Userspace RCU Implementation by Mathieu Desnoyers and Paul E. McKenney BUILDING -------- ./bootstrap (skip if using tarball) ./configure make make install QUICK START GUIDE ----------------- Usage of all urcu libraries * Define _LGPL_SOURCE (only) if your code is LGPL or GPL compatible before including the urcu.h or urcu-qsbr.h header. If your application is distributed under another license, function calls will be generated instead of inlines, so your application can link with the library. * Linking with one of the libraries below is always necessary even for LGPL and GPL applications. Usage of liburcu * #include * Link the application with "-lurcu". * This is the preferred version of the library, both in terms of speed and flexibility. Requires a signal, typically SIGUSR1. Can be overridden with -DSIGURCU by modifying Makefile.build.inc. Usage of liburcu-mb * #include * Compile any _LGPL_SOURCE code using this library with "-DURCU_MB". * Link with "-lurcu-mb". * This version of the urcu library does not need to reserve a signal number. URCU_MB uses full memory barriers for readers. This eliminates the need for signals but results in slower reads. Usage of liburcu-qsbr * #include * Link with "-lurcu-qsbr". * The QSBR flavor of RCU needs to have each reader thread executing rcu_quiescent_state() periodically to progress. rcu_thread_online() and rcu_thread_offline() can be used to mark long periods for which the threads are not active. It provides the fastest read-side at the expense of more intrusiveness in the application code. Usage of liburcu-bp * #include * Link with "-lurcu-bp". * The BP library flavor stands for "bulletproof". It is specifically designed to help tracing library to hook on applications without requiring to modify these applications. urcu_init(), rcu_register_thread() and rcu_unregister_thread() all become nops. The state is dealt with by the library internally at the expense of read-side and write-side performance. Initialization Each thread that has reader critical sections (that uses rcu_read_lock()/rcu_read_unlock() must first register to the URCU library. This is done by calling rcu_register_thread(). Unregistration must be performed before exiting the thread by using rcu_unregister_thread(). Reading Reader critical sections must be protected by locating them between calls to rcu_read_lock() and rcu_read_unlock(). Inside that lock, rcu_dereference() may be called to read an RCU protected pointer. Writing rcu_assign_pointer() and rcu_xchg_pointer() may be called anywhere. After, synchronize_rcu() must be called. When it returns, the old values are not in usage anymore. Usage of liburcu-defer * #include * Link with "-lurcu-defer" * Provides call_rcu() primitive to enqueue delayed callbacks. Queued callbacks are executed in batch periodically after a grace period. Do _not_ use call_rcu() within a read-side critical section, because it may call synchronize_rcu() if the thread queue is full. Being careful with signals The liburcu library uses signals internally. The signal handler is registered with the SA_RESTART flag. However, these signals may cause some non-restartable system calls to fail with errno = EINTR. Care should be taken to restart system calls manually if they fail with this error. A list of non-restartable system calls may be found in signal(7). The liburcu-mb and liburcu-qsbr versions of the Userspace RCU library do not require any signal. Read-side critical sections are allowed in a signal handler with liburcu and liburcu-mb. Be careful, however, to disable these signals between thread creation and calls to rcu_register_thread(), because a signal handler nesting on an unregistered thread would not be allowed to call rcu_read_lock(). Read-side critical sections are _not_ allowed in a signal handler with liburcu-qsbr, unless signals are disabled explicitly around each rcu_quiescent_state() calls, when threads are put offline and around calls to synchronize_rcu(). Even then, we do not recommend it. Usage of DEBUG_RCU DEBUG_RCU is used to add internal debugging self-checks to the RCU library. This define adds a performance penalty when enabled. Can be enabled by uncommenting the corresponding line in Makefile.build.inc. Usage of DEBUG_YIELD DEBUG_YIELD is used to add random delays in the code for testing purposes.