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
  12         Hints:  Forcing 32-bit build:
  13                 * CFLAGS="-m32 -g -O2" ./configure
  14
  15                 Forcing 64-bit build:
  16                 * CFLAGS="-m64 -g -O2" ./configure
  17
  18                 Forcing a 32-bit build with 386 backward compatibility:
  19                 * CFLAGS="-m32 -g -O2" ./configure --host=i386-pc-linux-gnu
  20
  21                 Forcing a 32-bit build for Sparcv9 (typical for Sparc v9)
  22                 * CFLAGS="-m32 -Wa,-Av9a -g -O2" ./configure
  23
  24 ARCHITECTURES SUPPORTED
  25 -----------------------
  26
  27 Currently, x86 (i386, i486, i586, i686), x86 64-bit, PowerPC 32/64, S390, S390x
  28 and Sparcv9 32/64 are supported. Only tested on Linux so far, but should
  29 theoretically work on other operating systems.
  30
  31 Alpha and ia64 architectures are supported, but depend on 4.x gcc with atomic
  32 builtins support.
  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
  43 QUICK START GUIDE
  44 -----------------
  45
  46 Usage of all urcu libraries
  47
  48         * Define _LGPL_SOURCE (only) if your code is LGPL or GPL compatible
  49           before including the urcu.h or urcu-qsbr.h header. If your application
  50           is distributed under another license, function calls will be generated
  51           instead of inlines, so your application can link with the library.
  52         * Linking with one of the libraries below is always necessary even for
  53           LGPL and GPL applications.
  54
  55 Usage of liburcu
  56
  57         * #include <urcu.h>
  58         * Link the application with "-lurcu".
  59         * This is the preferred version of the library, in terms of
  60           grace-period detection speed, read-side speed and flexibility.
  61           Dynamically detects kernel support for sys_membarrier(). Falls back
  62           on urcu-mb scheme if support is not present, which has slower
  63           read-side.
  64
  65 Usage of liburcu-qsbr
  66
  67         * #include <urcu-qsbr.h>
  68         * Link with "-lurcu-qsbr".
  69         * The QSBR flavor of RCU needs to have each reader thread executing
  70           rcu_quiescent_state() periodically to progress. rcu_thread_online()
  71           and rcu_thread_offline() can be used to mark long periods for which
  72           the threads are not active. It provides the fastest read-side at the
  73           expense of more intrusiveness in the application code.
  74
  75 Usage of liburcu-mb
  76
  77         * #include <urcu.h>
  78         * Compile any _LGPL_SOURCE code using this library with "-DRCU_MB".
  79         * Link with "-lurcu-mb".
  80         * This version of the urcu library uses memory barriers on the writer
  81           and reader sides. This results in faster grace-period detection, but
  82           results in slower reads.
  83
  84 Usage of liburcu-signal
  85
  86         * #include <urcu.h>
  87         * Compile any _LGPL_SOURCE code using this library with "-DRCU_SIGNAL".
  88         * Link the application with "-lurcu-signal".
  89         * Version of the library that requires a signal, typically SIGUSR1. Can
  90           be overridden with -DSIGRCU by modifying Makefile.build.inc.
  91
  92 Usage of liburcu-bp
  93
  94         * #include <urcu-bp.h>
  95         * Link with "-lurcu-bp".
  96         * The BP library flavor stands for "bulletproof". It is specifically
  97           designed to help tracing library to hook on applications without
  98           requiring to modify these applications. rcu_init(),
  99           rcu_register_thread() and rcu_unregister_thread() all become nops.
 100           The state is dealt with by the library internally at the expense of
 101           read-side and write-side performance.
 102
 103 Initialization
 104
 105         Each thread that has reader critical sections (that uses
 106         rcu_read_lock()/rcu_read_unlock() must first register to the URCU
 107         library. This is done by calling rcu_register_thread(). Unregistration
 108         must be performed before exiting the thread by using
 109         rcu_unregister_thread().
 110
 111 Reading
 112
 113         Reader critical sections must be protected by locating them between
 114         calls to rcu_read_lock() and rcu_read_unlock(). Inside that lock,
 115         rcu_dereference() may be called to read an RCU protected pointer.
 116
 117 Writing
 118
 119         rcu_assign_pointer() and rcu_xchg_pointer() may be called anywhere.
 120         After, synchronize_rcu() must be called. When it returns, the old
 121         values are not in usage anymore.
 122
 123 Usage of liburcu-defer
 124
 125         * #include <urcu-defer.h>
 126         * Link with "-lurcu-defer", and also with one of the urcu library
 127           (either urcu, urcu-bp, urcu-mb or urcu-qsbr).
 128         * Provides defer_rcu() primitive to enqueue delayed callbacks. Queued
 129           callbacks are executed in batch periodically after a grace period.
 130           Do _not_ use defer_rcu() within a read-side critical section, because
 131           it may call synchronize_rcu() if the thread queue is full.
 132         * Provides defer_rcu_ratelimit() primitive, which acts just like
 133           defer_rcu(), but takes an additional rate limiter callback forcing
 134           synchronized callback execution of the limiter returns non-zero.
 135         * Requires that rcu_defer_barrier() must be called in library destructor
 136           if a library queues callbacks and is expected to be unloaded with
 137           dlclose().
 138         * Its API is currently experimental. It may change in future library
 139           releases.
 140
 141 Being careful with signals
 142
 143         The liburcu library uses signals internally. The signal handler is
 144         registered with the SA_RESTART flag. However, these signals may cause
 145         some non-restartable system calls to fail with errno = EINTR. Care
 146         should be taken to restart system calls manually if they fail with this
 147         error. A list of non-restartable system calls may be found in
 148         signal(7). The liburcu-mb and liburcu-qsbr versions of the Userspace RCU
 149         library do not require any signal.
 150
 151         Read-side critical sections are allowed in a signal handler with
 152         liburcu and liburcu-mb. Be careful, however, to disable these signals
 153         between thread creation and calls to rcu_register_thread(), because a
 154         signal handler nesting on an unregistered thread would not be allowed to
 155         call rcu_read_lock().
 156
 157         Read-side critical sections are _not_ allowed in a signal handler with
 158         liburcu-qsbr, unless signals are disabled explicitly around each
 159         rcu_quiescent_state() calls, when threads are put offline and around
 160         calls to synchronize_rcu(). Even then, we do not recommend it.
 161
 162 Interaction with mutexes
 163
 164         One must be careful to do not cause deadlocks due to interaction of
 165         synchronize_rcu() and RCU read-side with mutexes. If synchronize_rcu()
 166         is called with a mutex held, this mutex (or any mutex which has this
 167         mutex in its dependency chain) should not be acquired from within a RCU
 168         read-side critical section.
 169
 170 Usage of DEBUG_RCU
 171
 172         DEBUG_RCU is used to add internal debugging self-checks to the
 173         RCU library. This define adds a performance penalty when enabled.
 174         Can be enabled by uncommenting the corresponding line in
 175         Makefile.build.inc.
 176
 177 Usage of DEBUG_YIELD
 178
 179         DEBUG_YIELD is used to add random delays in the code for testing
 180         purposes.
 181
 182 SMP support
 183
 184         By default the library is configured to use synchronization primitives
 185         adequate for SMP systems. On uniprocessor systems, support for SMP
 186         systems can be disabled with:
 187
 188                 ./configure --disable-smp-support
 189
 190         theoretically yielding slightly better performance.