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