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Quelle rcu.h Sprache: C |
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/** * \file rcu.h Read-Copy-Update (RCU) declarations * \ingroup rcu */ /***************************************************************************** * Copyright © 2021 Rémi Denis-Courmont * * This program is free software; you can redistribute it and/or modify it * under the terms of the GNU Lesser General Public License as published by * the Free Software Foundation; either version 2.1 of the License, or * (at your option) any later version. * * This program is distributed in the hope that it will be useful, * but WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the * GNU Lesser General Public License for more details. * * You should have received a copy of the GNU Lesser General Public License * along with this program; if not, write to the Free Software Foundation, * Inc., 51 Franklin Street, Fifth Floor, Boston MA 02110-1301, USA. *****************************************************************************/ #ifndef VLC_RCU_H_ #define VLC_RCU_H_ #include <stdatomic.h> /** * \defgroup rcu Read-Copy-Update synchronisation * \ingroup threads * The Read-Copy-Update (RCU) mechanism is a paradigm of memory synchronisation * first popularised by the Linux kernel. * It is essentially a mean to safely use atomic pointers, without encountering * use-after-free and other lifecycle bugs. * * It is an advantageous substitute for * the classic reader/writer lock with the following notable differences: * - The read side is completely lock-free and wait-free. * - The write side is guaranteed to make forward progress even if there are * perpetually active readers, provided that all read sides are each * individually finitely long. * - The protected data consists of a single atomic object, * typically an atomic pointer * (not to be confused with a pointer to an atomic object). * - There is no synchronisation object. RCU maintains its state globally, * and with a modicum of per-thread state. * * The practical use of RCU differs from reader/writer lock in the following * ways: * - There is no synchronisation object, so there are no needs for * initialisation and destruction. * - The directly protected objects are atom, so they must be accessed with * atomic type-generics. Specifically: * - Objects are read with atomic load-acquire. * - Objects are written with atomic store-release. * - There are no writer lock and unlock functions. Instead there is a * synchronisation function, vlc_rcu_synchronize(), which waits for all * earlier read-side critical sections to complete. * - Synchronisation between concurrent writers is not provided. If multiple * writers can race against each other, a separate mutual exclusion lock * for interlocking. * * @{ */ /** * Begins a read-side RCU critical section. * * This function marks the beginning of a read-side RCU critical section. * For the duration of the critical section, RCU-protected data is guaranteed * to remain valid, although it might be slightly stale. * * To access RCU-protect data, an atomic load-acquire should be used. * * \note Read-side RCU critical section can be nested. */ void vlc_rcu_read_lock(void); /** * Ends a read-side RCU critical section. * * This function marks the end of a read-side RCU critical section. After this * function is called, RCU-protected data may be destroyed and can no longer * be accessed safely. * * \note In the case of nested critical sections, this function notionally ends * the inner-most critical section. In practice, this makes no differences, as * the calling thread remains in a critical section until the number of * vlc_rcu_read_unlock() calls equals that of vlc_rcu_read_lock() calls. */ void vlc_rcu_read_unlock(void); /** * Checks if the thread is in an read-side RCU critical section. * * This function checks if the thread is in a middle of one or more read-side * RCU critical section(s). It has no side effects and is primarily meant for * self-debugging. * * \retval true the calling thread is in a read-side RCU critical section. * \retval false the calling thread is not in a read-side RCU critical section. */ VLC_USED bool vlc_rcu_read_held(void); /** * Waits for completion of earlier read-side RCU critical section. * * This functions waits until all read-side RCU critical sections that had * begun before to complete. Then it is safe to release resources associated * with the earlier value(s) of any RCU-protected atomic object. */ void vlc_rcu_synchronize(void); /** @} */ #endif /* !VLC_RCU_H_ */
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2026-11-01