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, x86 (i386, i486, i586, i686), x86 64-bit, PowerPC 32/64, S390, S390x,
  29 ARM, Alpha, ia64 and Sparcv9 32/64 are supported. Only tested on Linux so
  30 far, but should theoretically work on other operating systems.
  31
  32 ARM depends on running a Linux kernel 2.6.15 or better.
  33
  34 The gcc compiler versions 3.3, 3.4, 4.0, 4.1, 4.2, 4.3, 4.4 and 4.5 are
  35 supported, with the following exceptions:
  36
  37 - gcc 3.3 and 3.4 have a bug that prevents them from generating volatile
  38   accesses to offsets in a TLS structure on 32-bit x86. These versions are
  39   therefore not compatible with liburcu on x86 32-bit (i386, i486, i586, i686).
  40   The problem has been reported to the gcc community:
  41     http://www.mail-archive.com/gcc-bugs@gcc.gnu.org/msg281255.html
  42 - gcc 3.3 cannot match the "xchg" instruction on 32-bit x86 build.
  43   See: http://kerneltrap.org/node/7507
  44 - Alpha, ia64 and ARM architectures depend on 4.x gcc with atomic builtins
  45   support.
  46
  47
  48 QUICK START GUIDE
  49 -----------------
  50
  51 Usage of all urcu libraries
  52
  53         * Define _LGPL_SOURCE (only) if your code is LGPL or GPL compatible
  54           before including the urcu.h or urcu-qsbr.h header. If your application
  55           is distributed under another license, function calls will be generated
  56           instead of inlines, so your application can link with the library.
  57         * Linking with one of the libraries below is always necessary even for
  58           LGPL and GPL applications.
  59
  60 Usage of liburcu
  61
  62         * #include <urcu.h>
  63         * Link the application with "-lurcu".
  64         * This is the preferred version of the library, in terms of
  65           grace-period detection speed, read-side speed and flexibility.
  66           Dynamically detects kernel support for sys_membarrier(). Falls back
  67           on urcu-mb scheme if support is not present, which has slower
  68           read-side.
  69
  70 Usage of liburcu-qsbr
  71
  72         * #include <urcu-qsbr.h>
  73         * Link with "-lurcu-qsbr".
  74         * The QSBR flavor of RCU needs to have each reader thread executing
  75           rcu_quiescent_state() periodically to progress. rcu_thread_online()
  76           and rcu_thread_offline() can be used to mark long periods for which
  77           the threads are not active. It provides the fastest read-side at the
  78           expense of more intrusiveness in the application code.
  79
  80 Usage of liburcu-mb
  81
  82         * #include <urcu.h>
  83         * Compile any _LGPL_SOURCE code using this library with "-DRCU_MB".
  84         * Link with "-lurcu-mb".
  85         * This version of the urcu library uses memory barriers on the writer
  86           and reader sides. This results in faster grace-period detection, but
  87           results in slower reads.
  88
  89 Usage of liburcu-signal
  90
  91         * #include <urcu.h>
  92         * Compile any _LGPL_SOURCE code using this library with "-DRCU_SIGNAL".
  93         * Link the application with "-lurcu-signal".
  94         * Version of the library that requires a signal, typically SIGUSR1. Can
  95           be overridden with -DSIGRCU by modifying Makefile.build.inc.
  96
  97 Usage of liburcu-bp
  98
  99         * #include <urcu-bp.h>
 100         * Link with "-lurcu-bp".
 101         * The BP library flavor stands for "bulletproof". It is specifically
 102           designed to help tracing library to hook on applications without
 103           requiring to modify these applications. rcu_init(),
 104           rcu_register_thread() and rcu_unregister_thread() all become nops.
 105           The state is dealt with by the library internally at the expense of
 106           read-side and write-side performance.
 107
 108 Initialization
 109
 110         Each thread that has reader critical sections (that uses
 111         rcu_read_lock()/rcu_read_unlock() must first register to the URCU
 112         library. This is done by calling rcu_register_thread(). Unregistration
 113         must be performed before exiting the thread by using
 114         rcu_unregister_thread().
 115
 116 Reading
 117
 118         Reader critical sections must be protected by locating them between
 119         calls to rcu_read_lock() and rcu_read_unlock(). Inside that lock,
 120         rcu_dereference() may be called to read an RCU protected pointer.
 121
 122 Writing
 123
 124         rcu_assign_pointer() and rcu_xchg_pointer() may be called anywhere.
 125         After, synchronize_rcu() must be called. When it returns, the old
 126         values are not in usage anymore.
 127
 128 Usage of liburcu-defer
 129
 130         * Follow instructions for either liburcu, liburcu-qsbr,
 131           liburcu-mb, liburcu-signal, or liburcu-bp above.
 132           The liburcu-defer functionality is pulled into each of
 133           those library modules.
 134         * Provides defer_rcu() primitive to enqueue delayed callbacks. Queued
 135           callbacks are executed in batch periodically after a grace period.
 136           Do _not_ use defer_rcu() within a read-side critical section, because
 137           it may call synchronize_rcu() if the thread queue is full.
 138           This can lead to deadlock or worse.
 139         * Requires that rcu_defer_barrier() must be called in library destructor
 140           if a library queues callbacks and is expected to be unloaded with
 141           dlclose().
 142         * Its API is currently experimental. It may change in future library
 143           releases.
 144
 145 Usage of urcu-call-rcu
 146
 147         * Follow instructions for either liburcu, liburcu-qsbr,
 148           liburcu-mb, liburcu-signal, or liburcu-bp above.
 149           The urcu-call-rcu functionality is provided for each of
 150           these library modules.
 151         * Provides the call_rcu() primitive to enqueue delayed callbacks
 152           in a manner similar to defer_rcu(), but without ever delaying
 153           for a grace period.  On the other hand, call_rcu()'s best-case
 154           overhead is not quite as good as that of defer_rcu().
 155         * Provides call_rcu() to allow asynchronous handling of RCU
 156           grace periods.  A number of additional functions are provided
 157           to manage the helper threads used by call_rcu(), but reasonable
 158           defaults are used if these additional functions are not invoked.
 159           See API.txt for more details.
 160
 161 Being careful with signals
 162
 163         The liburcu library uses signals internally. The signal handler is
 164         registered with the SA_RESTART flag. However, these signals may cause
 165         some non-restartable system calls to fail with errno = EINTR. Care
 166         should be taken to restart system calls manually if they fail with this
 167         error. A list of non-restartable system calls may be found in
 168         signal(7). The liburcu-mb and liburcu-qsbr versions of the Userspace RCU
 169         library do not require any signal.
 170
 171         Read-side critical sections are allowed in a signal handler with
 172         liburcu and liburcu-mb. Be careful, however, to disable these signals
 173         between thread creation and calls to rcu_register_thread(), because a
 174         signal handler nesting on an unregistered thread would not be allowed to
 175         call rcu_read_lock().
 176
 177         Read-side critical sections are _not_ allowed in a signal handler with
 178         liburcu-qsbr, unless signals are disabled explicitly around each
 179         rcu_quiescent_state() calls, when threads are put offline and around
 180         calls to synchronize_rcu(). Even then, we do not recommend it.
 181
 182 Interaction with mutexes
 183
 184         One must be careful to do not cause deadlocks due to interaction of
 185         synchronize_rcu() and RCU read-side with mutexes. If synchronize_rcu()
 186         is called with a mutex held, this mutex (or any mutex which has this
 187         mutex in its dependency chain) should not be acquired from within a RCU
 188         read-side critical section.
 189
 190         This is especially important to understand in the context of the
 191         QSBR flavor: a registered reader thread being "online" by
 192         default should be considered as within a RCU read-side critical
 193         section unless explicitly put "offline". Therefore, if
 194         synchronize_rcu() is called with a mutex held, this mutex, as
 195         well as any mutex which has this mutex in its dependency chain
 196         should only be taken when the RCU reader thread is "offline"
 197         (this can be performed by calling rcu_thread_offline()).
 198
 199 Usage of DEBUG_RCU
 200
 201         DEBUG_RCU is used to add internal debugging self-checks to the
 202         RCU library. This define adds a performance penalty when enabled.
 203         Can be enabled by uncommenting the corresponding line in
 204         Makefile.build.inc.
 205
 206 Usage of DEBUG_YIELD
 207
 208         DEBUG_YIELD is used to add random delays in the code for testing
 209         purposes.
 210
 211 SMP support
 212
 213         By default the library is configured to use synchronization primitives
 214         adequate for SMP systems. On uniprocessor systems, support for SMP
 215         systems can be disabled with:
 216
 217                 ./configure --disable-smp-support
 218
 219         theoretically yielding slightly better performance.
 220
 221 Interaction with fork()
 222
 223         Special care must be taken for applications performing fork() without
 224         any following exec(). This is caused by the fact that Linux only clones
 225         the thread calling fork(), and thus never replicates any of the other
 226         parent thread into the child process. Most liburcu implementations
 227         require that all registrations (as reader, defer_rcu and call_rcu
 228         threads) should be released before a fork() is performed, except for the
 229         rather common scenario where fork() is immediately followed by exec() in
 230         the child process. The only implementation not subject to that rule is
 231         liburcu-bp, which is designed to handle fork() by calling
 232         rcu_bp_before_fork, rcu_bp_after_fork_parent and
 233         rcu_bp_after_fork_child.
 234
 235         Applications that use call_rcu() and that fork() without
 236         doing an immediate exec() must take special action.  The parent
 237         must invoke call_rcu_before_fork() before the fork() and
 238         call_rcu_after_fork_parent() after the fork().  The child
 239         process must invoke call_rcu_after_fork_child().
 240         These three APIs are suitable for passing to pthread_atfork().