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Diffstat (limited to 'lib/frrcu.h')
| -rw-r--r-- | lib/frrcu.h | 172 |
1 files changed, 172 insertions, 0 deletions
diff --git a/lib/frrcu.h b/lib/frrcu.h new file mode 100644 index 0000000..e7a54dc --- /dev/null +++ b/lib/frrcu.h @@ -0,0 +1,172 @@ +// 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; + +/* 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 */ |