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
  26 ARCHITECTURES SUPPORTED
  27 -----------------------
  28
  29 Currently, Linux x86 (i386, i486, i586, i686), x86 64-bit, PowerPC 32/64,
  30 S390, S390x, ARM 32/64, MIPS, Alpha, ia64, Sparcv9 32/64, Tilera, and
  31 hppa/PA-RISC are supported.
  32 Tested on Linux, FreeBSD 8.2/8.3/9.0/9.1/10.0 i386/amd64, and Cygwin.
  33 Should also work on: Android, NetBSD 5, OpenBSD, Darwin (more testing
  34 needed before claiming support for these OS).
  35
  36 Linux ARM depends on running a Linux kernel 2.6.15 or better, GCC 4.4 or
  37 better.
  38
  39 The gcc compiler versions 3.3, 3.4, 4.0, 4.1, 4.2, 4.3, 4.4 and 4.5 are
  40 supported, with the following exceptions:
  41
  42 - gcc 3.3 and 3.4 have a bug that prevents them from generating volatile
  43   accesses to offsets in a TLS structure on 32-bit x86. These versions are
  44   therefore not compatible with liburcu on x86 32-bit (i386, i486, i586, i686).
  45   The problem has been reported to the gcc community:
  46     http://www.mail-archive.com/gcc-bugs@gcc.gnu.org/msg281255.html
  47 - gcc 3.3 cannot match the "xchg" instruction on 32-bit x86 build.
  48   See: http://kerneltrap.org/node/7507
  49 - Alpha, ia64 and ARM architectures depend on gcc 4.x with atomic builtins
  50   support. For ARM this was introduced with gcc 4.4:
  51     http://gcc.gnu.org/gcc-4.4/changes.html
  52
  53 Clang version 3.0 (based on LLVM 3.0) is supported.
  54
  55 Building on MacOS X (Darwin) requires a work-around for processor
  56 detection:
  57   # 32-bit
  58   ./configure --build=i686-apple-darwin11
  59   # 64-bit
  60   ./configure --build=x86_64-apple-darwin11
  61
  62 For developers using the git tree:
  63
  64 This source tree is based on the autotools suite from GNU to simplify
  65 portability. Here are some things you should have on your system in order to
  66 compile the git repository tree :
  67
  68 - GNU autotools (automake >=1.10, autoconf >=2.50, autoheader >=2.50)
  69   (make sure your system wide "automake" points to a recent version!)
  70 - GNU Libtool >=2.2
  71   (for more information, go to http://www.gnu.org/software/autoconf/)
  72
  73 If you get the tree from the repository, you will need to use the "bootstrap"
  74 script in the root of the tree. It calls all the GNU tools needed to prepare the
  75 tree configuration.
  76
  77 Test scripts provided in the tests/ directory of the source tree depend
  78 on "bash" and the "seq" program.
  79
  80
  81 API
  82 ---
  83
  84 See the relevant API documentation files in doc/. The APIs provided by
  85 Userspace RCU are, by prefix:
  86
  87 - rcu_ : Read-Copy Update (see doc/rcu-api.txt)
  88 - cmm_ : Concurrent Memory Model
  89 - caa_ : Concurrent Architecture Abstraction
  90 - cds_ : Concurrent Data Structures (see doc/cds-api.txt)
  91 - uatomic_: Userspace Atomic (see doc/uatomic-api.txt)
  92
  93
  94 QUICK START GUIDE
  95 -----------------
  96
  97 Usage of all urcu libraries
  98
  99         * Define _LGPL_SOURCE (only) if your code is LGPL or GPL compatible
 100           before including the urcu.h or urcu-qsbr.h header. If your application
 101           is distributed under another license, function calls will be generated
 102           instead of inlines, so your application can link with the library.
 103         * Linking with one of the libraries below is always necessary even for
 104           LGPL and GPL applications.
 105         * Define URCU_INLINE_SMALL_FUNCTIONS before including Userspace RCU
 106           headers if you want Userspace RCU to inline small functions (10
 107           lines or less) into the application. It can be used by applications
 108           distributed under any kind of license, and does *not* make the
 109           application a derived work of Userspace RCU.
 110
 111           Those small inlined functions are guaranteed to match the library
 112           content as long as the library major version is unchanged.
 113           Therefore, the application *must* be compiled with headers matching
 114           the library major version number. Applications using
 115           URCU_INLINE_SMALL_FUNCTIONS may be unable to use debugging
 116           features of Userspace RCU without being recompiled.
 117
 118
 119 Usage of liburcu
 120
 121         * #include <urcu.h>
 122         * Link the application with "-lurcu".
 123         * This is the preferred version of the library, in terms of
 124           grace-period detection speed, read-side speed and flexibility.
 125           Dynamically detects kernel support for sys_membarrier(). Falls back
 126           on urcu-mb scheme if support is not present, which has slower
 127           read-side.
 128
 129 Usage of liburcu-qsbr
 130
 131         * #include <urcu-qsbr.h>
 132         * Link with "-lurcu-qsbr".
 133         * The QSBR flavor of RCU needs to have each reader thread executing
 134           rcu_quiescent_state() periodically to progress. rcu_thread_online()
 135           and rcu_thread_offline() can be used to mark long periods for which
 136           the threads are not active. It provides the fastest read-side at the
 137           expense of more intrusiveness in the application code.
 138
 139 Usage of liburcu-mb
 140
 141         * #include <urcu.h>
 142         * Compile any _LGPL_SOURCE code using this library with "-DRCU_MB".
 143         * Link with "-lurcu-mb".
 144         * This version of the urcu library uses memory barriers on the writer
 145           and reader sides. This results in faster grace-period detection, but
 146           results in slower reads.
 147
 148 Usage of liburcu-signal
 149
 150         * #include <urcu.h>
 151         * Compile any _LGPL_SOURCE code using this library with "-DRCU_SIGNAL".
 152         * Link the application with "-lurcu-signal".
 153         * Version of the library that requires a signal, typically SIGUSR1. Can
 154           be overridden with -DSIGRCU by modifying Makefile.build.inc.
 155
 156 Usage of liburcu-bp
 157
 158         * #include <urcu-bp.h>
 159         * Link with "-lurcu-bp".
 160         * The BP library flavor stands for "bulletproof". It is specifically
 161           designed to help tracing library to hook on applications without
 162           requiring to modify these applications. rcu_init(),
 163           rcu_register_thread() and rcu_unregister_thread() all become nops.
 164           The state is dealt with by the library internally at the expense of
 165           read-side and write-side performance.
 166
 167 Initialization
 168
 169         Each thread that has reader critical sections (that uses
 170         rcu_read_lock()/rcu_read_unlock() must first register to the URCU
 171         library. This is done by calling rcu_register_thread(). Unregistration
 172         must be performed before exiting the thread by using
 173         rcu_unregister_thread().
 174
 175 Reading
 176
 177         Reader critical sections must be protected by locating them between
 178         calls to rcu_read_lock() and rcu_read_unlock(). Inside that lock,
 179         rcu_dereference() may be called to read an RCU protected pointer.
 180
 181 Writing
 182
 183         rcu_assign_pointer() and rcu_xchg_pointer() may be called anywhere.
 184         After, synchronize_rcu() must be called. When it returns, the old
 185         values are not in usage anymore.
 186
 187 Usage of liburcu-defer
 188
 189         * Follow instructions for either liburcu, liburcu-qsbr,
 190           liburcu-mb, liburcu-signal, or liburcu-bp above.
 191           The liburcu-defer functionality is pulled into each of
 192           those library modules.
 193         * Provides defer_rcu() primitive to enqueue delayed callbacks. Queued
 194           callbacks are executed in batch periodically after a grace period.
 195           Do _not_ use defer_rcu() within a read-side critical section, because
 196           it may call synchronize_rcu() if the thread queue is full.
 197           This can lead to deadlock or worse.
 198         * Requires that rcu_defer_barrier() must be called in library destructor
 199           if a library queues callbacks and is expected to be unloaded with
 200           dlclose().
 201         * Its API is currently experimental. It may change in future library
 202           releases.
 203
 204 Usage of urcu-call-rcu
 205
 206         * Follow instructions for either liburcu, liburcu-qsbr,
 207           liburcu-mb, liburcu-signal, or liburcu-bp above.
 208           The urcu-call-rcu functionality is provided for each of
 209           these library modules.
 210         * Provides the call_rcu() primitive to enqueue delayed callbacks
 211           in a manner similar to defer_rcu(), but without ever delaying
 212           for a grace period.  On the other hand, call_rcu()'s best-case
 213           overhead is not quite as good as that of defer_rcu().
 214         * Provides call_rcu() to allow asynchronous handling of RCU
 215           grace periods.  A number of additional functions are provided
 216           to manage the helper threads used by call_rcu(), but reasonable
 217           defaults are used if these additional functions are not invoked.
 218           See rcu-api.txt in userspace-rcu documentation for more details.
 219
 220 Being careful with signals
 221
 222         The liburcu library uses signals internally. The signal handler is
 223         registered with the SA_RESTART flag. However, these signals may cause
 224         some non-restartable system calls to fail with errno = EINTR. Care
 225         should be taken to restart system calls manually if they fail with this
 226         error. A list of non-restartable system calls may be found in
 227         signal(7). The liburcu-mb and liburcu-qsbr versions of the Userspace RCU
 228         library do not require any signal.
 229
 230         Read-side critical sections are allowed in a signal handler,
 231         except those setup with sigaltstack(2), with liburcu and
 232         liburcu-mb. Be careful, however, to disable these signals
 233         between thread creation and calls to rcu_register_thread(), because a
 234         signal handler nesting on an unregistered thread would not be
 235         allowed to call rcu_read_lock().
 236
 237         Read-side critical sections are _not_ allowed in a signal handler with
 238         liburcu-qsbr, unless signals are disabled explicitly around each
 239         rcu_quiescent_state() calls, when threads are put offline and around
 240         calls to synchronize_rcu(). Even then, we do not recommend it.
 241
 242 Interaction with mutexes
 243
 244         One must be careful to do not cause deadlocks due to interaction of
 245         synchronize_rcu() and RCU read-side with mutexes. If synchronize_rcu()
 246         is called with a mutex held, this mutex (or any mutex which has this
 247         mutex in its dependency chain) should not be acquired from within a RCU
 248         read-side critical section.
 249
 250         This is especially important to understand in the context of the
 251         QSBR flavor: a registered reader thread being "online" by
 252         default should be considered as within a RCU read-side critical
 253         section unless explicitly put "offline". Therefore, if
 254         synchronize_rcu() is called with a mutex held, this mutex, as
 255         well as any mutex which has this mutex in its dependency chain
 256         should only be taken when the RCU reader thread is "offline"
 257         (this can be performed by calling rcu_thread_offline()).
 258
 259 Interaction with fork()
 260
 261         Special care must be taken for applications performing fork() without
 262         any following exec(). This is caused by the fact that Linux only clones
 263         the thread calling fork(), and thus never replicates any of the other
 264         parent thread into the child process. Most liburcu implementations
 265         require that all registrations (as reader, defer_rcu and call_rcu
 266         threads) should be released before a fork() is performed, except for the
 267         rather common scenario where fork() is immediately followed by exec() in
 268         the child process. The only implementation not subject to that rule is
 269         liburcu-bp, which is designed to handle fork() by calling
 270         rcu_bp_before_fork, rcu_bp_after_fork_parent and
 271         rcu_bp_after_fork_child.
 272
 273         Applications that use call_rcu() and that fork() without
 274         doing an immediate exec() must take special action.  The parent
 275         must invoke call_rcu_before_fork() before the fork() and
 276         call_rcu_after_fork_parent() after the fork().  The child
 277         process must invoke call_rcu_after_fork_child().
 278         Even though these three APIs are suitable for passing to
 279         pthread_atfork(), use of pthread_atfork() is *STRONGLY
 280         DISCOURAGED* for programs calling the glibc memory allocator
 281         (malloc(), calloc(), free(), ...) within call_rcu callbacks.
 282         This is due to limitations in the way glibc memory allocator
 283         handles calls to the memory allocator from concurrent threads
 284         while the pthread_atfork() handlers are executing.
 285         Combining e.g.:
 286         * call to free() from callbacks executed within call_rcu worker
 287           threads,
 288         * executing call_rcu atfork handlers within the glibc pthread
 289           atfork mechanism,
 290         will sometimes trigger interesting process hangs. This usually
 291         hangs on a memory allocator lock within glibc.
 292
 293 Thread Local Storage (TLS)
 294
 295         Userspace RCU can fall back on pthread_getspecific() to emulate
 296         TLS variables on systems where it is not available. This behavior
 297         can be forced by specifying --disable-compiler-tls as configure
 298         argument.
 299
 300 Usage of DEBUG_RCU
 301
 302         DEBUG_RCU is used to add internal debugging self-checks to the
 303         RCU library. This define adds a performance penalty when enabled.
 304         Can be enabled by uncommenting the corresponding line in
 305         Makefile.build.inc.
 306
 307 Usage of DEBUG_YIELD
 308
 309         DEBUG_YIELD is used to add random delays in the code for testing
 310         purposes.
 311
 312 SMP support
 313
 314         By default the library is configured to use synchronization primitives
 315         adequate for SMP systems. On uniprocessor systems, support for SMP
 316         systems can be disabled with:
 317
 318                 ./configure --disable-smp-support
 319
 320         theoretically yielding slightly better performance.
 321
 322 MAKE TARGETS
 323 ------------
 324
 325 In addition to the usual "make check" target, Userspace RCU features
 326 "make regtest" and "make bench" targets.
 327
 328 make check:   Short tests, meant to be run when rebuilding or porting
 329               Userspace RCU.
 330
 331 make regtest: Long (many hours) test, meant to be run when modifying
 332               Userspace RCU or porting it to a new architecture or
 333               operating system.
 334
 335 make bench:   Long (many hours) benchmarks.
 336
 337 CONTACTS
 338 --------
 339
 340 You can contact the maintainers on the following mailing list:
 341 lttng-dev@lists.lttng.org