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// SPDX-License-Identifier: ISC
/*
* Copyright (c) 2017-19 David Lamparter, for NetDEF, Inc.
*/
#ifndef _FRRCU_H
#define _FRRCU_H
#include <assert.h>
#include "memory.h"
#include "atomlist.h"
#ifdef __cplusplus
extern "C" {
#endif
/* quick RCU primer:
* There's a global sequence counter. Whenever a thread does a
* rcu_read_lock(), it is marked as holding the current sequence counter.
* When something is cleaned with RCU, the global sequence counter is
* increased and the item is queued for cleanup - *after* all threads are
* at a more recent sequence counter (or no sequence counter / unheld).
*
* So, by delaying resource cleanup, RCU ensures that things don't go away
* while another thread may hold a (stale) reference.
*
* Note that even if a thread is in rcu_read_lock(), it is invalid for that
* thread to access bits after rcu_free() & co on them. This is a design
* choice to allow no-op'ing out the entire RCU mechanism if we're running
* singlethreaded. (Also allows some optimization on the counter bumping.)
*
* differences from Linux Kernel RCU:
* - there's no rcu_synchronize(), if you really need to defer something
* use rcu_call() (and double check it's really necessary)
* - rcu_dereference() and rcu_assign_pointer() don't exist, use atomic_*
* instead (ATOM* list structures do the right thing)
*/
/* opaque */
struct rcu_thread;
/* sets up rcu thread info
*
* return value must be passed into the thread's call to rcu_thread_start()
*/
extern struct rcu_thread *rcu_thread_new(void *arg);
/* called before new thread creation, sets up rcu thread info for new thread
* before it actually exits. This ensures possible RCU references are held
* for thread startup.
*
* return value must be passed into the new thread's call to rcu_thread_start()
*/
extern struct rcu_thread *rcu_thread_prepare(void);
/* cleanup in case pthread_create() fails */
extern void rcu_thread_unprepare(struct rcu_thread *rcu_thread);
/* called early in the new thread, with the return value from the above.
* NB: new thread is initially in RCU-held state! (at depth 1)
*
* TBD: maybe inherit RCU state from rcu_thread_prepare()?
*/
extern void rcu_thread_start(struct rcu_thread *rcu_thread);
/* thread exit is handled through pthread_key_create's destructor function */
/* global RCU shutdown - must be called with only 1 active thread left. waits
* until remaining RCU actions are done & RCU thread has exited.
*
* This is mostly here to get a clean exit without memleaks.
*/
extern void rcu_shutdown(void);
/* enter / exit RCU-held state. counter-based, so can be called nested. */
extern void rcu_read_lock(void);
extern void rcu_read_unlock(void);
/* for debugging / safety checks */
extern void rcu_assert_read_locked(void);
extern void rcu_assert_read_unlocked(void);
enum rcu_action_type {
RCUA_INVALID = 0,
/* used internally by the RCU code, shouldn't ever show up outside */
RCUA_NEXT,
RCUA_END,
/* normal RCU actions, for outside use */
RCUA_FREE,
RCUA_CLOSE,
RCUA_CALL,
};
/* since rcu_head is intended to be embedded into structs which may exist
* with lots of copies, rcu_head is shrunk down to its absolute minimum -
* the atomlist pointer + a pointer to this action struct.
*/
struct rcu_action {
enum rcu_action_type type;
union {
struct {
struct memtype *mt;
ptrdiff_t offset;
} free;
struct {
void (*fptr)(void *arg);
ptrdiff_t offset;
} call;
} u;
};
/* RCU cleanup function queue item */
PREDECL_ATOMLIST(rcu_heads);
struct rcu_head {
struct rcu_heads_item head;
const struct rcu_action *action;
};
/* special RCU head for delayed fd-close */
struct rcu_head_close {
struct rcu_head rcu_head;
int fd;
};
/* enqueue RCU action - use the macros below to get the rcu_action set up */
extern void rcu_enqueue(struct rcu_head *head, const struct rcu_action *action);
/* RCU free() and file close() operations.
*
* freed memory / closed fds become _immediately_ unavailable to the calling
* thread, but will remain available for other threads until they have passed
* into RCU-released state.
*/
/* may be called with NULL mt to do non-MTYPE free() */
#define rcu_free(mtype, ptr, field) \
do { \
typeof(ptr) _ptr = (ptr); \
if (!_ptr) \
break; \
struct rcu_head *_rcu_head = &_ptr->field; \
static const struct rcu_action _rcu_action = { \
.type = RCUA_FREE, \
.u.free = { \
.mt = mtype, \
.offset = offsetof(typeof(*_ptr), field), \
}, \
}; \
rcu_enqueue(_rcu_head, &_rcu_action); \
} while (0)
/* use this sparingly, it runs on (and blocks) the RCU thread */
#define rcu_call(func, ptr, field) \
do { \
typeof(ptr) _ptr = (ptr); \
void (*fptype)(typeof(ptr)); \
struct rcu_head *_rcu_head = &_ptr->field; \
static const struct rcu_action _rcu_action = { \
.type = RCUA_CALL, \
.u.call = { \
.fptr = (void *)func, \
.offset = offsetof(typeof(*_ptr), field), \
}, \
}; \
(void)(_fptype = func); \
rcu_enqueue(_rcu_head, &_rcu_action); \
} while (0)
extern void rcu_close(struct rcu_head_close *head, int fd);
#ifdef __cplusplus
}
#endif
#endif /* _FRRCU_H */
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