikely(READ_ONCE(s->sequence) != start);
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}
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/**
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* read_seqcount_retry() - end a seqcount_t read critical section
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* @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
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* @start: count, from read_seqcount_begin()
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*
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* read_seqcount_retry closes the read critical section of given
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* seqcount_t. If the critical section was invalid, it must be ignored
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* (and typically retried).
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*
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* Return: true if a read section retry is required, else false
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*/
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#define read_seqcount_retry(s, start) \
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do_read_seqcount_retry(seqprop_ptr(s), start)
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static inline int do_read_seqcount_retry(const seqcount_t *s, unsigned start)
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{
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smp_rmb();
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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)
|
|
{
|
|
do_raw_write_seqcount_begin(s);
|
|
seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
|
|
}
|
|
|
|
/**
|
|
* 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;
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|
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spin_lock_irqsave(&sl->lock, flags);
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do_write_seqcount_begin(&sl->seqcount.seqcount);
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return flags;
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|
}
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|
|
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/**
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|
* write_seqlock_irqsave() - start a non-interruptible seqlock_t write
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|
* section
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* @lock: Pointer to seqlock_t
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* @flags: Stack-allocated storage for saving caller's local interrupt
|
|
* state, to be passed to write_sequnlock_irqrestore().
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*
|
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* _irqsave variant of write_seqlock(). Use it only if the read side
|
|
* section, or other write sections, can be invoked from hardirq context.
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|
*/
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#define write_seqlock_irqsave(lock, flags) \
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do { flags = __write_seqlock_irqsave(lock); } while (0)
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|
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/**
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* write_sequnlock_irqrestore() - end non-interruptible seqlock_t write
|
|
* section
|
|
* @sl: Pointer to seqlock_t
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|
* @flags: Caller's saved interrupt state, from write_seqlock_irqsave()
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|
*
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|
* write_sequnlock_irqrestore closes the serialized and non-interruptible
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|
* seqlock_t write section previously opened with write_seqlock_irqsave().
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|
*/
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static inline void
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|
write_sequnlock_irqrestore(seqlock_t *sl, unsigned long flags)
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|
{
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|
do_write_seqcount_end(&sl->seqcount.seqcount);
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spin_unlock_irqrestore(&sl->lock, flags);
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|
}
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|
|
|
/**
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|
* read_seqlock_excl() - begin a seqlock_t locking reader section
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|
* @sl: Pointer to seqlock_t
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|
*
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* read_seqlock_excl opens a seqlock_t locking reader critical section. A
|
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* 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().
|
|
*
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|
* 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);
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|
}
|
|
|
|
/**
|
|
* 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 */
|