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