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