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