Userspace RCU Implementation by Mathieu Desnoyers and Paul E. McKenney BUILDING -------- ./bootstrap (skip if using tarball) ./configure make make install ldconfig Hints: Forcing 32-bit build: * CFLAGS="-m32 -g -O2" ./configure Forcing 64-bit build: * CFLAGS="-m64 -g -O2" ./configure Forcing a 32-bit build with 386 backward compatibility: * CFLAGS="-m32 -g -O2" ./configure --host=i386-pc-linux-gnu Forcing a 32-bit build for Sparcv9 (typical for Sparc v9) * CFLAGS="-m32 -Wa,-Av9a -g -O2" ./configure ARCHITECTURES SUPPORTED ----------------------- Currently, Linux x86 (i386, i486, i586, i686), x86 64-bit, PowerPC 32/64, S390, S390x, ARM, Alpha, ia64 and Sparcv9 32/64 are supported. Tested on Linux, FreeBSD 8.2/9.0, and Cygwin. Should also work on: Android, NetBSD 5, OpenBSD, Darwin (more testing needed before claiming support for these OS). Linux ARM depends on running a Linux kernel 2.6.15 or better, GCC 4.4 or better. The gcc compiler versions 3.3, 3.4, 4.0, 4.1, 4.2, 4.3, 4.4 and 4.5 are supported, with the following exceptions: - gcc 3.3 and 3.4 have a bug that prevents them from generating volatile accesses to offsets in a TLS structure on 32-bit x86. These versions are therefore not compatible with liburcu on x86 32-bit (i386, i486, i586, i686). The problem has been reported to the gcc community: http://www.mail-archive.com/gcc-bugs@gcc.gnu.org/msg281255.html - gcc 3.3 cannot match the "xchg" instruction on 32-bit x86 build. See: http://kerneltrap.org/node/7507 - Alpha, ia64 and ARM architectures depend on gcc 4.x with atomic builtins support. For ARM this was introduced with gcc 4.4: http://gcc.gnu.org/gcc-4.4/changes.html Clang version 3.0 (based on LLVM 3.0) is supported. Building on MacOS X (Darwin) requires a work-around for processor detection: # 32-bit ./configure --build=i686-apple-darwin11 # 64-bit ./configure --build=x86_64-apple-darwin11 For developers using the git tree: This source tree is based on the autotools suite from GNU to simplify portability. Here are some things you should have on your system in order to compile the git repository tree : - GNU autotools (automake >=1.10, autoconf >=2.50, autoheader >=2.50) (make sure your system wide "automake" points to a recent version!) - GNU Libtool >=2.2 (for more information, go to http://www.gnu.org/software/autoconf/) If you get the tree from the repository, you will need to use the "bootstrap" script in the root of the tree. It calls all the GNU tools needed to prepare the tree configuration. Test scripts provided in the tests/ directory of the source tree depend on "bash" and the "seq" program. 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, in terms of grace-period detection speed, read-side speed and flexibility. Dynamically detects kernel support for sys_membarrier(). Falls back on urcu-mb scheme if support is not present, which has slower read-side. 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-mb * #include * Compile any _LGPL_SOURCE code using this library with "-DRCU_MB". * Link with "-lurcu-mb". * This version of the urcu library uses memory barriers on the writer and reader sides. This results in faster grace-period detection, but results in slower reads. Usage of liburcu-signal * #include * Compile any _LGPL_SOURCE code using this library with "-DRCU_SIGNAL". * Link the application with "-lurcu-signal". * Version of the library that requires a signal, typically SIGUSR1. Can be overridden with -DSIGRCU by modifying Makefile.build.inc. 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. rcu_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 * Follow instructions for either liburcu, liburcu-qsbr, liburcu-mb, liburcu-signal, or liburcu-bp above. The liburcu-defer functionality is pulled into each of those library modules. * Provides defer_rcu() primitive to enqueue delayed callbacks. Queued callbacks are executed in batch periodically after a grace period. Do _not_ use defer_rcu() within a read-side critical section, because it may call synchronize_rcu() if the thread queue is full. This can lead to deadlock or worse. * Requires that rcu_defer_barrier() must be called in library destructor if a library queues callbacks and is expected to be unloaded with dlclose(). * Its API is currently experimental. It may change in future library releases. Usage of urcu-call-rcu * Follow instructions for either liburcu, liburcu-qsbr, liburcu-mb, liburcu-signal, or liburcu-bp above. The urcu-call-rcu functionality is provided for each of these library modules. * Provides the call_rcu() primitive to enqueue delayed callbacks in a manner similar to defer_rcu(), but without ever delaying for a grace period. On the other hand, call_rcu()'s best-case overhead is not quite as good as that of defer_rcu(). * Provides call_rcu() to allow asynchronous handling of RCU grace periods. A number of additional functions are provided to manage the helper threads used by call_rcu(), but reasonable defaults are used if these additional functions are not invoked. See rcu-api.txt in userspace-rcu documentation for more details. 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, except those setup with sigaltstack(2), 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. Interaction with mutexes One must be careful to do not cause deadlocks due to interaction of synchronize_rcu() and RCU read-side with mutexes. If synchronize_rcu() is called with a mutex held, this mutex (or any mutex which has this mutex in its dependency chain) should not be acquired from within a RCU read-side critical section. This is especially important to understand in the context of the QSBR flavor: a registered reader thread being "online" by default should be considered as within a RCU read-side critical section unless explicitly put "offline". Therefore, if synchronize_rcu() is called with a mutex held, this mutex, as well as any mutex which has this mutex in its dependency chain should only be taken when the RCU reader thread is "offline" (this can be performed by calling rcu_thread_offline()). 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. SMP support By default the library is configured to use synchronization primitives adequate for SMP systems. On uniprocessor systems, support for SMP systems can be disabled with: ./configure --disable-smp-support theoretically yielding slightly better performance. Interaction with fork() Special care must be taken for applications performing fork() without any following exec(). This is caused by the fact that Linux only clones the thread calling fork(), and thus never replicates any of the other parent thread into the child process. Most liburcu implementations require that all registrations (as reader, defer_rcu and call_rcu threads) should be released before a fork() is performed, except for the rather common scenario where fork() is immediately followed by exec() in the child process. The only implementation not subject to that rule is liburcu-bp, which is designed to handle fork() by calling rcu_bp_before_fork, rcu_bp_after_fork_parent and rcu_bp_after_fork_child. Applications that use call_rcu() and that fork() without doing an immediate exec() must take special action. The parent must invoke call_rcu_before_fork() before the fork() and call_rcu_after_fork_parent() after the fork(). The child process must invoke call_rcu_after_fork_child(). Even though these three APIs are suitable for passing to pthread_atfork(), use of pthread_atfork() is *STRONGLY DISCOURAGED* for programs calling the glibc memory allocator (malloc(), calloc(), free(), ...) within call_rcu callbacks. This is due to limitations in the way glibc memory allocator handles calls to the memory allocator from concurrent threads while the pthread_atfork() handlers are executing. Combining e.g.: * call to free() from callbacks executed within call_rcu worker threads, * executing call_rcu atfork handlers within the glibc pthread atfork mechanism, will sometimes trigger interesting process hangs. This usually hangs on a memory allocator lock within glibc. Thread Local Storage (TLS) Userspace RCU can fall back on pthread_getspecific() to emulate TLS variables on systems where it is not available. This behavior can be forced by specifying --disable-compiler-tls as configure argument.