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![Christian König](/assets/img/avatar_default.png)
Daniel pointed out that this series removes the last user of seqcount_ww_mutex_t, so let's drop this. Signed-off-by: Christian König <christian.koenig@amd.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Ingo Molnar <mingo@redhat.com> Cc: Will Deacon <will@kernel.org> Cc: Waiman Long <longman@redhat.com> Cc: Boqun Feng <boqun.feng@gmail.com> Cc: linux-kernel@vger.kernel.org Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org> Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch> Link: https://patchwork.freedesktop.org/patch/msgid/20220407085946.744568-16-christian.koenig@amd.com
1222 lines
38 KiB
C
1222 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/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 or 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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/*
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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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/*
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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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#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);
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}
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/**
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* raw_write_seqcount_begin() - start a seqcount_t write section w/o lockdep
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* @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;
|
|
|
|
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 */
|