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// SPDX-License-Identifier: LGPL-2.1-or-later
/*
* "Sequence" lock primitive
*
* Copyright (C) 2015 David Lamparter <equinox@diac24.net>
*/
#ifndef _SEQLOCK_H
#define _SEQLOCK_H
#include <stdbool.h>
#include <stdint.h>
#include <pthread.h>
#include "frratomic.h"
#ifdef __cplusplus
extern "C" {
#endif
/*
* this locking primitive is intended to use in a 1:N setup.
*
* - one "counter" seqlock issuing increasing numbers
* - multiple seqlock users hold references on these numbers
*
* this is intended for implementing RCU reference-holding. There is one
* global counter, with threads locking a seqlock whenever they take a
* reference. A seqlock can also be idle/unlocked.
*
* The "counter" seqlock will always stay locked; the RCU cleanup thread
* continuously counts it up, waiting for threads to release or progress to a
* sequence number further ahead. If all threads are > N, references dropped
* in N can be free'd.
*
* generally, the lock function is:
*
* Thread-A Thread-B
*
* seqlock_acquire(a)
* | running seqlock_wait(b) -- a <= b
* seqlock_release() | blocked
* OR: seqlock_acquire(a') | -- a' > b
* (resumes)
*/
/* use sequentially increasing "ticket numbers". lowest bit will always
* be 1 to have a 'cleared' indication (i.e., counts 1,5,9,13,etc. )
* 2nd lowest bit is used to indicate we have waiters.
*/
typedef _Atomic uint32_t seqlock_ctr_t;
typedef uint32_t seqlock_val_t;
#define seqlock_assert_valid(val) assert((val) & SEQLOCK_HELD)
/* NB: SEQLOCK_WAITERS is only allowed if SEQLOCK_HELD is also set; can't
* have waiters on an unheld seqlock
*/
#define SEQLOCK_HELD (1U << 0)
#define SEQLOCK_WAITERS (1U << 1)
#define SEQLOCK_VAL(n) ((n) & ~SEQLOCK_WAITERS)
#define SEQLOCK_STARTVAL 1U
#define SEQLOCK_INCR 4U
/* TODO: originally, this was using "atomic_fetch_add", which is the reason
* bit 0 is used to indicate held state. With SEQLOCK_WAITERS added, there's
* no fetch_add anymore (cmpxchg loop instead), so we don't need to use bit 0
* for this anymore & can just special-case the value 0 for it and skip it in
* counting.
*/
struct seqlock {
/* always used */
seqlock_ctr_t pos;
/* used when futexes not available: (i.e. non-linux) */
pthread_mutex_t lock;
pthread_cond_t wake;
};
/* sqlo = 0 - init state: not held */
extern void seqlock_init(struct seqlock *sqlo);
/* basically: "while (sqlo <= val) wait();"
* returns when sqlo > val || !seqlock_held(sqlo)
*/
extern void seqlock_wait(struct seqlock *sqlo, seqlock_val_t val);
/* same, but time-limited (limit is an absolute CLOCK_MONOTONIC value) */
extern bool seqlock_timedwait(struct seqlock *sqlo, seqlock_val_t val,
const struct timespec *abs_monotime_limit);
/* one-shot test, returns true if seqlock_wait would return immediately */
extern bool seqlock_check(struct seqlock *sqlo, seqlock_val_t val);
static inline bool seqlock_held(struct seqlock *sqlo)
{
return !!atomic_load_explicit(&sqlo->pos, memory_order_relaxed);
}
/* sqlo - get seqlock position -- for the "counter" seqlock */
extern seqlock_val_t seqlock_cur(struct seqlock *sqlo);
/* ++sqlo (but atomic & wakes waiters) - returns value that we bumped to.
*
* guarantees:
* - each seqlock_bump call bumps the position by exactly one SEQLOCK_INCR.
* There are no skipped/missed or multiple increments.
* - each return value is only returned from one seqlock_bump() call
*/
extern seqlock_val_t seqlock_bump(struct seqlock *sqlo);
/* sqlo = val - can be used on held seqlock. */
extern void seqlock_acquire_val(struct seqlock *sqlo, seqlock_val_t val);
/* sqlo = ref - standard pattern: acquire relative to other seqlock */
static inline void seqlock_acquire(struct seqlock *sqlo, struct seqlock *ref)
{
seqlock_acquire_val(sqlo, seqlock_cur(ref));
}
/* sqlo = 0 - set seqlock position to 0, marking as non-held */
extern void seqlock_release(struct seqlock *sqlo);
/* release should normally be followed by a bump on the "counter", if
* anything other than reading RCU items was done
*/
#ifdef __cplusplus
}
#endif
#endif /* _SEQLOCK_H */
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