README

   1 Userspace RCU Implementation
   2 by Mathieu Desnoyers and Paul E. McKenney
   3
   4 BUILDING
   5 --------
   6
   7         ./bootstrap (skip if using tarball)
   8         ./configure
   9         make
  10         make install
  11         ldconfig
  12
  13         Hints:  Forcing 32-bit build:
  14                 * CFLAGS="-m32 -g -O2" ./configure
  15
  16                 Forcing 64-bit build:
  17                 * CFLAGS="-m64 -g -O2" ./configure
  18
  19                 Forcing a 32-bit build with 386 backward compatibility:
  20                 * CFLAGS="-m32 -g -O2" ./configure --host=i386-pc-linux-gnu
  21
  22                 Forcing a 32-bit build for Sparcv9 (typical for Sparc v9)
  23                 * CFLAGS="-m32 -Wa,-Av9a -g -O2" ./configure
  24
  25 ARCHITECTURES SUPPORTED
  26 -----------------------
  27
  28 Currently, Linux x86 (i386, i486, i586, i686), x86 64-bit, PowerPC 32/64,
  29 S390, S390x, ARM, Alpha, ia64 and Sparcv9 32/64 are supported. Tested on
  30 Linux, FreeBSD 8.2/9.0, and Cygwin. Should also work on: Android, NetBSD 5,
  31 OpenBSD, Darwin (more testing needed before claiming support for these OS).
  32
  33 Linux ARM depends on running a Linux kernel 2.6.15 or better, GCC 4.4 or
  34 better.
  35
  36 The gcc compiler versions 3.3, 3.4, 4.0, 4.1, 4.2, 4.3, 4.4 and 4.5 are
  37 supported, with the following exceptions:
  38
  39 - gcc 3.3 and 3.4 have a bug that prevents them from generating volatile
  40   accesses to offsets in a TLS structure on 32-bit x86. These versions are
  41   therefore not compatible with liburcu on x86 32-bit (i386, i486, i586, i686).
  42   The problem has been reported to the gcc community:
  43     http://www.mail-archive.com/gcc-bugs@gcc.gnu.org/msg281255.html
  44 - gcc 3.3 cannot match the "xchg" instruction on 32-bit x86 build.
  45   See: http://kerneltrap.org/node/7507
  46 - Alpha, ia64 and ARM architectures depend on gcc 4.x with atomic builtins
  47   support. For ARM this was introduced with gcc 4.4:
  48     http://gcc.gnu.org/gcc-4.4/changes.html
  49
  50 Clang version 3.0 (based on LLVM 3.0) is supported.
  51
  52 Building on MacOS X (Darwin) requires a work-around for processor
  53 detection:
  54   # 32-bit
  55   ./configure --build=i686-apple-darwin11
  56   # 64-bit
  57   ./configure --build=x86_64-apple-darwin11
  58
  59 For developers using the git tree:
  60
  61 This source tree is based on the autotools suite from GNU to simplify
  62 portability. Here are some things you should have on your system in order to
  63 compile the git repository tree :
  64
  65 - GNU autotools (automake >=1.10, autoconf >=2.50, autoheader >=2.50)
  66   (make sure your system wide "automake" points to a recent version!)
  67 - GNU Libtool >=2.2
  68   (for more information, go to http://www.gnu.org/software/autoconf/)
  69
  70 If you get the tree from the repository, you will need to use the "bootstrap"
  71 script in the root of the tree. It calls all the GNU tools needed to prepare the
  72 tree configuration.
  73
  74 Test scripts provided in the tests/ directory of the source tree depend
  75 on "bash" and the "seq" program.
  76
  77
  78 QUICK START GUIDE
  79 -----------------
  80
  81 Usage of all urcu libraries
  82
  83         * Define _LGPL_SOURCE (only) if your code is LGPL or GPL compatible
  84           before including the urcu.h or urcu-qsbr.h header. If your application
  85           is distributed under another license, function calls will be generated
  86           instead of inlines, so your application can link with the library.
  87         * Linking with one of the libraries below is always necessary even for
  88           LGPL and GPL applications.
  89
  90 Usage of liburcu
  91
  92         * #include <urcu.h>
  93         * Link the application with "-lurcu".
  94         * This is the preferred version of the library, in terms of
  95           grace-period detection speed, read-side speed and flexibility.
  96           Dynamically detects kernel support for sys_membarrier(). Falls back
  97           on urcu-mb scheme if support is not present, which has slower
  98           read-side.
  99
 100 Usage of liburcu-qsbr
 101
 102         * #include <urcu-qsbr.h>
 103         * Link with "-lurcu-qsbr".
 104         * The QSBR flavor of RCU needs to have each reader thread executing
 105           rcu_quiescent_state() periodically to progress. rcu_thread_online()
 106           and rcu_thread_offline() can be used to mark long periods for which
 107           the threads are not active. It provides the fastest read-side at the
 108           expense of more intrusiveness in the application code.
 109
 110 Usage of liburcu-mb
 111
 112         * #include <urcu.h>
 113         * Compile any _LGPL_SOURCE code using this library with "-DRCU_MB".
 114         * Link with "-lurcu-mb".
 115         * This version of the urcu library uses memory barriers on the writer
 116           and reader sides. This results in faster grace-period detection, but
 117           results in slower reads.
 118
 119 Usage of liburcu-signal
 120
 121         * #include <urcu.h>
 122         * Compile any _LGPL_SOURCE code using this library with "-DRCU_SIGNAL".
 123         * Link the application with "-lurcu-signal".
 124         * Version of the library that requires a signal, typically SIGUSR1. Can
 125           be overridden with -DSIGRCU by modifying Makefile.build.inc.
 126
 127 Usage of liburcu-bp
 128
 129         * #include <urcu-bp.h>
 130         * Link with "-lurcu-bp".
 131         * The BP library flavor stands for "bulletproof". It is specifically
 132           designed to help tracing library to hook on applications without
 133           requiring to modify these applications. rcu_init(),
 134           rcu_register_thread() and rcu_unregister_thread() all become nops.
 135           The state is dealt with by the library internally at the expense of
 136           read-side and write-side performance.
 137
 138 Initialization
 139
 140         Each thread that has reader critical sections (that uses
 141         rcu_read_lock()/rcu_read_unlock() must first register to the URCU
 142         library. This is done by calling rcu_register_thread(). Unregistration
 143         must be performed before exiting the thread by using
 144         rcu_unregister_thread().
 145
 146 Reading
 147
 148         Reader critical sections must be protected by locating them between
 149         calls to rcu_read_lock() and rcu_read_unlock(). Inside that lock,
 150         rcu_dereference() may be called to read an RCU protected pointer.
 151
 152 Writing
 153
 154         rcu_assign_pointer() and rcu_xchg_pointer() may be called anywhere.
 155         After, synchronize_rcu() must be called. When it returns, the old
 156         values are not in usage anymore.
 157
 158 Usage of liburcu-defer
 159
 160         * Follow instructions for either liburcu, liburcu-qsbr,
 161           liburcu-mb, liburcu-signal, or liburcu-bp above.
 162           The liburcu-defer functionality is pulled into each of
 163           those library modules.
 164         * Provides defer_rcu() primitive to enqueue delayed callbacks. Queued
 165           callbacks are executed in batch periodically after a grace period.
 166           Do _not_ use defer_rcu() within a read-side critical section, because
 167           it may call synchronize_rcu() if the thread queue is full.
 168           This can lead to deadlock or worse.
 169         * Requires that rcu_defer_barrier() must be called in library destructor
 170           if a library queues callbacks and is expected to be unloaded with
 171           dlclose().
 172         * Its API is currently experimental. It may change in future library
 173           releases.
 174
 175 Usage of urcu-call-rcu
 176
 177         * Follow instructions for either liburcu, liburcu-qsbr,
 178           liburcu-mb, liburcu-signal, or liburcu-bp above.
 179           The urcu-call-rcu functionality is provided for each of
 180           these library modules.
 181         * Provides the call_rcu() primitive to enqueue delayed callbacks
 182           in a manner similar to defer_rcu(), but without ever delaying
 183           for a grace period.  On the other hand, call_rcu()'s best-case
 184           overhead is not quite as good as that of defer_rcu().
 185         * Provides call_rcu() to allow asynchronous handling of RCU
 186           grace periods.  A number of additional functions are provided
 187           to manage the helper threads used by call_rcu(), but reasonable
 188           defaults are used if these additional functions are not invoked.
 189           See rcu-api.txt in userspace-rcu documentation for more details.
 190
 191 Being careful with signals
 192
 193         The liburcu library uses signals internally. The signal handler is
 194         registered with the SA_RESTART flag. However, these signals may cause
 195         some non-restartable system calls to fail with errno = EINTR. Care
 196         should be taken to restart system calls manually if they fail with this
 197         error. A list of non-restartable system calls may be found in
 198         signal(7). The liburcu-mb and liburcu-qsbr versions of the Userspace RCU
 199         library do not require any signal.
 200
 201         Read-side critical sections are allowed in a signal handler,
 202         except those setup with sigaltstack(2), with liburcu and
 203         liburcu-mb. Be careful, however, to disable these signals
 204         between thread creation and calls to rcu_register_thread(), because a
 205         signal handler nesting on an unregistered thread would not be
 206         allowed to call rcu_read_lock().
 207
 208         Read-side critical sections are _not_ allowed in a signal handler with
 209         liburcu-qsbr, unless signals are disabled explicitly around each
 210         rcu_quiescent_state() calls, when threads are put offline and around
 211         calls to synchronize_rcu(). Even then, we do not recommend it.
 212
 213 Interaction with mutexes
 214
 215         One must be careful to do not cause deadlocks due to interaction of
 216         synchronize_rcu() and RCU read-side with mutexes. If synchronize_rcu()
 217         is called with a mutex held, this mutex (or any mutex which has this
 218         mutex in its dependency chain) should not be acquired from within a RCU
 219         read-side critical section.
 220
 221         This is especially important to understand in the context of the
 222         QSBR flavor: a registered reader thread being "online" by
 223         default should be considered as within a RCU read-side critical
 224         section unless explicitly put "offline". Therefore, if
 225         synchronize_rcu() is called with a mutex held, this mutex, as
 226         well as any mutex which has this mutex in its dependency chain
 227         should only be taken when the RCU reader thread is "offline"
 228         (this can be performed by calling rcu_thread_offline()).
 229
 230 Usage of DEBUG_RCU
 231
 232         DEBUG_RCU is used to add internal debugging self-checks to the
 233         RCU library. This define adds a performance penalty when enabled.
 234         Can be enabled by uncommenting the corresponding line in
 235         Makefile.build.inc.
 236
 237 Usage of DEBUG_YIELD
 238
 239         DEBUG_YIELD is used to add random delays in the code for testing
 240         purposes.
 241
 242 SMP support
 243
 244         By default the library is configured to use synchronization primitives
 245         adequate for SMP systems. On uniprocessor systems, support for SMP
 246         systems can be disabled with:
 247
 248                 ./configure --disable-smp-support
 249
 250         theoretically yielding slightly better performance.
 251
 252 Interaction with fork()
 253
 254         Special care must be taken for applications performing fork() without
 255         any following exec(). This is caused by the fact that Linux only clones
 256         the thread calling fork(), and thus never replicates any of the other
 257         parent thread into the child process. Most liburcu implementations
 258         require that all registrations (as reader, defer_rcu and call_rcu
 259         threads) should be released before a fork() is performed, except for the
 260         rather common scenario where fork() is immediately followed by exec() in
 261         the child process. The only implementation not subject to that rule is
 262         liburcu-bp, which is designed to handle fork() by calling
 263         rcu_bp_before_fork, rcu_bp_after_fork_parent and
 264         rcu_bp_after_fork_child.
 265
 266         Applications that use call_rcu() and that fork() without
 267         doing an immediate exec() must take special action.  The parent
 268         must invoke call_rcu_before_fork() before the fork() and
 269         call_rcu_after_fork_parent() after the fork().  The child
 270         process must invoke call_rcu_after_fork_child().
 271         Even though these three APIs are suitable for passing to
 272         pthread_atfork(), use of pthread_atfork() is *STRONGLY
 273         DISCOURAGED* for programs calling the glibc memory allocator
 274         (malloc(), calloc(), free(), ...) within call_rcu callbacks.
 275         This is due to limitations in the way glibc memory allocator
 276         handles calls to the memory allocator from concurrent threads
 277         while the pthread_atfork() handlers are executing.
 278         Combining e.g.:
 279         * call to free() from callbacks executed within call_rcu worker
 280           threads,
 281         * executing call_rcu atfork handlers within the glibc pthread
 282           atfork mechanism,
 283         will sometimes trigger interesting process hangs. This usually
 284         hangs on a memory allocator lock within glibc.
 285
 286 Thread Local Storage (TLS)
 287
 288         Userspace RCU can fall back on pthread_getspecific() to emulate
 289         TLS variables on systems where it is not available. This behavior
 290         can be forced by specifying --disable-compiler-tls as configure
 291         argument.